• Skip to main content
  • Skip to primary sidebar

Homemade Circuit Projects

Get free circuit help 24/7

New Projects | Privacy Policy | About us | Contact | Disclaimer | Copyright | Videos | Circuits for Beginners | Basic Circuits | Hobby Projects | Transistor Circuits | LED Drivers 

You are here: Home / Battery Chargers / Low Battery Indicator Circuit Using Two Transistors Only

Low Battery Indicator Circuit Using Two Transistors Only

Last Updated on December 22, 2022 by Swagatam 199 Comments

caution electricity can be dangerous

The following post describes a simple low battery indicator circuit by using just two inexpensive NPN transistors. The main feature of this circuit is its very low stand by current consumption.

The Circuit Concept

We have so far seen how to make a low battery indicator circuits using a 741 IC and a 555 IC, which are no doubt outstanding with their abilities of detecting and indicating low battery voltage thresholds.

However the following post relates yet another similar circuit which is much cheaper and employs just a couple of NPN transistors for producing the required low battery indications.

Advantage of Transistor over IC

The main advantage of the proposed two transistor low battery indicator circuit is its very low current consumption compared to the IC counterparts which consume relatively higher currents.

A IC 555 would consume around 5mA, a IC741 around 3 mA, while the present circuit would just consume around 1.5mA current.

Thus the present circuit becomes more efficient especially in cases where stand by current consumption tend to become an issue, example suppose in units which depend on low current battery supplies such as a 9V PP3 battery.

Circuit can Operate at 1.5V

Another advantage of this circuit is it's ability to work even at voltages around 1.5V which gives it a clear edge over the IC based circuits.

As shown in the following circuit diagram, the two transistors are configured as voltage sensor and inverter.

The first transistor on the left senses the threshold voltage level as per the setting of the 47K preset. As long as this transistor conducts, the second transistor on the right is held switched OFF, which also keeps the LED switched OFF.

As soon as the battery voltage falls below the set threshold level, the left transistor is no longer able to conduct.

This situation instantly switches ON the right hand side transistor, enabling the LED to illuminate.

The LED switches ON and provides the required indications of a low battery warning.

Circuit Diagram

Video Demonstration:

The above circuit was successfully built and installed by Mr. Allan in his paranormal depletion detector unit. The following video presents the implementation results:

Upgrading the above Transistorized Low Battery circuit into a Low Battery Cut-off Circuit

Referring to the above diagram, the low battery indicator is formed by the two NPN transistors, while the additional BC557 and the relay are used for cutting OFF the battery from the load when it reaches the lower threshold, in this state the relay connects the battery to the available charging input.

However when the battery is in its normal state the relay connects the battery with the load and allows the load to operate through battery power.

Adding Hysteresis

One drawback of the above design could be the chattering of the relay at the threshold voltage levels, due to the battery voltage dropping immediately during the relay changeover process.

This can be prevented by adding a 100uF at the base of the middle BC547. However, this still wouldn't stop the relay from constantly switching ON/OFF at the low battery changeover threshold.

In order to rectify this, a hysteresis effect will need to be introduced which can be accomplished through a feedback resistor between the collector of the BC557 and the middle BC547 transistor.

The modified design for implementing the above condition can be seen in the following diagram:

The two resistors, one at the base of BC547 and the other at the collector of BC557 decide the other threshold of the relay changeover, meaning the full charge cut off threshold of the battery. Here, the values are arbitrarily selected, for accurate results these values will need to be optimized with some trial and error.

Low Battery Indicator using a PUT

This low battery indicator circuit is used with a programmable unijunction transistor (PUT), since the threshold characteristics of the UJT could be effectively defined, and can be designed to flash a connected LED indicator.

The PUT (Q1) is configured like a relaxation oscillator circuit. As the supply voltage which is being monitored (Vmon) starts dropping, the gate voltage of the PUT (Vg) also begins dropping, while its anode voltage (Va) basically stays constant.

The PUT begins oscillating only as soon as the gate voltage drops below Va by 0.6 volts. As Vmon comes down further, Vg also drops accordingly and this situation triggers ON the PUT. Therefore, the period of the cycle becomes lesser and this causes an increase in the frequency of flashing indicating that the battery has become too low and needs to be changed.

Parts List

You'll also like:

  • 1.  Cell Phone Emergency Charger Pack using Ni-Cd Batteries
  • 2.  3 LED Battery Level Indicator Circuit
  • 3.  Make this Fast Battery Charger Circuit
  • 4.  How to Switch Two Batteries Manually using Opto Coupler
  • 5.  Single Transformer Inverter/Charger Circuit
  • 6.  Lead Acid Battery Charger Circuits

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

Reader Interactions

Comments

    Have Questions? Please post your comments below for quick replies! Comments should be related to the above artcile Cancel reply

    Your email address will not be published. Required fields are marked *

  1. josh says

    December 26, 2022

    Hello again and thanks another reply. Now I understand. I didn’t realize that the potentiometer it’s self functions as a voltage divider. My only experience with them before now was when wiring my guitars and I always thought of them as them as purely resistive components. Anyways, appreciate the responsiveness

    Reply
    • Swagatam says

      December 26, 2022

      No problem! I am glad I could help. All the best to you.

      Reply
  2. Josh L Standridge says

    December 22, 2022

    Thanks again for the reply. I understand the concept of the circuit, just not how with the given values it would work. If the 33k and 47k potentiometer/resistor serve as a voltage divider, then wouldn’t the voltage have to drop to around 1.5 volts before the first transistor would stop conducting?

    Reply
    • Swagatam says

      December 23, 2022

      The potentiometer can be used to adjust the cut off threshold to any desired value with respect to the battery voltage. So if you want the left BC547 to cut off at 11 V, you can adjust the potentiometer accordingly, similarly you can do the same for any other battrety voltage such as 10 V, 10.5 V or 9V etc. The potentiometer gives you the full freedom to adjust the cut threshold to any desired values, as required.

      Reply
  3. josh says

    December 21, 2022

    Hello, not understanding the circuit function of the first 3 diagrams. Wouldn’t more voltage drop across the 33-k resistor biasing the led transistor since there’s the additional 48 k at the base of the first? Also why is there any arrow pointing outward from the base of the first transistor being an npn? Thanks

    Reply
    • Swagatam says

      December 22, 2022

      Hello, the 47K resistors is a potentiometer, or it can be a preset, or a trimpot. The arrow indicates the wiper arm of the potentiometer which is connected with the base of the transistor.

      Reply
      • josh says

        December 22, 2022

        Thank you for the response. I’m still not understanding how the first transistor stops conducting at the threshold voltage with it being above 0.7 volts or how the other transistor doesn’t conduct first being that there’s less resistance to its base? I’m new to electronics so I’m probably missing something obvious here.

        Reply
        • Swagatam says

          December 22, 2022

          Let’s assume the 47K trimpot is adjusted such that the base voltage of the left BC547 drops below 0.7 V when the battery voltage drops below 11 V. This means, as long as the battery voltage is above 11 V, the left BC547 remains switched ON, which causes the base voltage of the right side BC547 to be grounded. Due to this the right side BC547 remains turned OFF, and the LED also remains shut off.
          Now, in a situation when the battery voltage drops below 11 V, the base voltage of the left BC547 also correspondingly drops below 0.7 V, which causes it to shut off. When this happens, the right side BC547 base starts getting the biasing voltage through its 33 K resistor and it switches ON. This also allows the LED to switch ON and indicate the low battery condition.

          Reply
  4. ACG says

    September 14, 2022

    Nice, but a bit dated.
    PMIC supervisor IC chips have long since advanced the art of low voltage detection. The operating currents are in the 10uA to 1uA (micro) range. The pin counts are down to 3 pins. Hysteresis is built in. Price is under 50 seconds for singles and dirt cheap for larger quantities.

    Reply
  5. CHINOMSO says

    October 28, 2021

    Hello sir, please I need your urgent help with this; Is there a way you could modify your above circuit further to include another stage of switching so as to sound a buzzer alarm at 11 volts (for a case of 12v power supply) to warn of low battery before switching of the relay at about 10.5v. I have really tried this with a trial and error method several times, wasting a lot of transistors in the process. Please sir, I would be glad you illustrate with with a diagram. Thanks in anticipation.

    Reply
    • Swagatam says

      October 28, 2021

      Hello Chinomso, there’s no question of wasting transistors in the above circuits, because the transistors are safe and can never burn.

      If you want a sharp response then instead of transistors you can use an opamp based design, as shown below:

      https://www.homemade-circuits.com/wp-content/uploads/2021/06/SG3525-battery-low-voltage-latching-compressed.jpg

      This had been designed for SG3525 inverter, therefore ignore the pin#10 indication.

      Also you can remove the 4k7 and 1k from the output of the opamp and connect the buzzer directly from battery positive to pin#6 of the op amp.

      You can remove the hysteresis network which uses 1N4148 diode and series 1K.

      You can make another similar circuit for activating a relay through a driver transistor. If you are using a transistor with the op amp then the 4k7 and 1k at the output should be included in the circuit, and the transistor base connected to the 4k7 / 1K junction.

      Reply
      • CHINOMSO says

        October 29, 2021

        Thank you so much for the circuit, I really appreciate. However sir, i chose the transistor based because I really want to save as much power as I could. Hence, the circuitry should be such that it would be required to have a two stage switching, such that a set of transistors in TTL mode switches the buzzer on at 11 volts, then as the battery voltage reduces further to 10.5 volts, another set of transistor switches off the relay. I need this in my inverter circuit and would so much appreciate it I could get this from you. Thanks alot

        Reply
        • Swagatam says

          October 29, 2021

          Hello Chinomso, transistor circuit will not be able to provide precise cut offs, sometimes it may be at 10.5V and sometimes 10V or sometimes 10.8V

          Reply
          • CHINOMSO says

            October 30, 2021

            I really would be completely blind to these things If not for your help. I really appreciate you sir

            Reply
            • Swagatam says

              October 30, 2021

              I am glad to help Chinomso!

              Reply
      • CHINOMSO says

        October 30, 2021

        Hello sir, can LM393 opamp be used in the two stage mode, such that : the pin 1 of the ic, for instance, controls the relay, while the pin 7 controls the buzzer for low battery warning?. If it can, please i would be very happy if you could provide me with good diagram to that design. I would love to use 741 opamp as you adviced, but considering space, power consumption and circuit ambiguity, I would want it on a single source. Thanks

        Reply
        • Swagatam says

          October 30, 2021

          Hello Chinomso, the IC 741 can be used with a single power supply, the -battery indicates the 0V or the ground of the Dc supply.

          You can try the following configurations using the LM393 IC:

          low battery cut off with battrey using IC LM393

          Reply
          • CHINOMSO says

            November 1, 2021

            Thanks alot sir, I’ll try this

            Reply
            • Swagatam says

              November 1, 2021

              You are welcome Chinomso, there’s a small mistake in the diagram, the feedback diode 1N4148 must be connected to pun#3 of the opamp and its polarity must be reversed.

              Reply
  6. Joe says

    October 10, 2021

    Can BC337 be used instead of BC547 in first circuit?

    Reply
    • Swagatam says

      October 10, 2021

      Yes, can be used.

      Reply
  7. Gogulanath Sivaprakasam says

    August 2, 2021

    Hi sir
    Can this be optimized for 24 v system

    Reply
    • Swagatam says

      August 2, 2021

      Hi Gogulanath, you can use the same circuit without any customization for 24V also….for the relay circuit, you may have to replace the 12V relay with a 24V relay.

      Reply
  8. Patrick says

    May 20, 2021

    Thanks for this site and for posting this circuit. I have been getting back into electronics with my kids after many years off and your site is great for that purpose. I have a couple of questions about the first circuit (the one that only lights the LED).

    1. In the 12.5V +/- 1V range using a 10k LED for the load this circuit will power the LED with 1.2mA. What LED works at 1.2mA? I usually see current ranges in the 20-30mA. I think I need to lower both the LED driving transistors base and collectors resistors substantially. According to LTSpice 10k and 390 are working values for those two resistors.

    2. How stable do you think the triggering threshold be over time? Let’s say that it triggers at 12.5V at room temp using a BC547B (which may or may not have higher gain than BC547). How likely is it to trigger at the same over a long period of time and a range of different temperatures?

    Thanks

    Reply
    • Swagatam says

      May 20, 2021

      Thank you for asking this question:
      1) I normally use 10K for my red LEDs since they look too bright for any other lower resistor values. The resistor values can be tweaked, there are no criticality regarding the values of the resistors, because ultimately it’s the preset adjustment that decides the detection thresholds.

      2) The transistors will start giving wrong results after around 37 or 40 degrees Celsius, under this the results can be expected to be fairly consistent. The preset quality will also affect the results over a long period of time, since the preset contacts may get dirty and loose or might deviate from the initial adjustment.

      Reply
  9. Yogesh says

    May 5, 2021

    Thanks a lot sir, This circuit is very helpful to me and it is easy to understand ๐Ÿ˜ƒ

    Reply
    • Swagatam says

      May 5, 2021

      You are welcome Yogesh, I appreciate your kind feedback!

      Reply
  10. Al says

    March 18, 2021

    Hello

    I have a battery holder that doesnt have a battery low indicator but I have access to the board.
    This solution is exactly what I am looking for. Can you help not an EE

    Reply
    • Swagatam says

      March 18, 2021

      What help do you need?

      Reply
  11. Dane says

    June 25, 2020

    As you know, I don’t know much but it looks like it might be easier to keep with your initial design for low battery but have it wired to an additional green LED that is manufactured to blink or flash. I think they make them like that. Am I right to stick with your old way and have the additional blinking green LED?

    Reply
    • Swagatam says

      June 25, 2020

      No Problem Dane!

      Reply
  12. Dane says

    June 24, 2020

    I forgot to mention I will be using a 9 volt battery

    Reply
  13. Dane says

    June 24, 2020

    I found a guy who is trying to design a PCB for me. I am very excited and will keep you posted.
    Thank you so much for your help. I was wondering if you could help me again.
    I am wondering if there is a way to adapt and change your low battery circuit to do what I need.
    I have a switch that operates from 1 volt to 24 volts. It has 3 connectors. One is a Common. The other is a Normally Open connector that goes to my Green LED when the switch is on. It has a Normally Closed connector that goes to my Red LED when the switch is off. I would like to avoid adding another LED for the low battery indicator (was going to use a yellow led). Instead, is there a way to make my Green LED blink on and off when the battery gets low? Using only 2 LEDโ€™s would really help. Thank you again for your input.

    Reply
    • Swagatam says

      June 24, 2020

      That great news! However, getting the LED to flash can make the design a bit complex. You can try the last circuit explained in the following article, hope this might help you accomplish your requirement:

      https://www.homemade-circuits.com/flashing-led-battery-status-indicator/

      Reply
  14. Dane says

    June 15, 2020

    I will look into finding a company that can do that here. Thank you again!

    Reply
    • Swagatam says

      June 15, 2020

      Sure, I hope you find an appropriate source soon.

      Reply
  15. Dane says

    June 15, 2020

    Great! Thanks for the clarification. I will start with the 2K2 and see if it’s bright enough. I have a hand drawn diagram of what I am trying to accomplish but it is terrible and I am too embarrassed to post it publicly. I think I am going to need someone to draw it out for me so that maybe I can have a small PCB made. Any ideas on who might be good to help with that?
    Thanks again!

    Reply
    • Swagatam says

      June 15, 2020

      Thanks very much, I understand your problem, however I do not have anybody who can design PCBs correctly, since it is a specialized job and must be done in accordance with the standard PCB designing rules. I guess Only a PCB manufacturer will be able to draw it correctly.

      Reply
  16. Dane says

    June 14, 2020

    Hi again!
    Could you please clarify the circuitry of your low battery warning design?
    The photo you sent me shows a 1k resistor (bottom right) but the diagram shows a 2k2 resistor.
    Which one should I use?

    Reply
    • Swagatam says

      June 14, 2020

      Hi, It can be any value between 680 ohms ohms and probably 10 k for supplies between 9V and 15 V, so you can use any resistor between this range although the light will decrease as the value is increased.

      Reply
  17. Dane says

    June 7, 2020

    Will do!

    Reply
  18. Dane says

    June 6, 2020

    Thanks for the offer. I had planned on sending you a picture of my completed project for your opinion. Any idea how to attach a picture for you and how to send?

    Reply
    • Swagatam says

      June 7, 2020

      OK, for the picture you can upload it to any “free image hosting site’, and provide the link to me, I’ll check it and verify the details for you

      Reply
  19. Dane says

    June 5, 2020

    Once again you have been very, very helpful. I have ordered all of the parts and will start testing the different resistors as soon as they all arrive. Thank you again so much and I hope you can some day make it to Florida!

    Reply
    • Swagatam says

      June 6, 2020

      The pleasure is all mine! please feel free to comment back if you face any problems with the projects!

      Reply
  20. Dane Shearer says

    June 4, 2020

    Wow! That picture really helped a lot! That solves my low battery problem very well and thank you so much. Now I need to use the rest of the info you gave me to complete my project. Since I don’t need my green LED to be very bright but always “On” which resistor would you recommend for longest battery life for my 9 volt battery?
    Thank you so much again!
    (PS. If you ever come to Florida, at least let me buy you dinner!)

    Reply
    • Swagatam says

      June 5, 2020

      Thanks very much, Glad you found it useful! The green LED illumination will need to be checked practically since different brands have different quality standards. You can start with a 470 ohm resistor and then try a few higher values, like 560 ohms or 680 ohms until you find the right one which provides just the right illumination yet uses a reasonably high resistance value. The 330 ohm is the lowest threshold, so the value must be higher than this value.

      Itโ€™ll be a pleasure meeting you if I ever happen to visit Florida, thanks for the invitation.

      Reply
  21. Dane Shearer says

    June 3, 2020

    Thank you and I think that all makes sense. You have been very helpful however I know nothing about electronics and I don’t understand the electrical drawing showing the arrow coming from #2 on the IC 741 to the 10k Preset. It doesn’t look soldered to the preset. It just points at it.
    Is there a way I can pay you to make one of what I am needing using the actual parts so that I can remake it here? I would LOVE to do that and pay for it.
    That way I know I could follow what you are so kindly recommending.
    Thank you again, Dane

    Reply
    • Swagatam says

      June 4, 2020

      Hi, thank you, I understand and appreciate your generosity, however payment may not be required, I’ll help you anyway until your goal is accomplished, because the concept is quite simple and basic.

      Please go to the previous IC 741 link, and check the first diagrams, I have updated it in a pictorial format using real part images. This will help you to understand how each part needs to be connected practically.
      Let me know if you have any more doubts.

      Reply
  22. Dane Shearer says

    June 2, 2020

    Thank you so much for your response.

    I need a lot of help designing a 9 volt alarm that we are going to make and resell. In this alarm, a piezo alarm will go off with a red LED light when switched to do so. ( I already have the switching device – it measures air pressure with normal NO NC and Common) The alarm will have a green LED that will be on all of the time unless air pressure is lost which is measured by the switch.

    I need help selecting the right snap in 5mm LEDโ€™s (and or resistors) so the LED’s will last at least 10 years and not drain much of the 9 volt battery. A green LED will be on all of the time unless switched off at which point the alarm and red light switches on.

    I need another properly designed LED (yellow) to light up when the 9 volt battery gets low and needs to be replaced. Either that or the green or red light could flash to warn of low battery. I donโ€™t know how to make a low battery light up a yellow light or make a green or red flash.

    Thank for your consideration and I would be happy to pay for your time

    Dane Shearer
    ddj100@aol.com

    Reply
    • Swagatam says

      June 2, 2020

      Hi, thanks, The requirement looks very basic so I will explain it to you here quickly. Assuming you are using the standard 20 mA 3.3 V LEDs, the resistor values that will limit the LEDs with 20 mA could be calculated using the following formula:

      R = V – LED fwd V / LED current, where V is the 9 V, LED fwd V is 3.3 V, and LED current is = 20 mA or 0.02 Amp
      Therefore R = 9 – 3.3 / .02 = 285 Ohms. However, choosing a higher value than this is recommended which will ensure lower current than the optimal 20 mA value and will enable the device to have a much longer life. So instead of 285 Ohms we can go for 300 Ohms or a 330 Ohms 1/4 watt 5% watt

      Flashing the red or green LED may require a more elaborate electronics and part count, so illuminating a separate yellow LED will be a better option.

      Since the response must be sharp and accurate I would suggest an op amp based low batt detector as shown in the first diagram from this article:
      https://www.homemade-circuits.com/how-to-make-simple-low-battery-voltage/

      The red LED should be replaced with a yellow. The LED resistor may be calculated as described in the previous paragraphs.

      Reply
      • Swagatam says

        June 2, 2020

        You can even go for higher resistance values than 330 ohms, may be up to 680 ohms, because today the modern LEDs are designed to produce high bright illumination even with minimal current supplies. Lower current or higher series resistor will mean longer life for the LEDs and the battery…

        Reply
  23. Dane Shearer says

    June 1, 2020

    Hi there,
    I am designing an alarm to be manufactured and resold by my wife and I. I know marketing not electronics. We need help designing a hopefully simple Green LED – “On”, Red – “Off” and Yellow (Or flashing Red) “Replace 9 volt battery” Are you available for such a project?
    Thank you, Dane

    Reply
    • Swagatam says

      June 1, 2020

      Hi, I can try. Please provide the working specifications in detail, I’ll try to figure it out for you.

      Reply
  24. Ahmed Alissa says

    May 17, 2020

    Thank you for your efforts ๐Ÿ™‚

    I’ve used the 1st circuit in a small project and it worked perfectly.
    Is it possible to make the led blink instead of always ON without affecting the 1.5mA draw current?, (my goal is to make it more noticeable and save power)

    Reply
    • Swagatam says

      May 18, 2020

      You can perhaps try integrating the following circuit across the collector of the low battery circuit
      https://www.homemade-circuits.com/how-to-make-single-transistor-led/

      Reply
      • Ahmed Alissa says

        May 20, 2020

        I looked into that before, and some other online circuits, my problem is that I need this flashing indicator for single 18650 (3.7v), the posted indicator circuit is working with low voltage but the flashing circuits I found doesn’t.

        Reply
        • Swagatam says

          May 20, 2020

          You can buy a readymade flashing LED that does not require any circuit.

          Reply
  25. harold ilano says

    January 13, 2020

    it’s a simple and a perfect one (for my project)! minimal components but very effective. thank you for this, sir!

    Reply
    • Swagatam says

      January 13, 2020

      Glad you liked it, thanks for your feedback…

      Reply
    • harold ilano says

      January 13, 2020

      you’re welcome, sir. take care!

      Reply
  26. Vaisakhan says

    August 1, 2019

    Also, I am charging the battery from a solar panel via charge controller. So during night time no means to charge the battery.

    Reply
    • Swagatam says

      August 1, 2019

      You can modify one of the circuits from this article:

      https://www.homemade-circuits.com/simple-dc-ups-circuit-for-modemrouter/

      Replace the battery with solar panel. And put the battery in place of the Modem/Router

      Reply
      • Chris Morey says

        February 17, 2020

        I have numerous 2S rc lipo’s 8.4v charged. We store them at 7.4 volts. I was wondering if this circuit would allow us to discharge the battery with a 5 amp load, and then have this circuit disconnect the battery at 7.4 volts. Y

        Reply
        • Swagatam says

          February 18, 2020

          yes you can try the last circuit for the mentioned purpose!

          Reply
    • LEO SMIT says

      October 15, 2019

      Dear sir,

      when the led is on, let’s say at 5V as in the video, BC 557 conducts, and the relay-voltage drops to 5V .
      the relay is supposed to go on, but the relay that is used is 12 V, so it is not sure that it’s gonna work at 5V…

      Reply
  27. Vaisakhan says

    August 1, 2019

    Good Day Swagat,

    I made the 2nd circuit and I am using it in a 12 v battery to light up an a 12 v led & I have an issue, that just before cut off voltage the relay start chattering. I selected cut off voltage 10.7V. How can I avoid this issue?

    Reply
    • Swagatam says

      August 1, 2019

      Good day Vaisakhan, you can try adding a 100uF/25V across the base/emitter of the BC557. negative terminal will go to the base.

      Reply
      • Vaisakhan says

        August 1, 2019

        Thanks for your quick response. Added the additional capacitor but not solved the issue. Once the load disconnected from battery due to low voltage, slowly battery voltage tends to rise & it is again connected the load. This cycle keep on repeating. Any way to solve this issue?

        Reply
        • Swagatam says

          August 1, 2019

          The remedy which I suggested was to prevent chattering of relay at the threshold, it won’t prevent the cyclic action when the battery voltage rises. If you wish to latch it, you can add a 100K resistor between the collector of BC557, and the middle BC547 base.

          Reply
    • Leo Smit says

      February 11, 2020

      Dear sir,
      I have the same problem with the relay and placed the resistor 100Kohm and 100uf/25v , but at the threshold-voltage the relay starts chattering (only when the charging voltage is also present).
      Due tot the fact that the charging voltage( 13,7 volt) takes over the working circuit , the circuit thinks that it can work as usual with full battery , it will try to switch the relay to normal (load) and then recognizes that battery is to low and then switches again to charger (and vice cersa)
      Don’t know how to prevent that, but maybe you have a suggestion?

      Reply
      • Swagatam says

        February 12, 2020

        Hi Leo, you are right, and this can be prevented by adding a hysteresis effect in the circuit. The hysteresis or latching effect will work if a resistor is introduced at the base of the middle transistor…I forgot to mention this earlier. So please put a 1k resistor in series with the base of the middle BC547, and now connect a 100k resistor between the collector of BC557 and the base of the middle BC547.

        Reply
        • leo smit says

          February 12, 2020

          will for sure try this, the 100kohm from bc557 to bc547 was allready placed, so the 1k must do the trick
          thank you!

          Reply
          • Swagatam says

            February 12, 2020

            No problem….also I have updated the new design at the bottom of the article.

            Reply
  28. Sam says

    April 23, 2019

    Please will this circuit work with a 4s lipo battery which is 14.8v. And 16.8v charged voltage?

    Reply
    • Swagatam says

      April 23, 2019

      Yes, it can work for all types of batteries

      Reply
      • Lisa Lisa says

        October 11, 2019

        Yes… I conform it does work at Lipo voltages… I wish their was something you can add for a little hysteresis…

        Reply
  29. Dhits Gomnic says

    December 20, 2018

    Good Day Sir! Looks like the circuit of 741 here is missing sir. ๐Ÿ™‚ The one that automatically switch the load to source and battery ๐Ÿ™‚

    Reply
    • Swagatam says

      December 21, 2018

      Hi Dhits, it was redirecting to a wrong post due to a site misconfiguration, here’s the actual you are looking for ๐Ÿ˜‰ please check this out

      https://www.homemade-circuits.com/2012/01/how-to-make-simple-low-battery-voltage.html

      Reply
  30. scott cacciamani says

    February 26, 2018

    sorry i’m having so much trouble with this. the center lug of 547 near the 47 k resister is soldered exactly where? The center lug of the 47k trim pot goes under which base, the base of itself or the 547. also where can i find a diagram on how to hook up a on off switch to this? You can tell i have no experience.

    thanks again

    Reply
    • Swagatam says

      February 26, 2018

      What is so complicated? Please make your assembly layout exactly as shown in the diagram, place the components on the PCB exactly as presented in the diagram, then you have any problems.

      The 47k preset will have 3 leads, the center one connects with the base…out of the other two, one goes to the ground, the other to the 33K.

      If you are making it over a breadboard then it can a lot confusing for you.

      And make sure to check the base, emitter, collector pins of the BC547 correctly

      Reply
  31. scott cacciamani says

    February 23, 2018

    sorry, forgot the last one. Is the 47k resester center lug soderd directly to the center lug of the bc-547?

    Reply
    • Swagatam says

      February 24, 2018

      it is directly connected to the base, because we already have a series resistor with the positive arm of the preset.

      Reply
  32. scott cacciamani says

    February 23, 2018

    Hi, again, when using a power supply to set the low voltage on the 47k trim pot. i assume i hook up the leads positive to positive and neg to neg, but what voltage should i set the power supply at before turning the screw on the trim pot. This would be for a low voltage indicator of a 9v battery

    Reply
    • Swagatam says

      February 24, 2018

      apply 8.25V to the circuit, and adjust the preset such that the LED just begins illuminating, that’s all.

      Reply
  33. scott cacciamani says

    February 17, 2018

    Hi, in the 9v depletion detector circuit the 47k variable resister is a preset for what voltage? With this circuit there is not pot to set correct? Also, my battery positive wire has a diode to prevent the battery from charging dc converter is plugged in, can I still use this circuit? Thanks Scott

    Reply
    • Swagatam says

      February 17, 2018

      the 47k preset can be used for adjusting the threshold at which the LED needs to light up and indicate the situation.

      The diode will not have any impact of the the above circuit, you can still use it…

      Reply
  34. jindro says

    November 20, 2017

    Sir I’m just a newbie hobbyist, I’m not yet well versed using Analog Multimeter and I’m really having a hard time using it. When you said series a bulb meaning, using 1 wire of the primary, breaking it then link it with both wires of the bulb?

    Reply
    • Swagatam says

      November 20, 2017

      Jindro, connecting in series means, break one of the input wures, and join the cut ends with the bulb terminals.

      To trafo wire <------(bulb)---------> 220V <--------------> To other trafo wire

      Reply
  35. jindro says

    November 20, 2017

    Sir can you help me to identify a certain scavenge transformer? sorry for asking here because it is not the topic here, but I don’t know where to go or ask, it is ok if you cant , tnx sir

    Reply
    • Swagatam says

      November 20, 2017

      you can feed 220V from the primary side, and check the AC voltage on the secondary side to verify the specs

      Reply
      • jindro says

        November 20, 2017

        tnx again Sir! for replying patiently, the transfo sir has 4 wires in the primary, the number from the first 2 wires from left is erased and the last 2 is 240 240

        Reply
        • Swagatam says

          November 20, 2017

          Jindro, first check the resistances across the wires, the pair which shows maximum resistance can be assumed to be the safest pair for the mains input.

          Still, for 100% safety, use a 100 watt bulb in series while connecting it to 220V input, and make sure the 100 watt bulb remains almost shut, if it is illuminating brightly then the wires may not be specified for the 220V input, then you may have to investigate the other sets of wires in the same manner.

          Reply
  36. jindro says

    November 20, 2017

    Sir I just scavenge the batts. from broken lap top batts. in junkshop, can I trust it to charge equally? just cut the charging using your fullcharge indicator?

    Reply
    • Swagatam says

      November 20, 2017

      if the two cells are not uniform with their charging specifications, then it can be a problem, and the results will be inefficient.

      you can try the following arrangement if possible:

      https://www.homemade-circuits.com/parallel-battery-chargerchangeover/

      the diodes will need to be Schottky type not the normal rectifier diodes.

      Reply
  37. jindro says

    November 20, 2017

    My problem Sir is, if I charge it individually and it is also wired in parallel as a powersource will it not charge also in parallel?

    Reply
    • Swagatam says

      November 20, 2017

      Jindro, ideally you must remove them and connect them separately while charging, however this may not be critical, you can charge them together in parallel, just make sure the supply voltage is strictly cut-off at 4.2V

      Reply
  38. jindro says

    November 19, 2017

    Sir Good Evening! I will build this for my power bank circuit, I’m using 2 li ion bat. Can I use 2n2222 as alternative Sir?

    Reply
    • Swagatam says

      November 19, 2017

      Good Evening Jindro,

      2N2222 can be used instead of BC547, no problem.

      Reply
      • jindro says

        November 20, 2017

        Good Morning Sir! Is ok to wire 2 li ion bat. in parallel for power source of phone charger then wire it individually to charge the battery individually? because I’m not sure if its ok to charge the battery in parallel.

        Reply
        • jindro says

          November 20, 2017

          Sir can I ask again what is the alternative for Bc 557? T.Y. very much Sir for patiently helping lots of people like me ๐Ÿ™‚

          Reply
          • Swagatam says

            November 20, 2017

            you can try 2N2907, it will be compatible with your 2N2222

            Reply
        • Swagatam says

          November 20, 2017

          Good Morning Jindro,

          “2 Li-ion batts in parallel for power source of phone charger” I did not understand this statement.

          Are you trying to make a power bank circuit?

          Please clarify?

          Reply
          • jindro says

            November 20, 2017

            tnx again Sir for replying! yes sir I’m trying to build a powerbank and give it to my friend, there’s no electricity in there place, my idea is to make your lm7805 phone charger then use solar panel to charge the battery.

            Reply
            • Swagatam says

              November 20, 2017

              Hi Jindro, in that case it is OK to use two cells in parallel as the source for the mobile phone, and once discharged you can charge them separately.

              Reply
  39. Pierre says

    September 2, 2017

    Hi,
    Thank you for sharing this schematic.
    I have few questions if you can help :
    – Why using 2 transistors and not only one?
    – I am trying to find a SMD sot-23 transistor but looking at the datasheets they all have a threshold voltage that has values like “min 0.6V, Typ 1V, max 1.4V” with such difference between min and max voltage threshold how is it possible to set correctly the (3.2V) low battery voltage?

    Thanks !

    Reply
    • Swagatam says

      September 2, 2017

      Hi, two transistors are used to achieve better sensitivity and higher accuracy margin between the cut off points.

      which specification are you referring to, is it the base/emitter voltage….??

      Reply
  40. Fajar says

    August 21, 2017

    Thank You

    Reply
  41. Swagatam says

    June 29, 2017

    yes it can be used

    Reply
  42. Swagatam says

    June 7, 2017

    the mentioned numbers will also work…..you can use them

    Reply
  43. Unknown says

    March 19, 2017

    Tip:
    Instead of using the potentiometer as a potentiometer, use it as a variable resistor by only using one side.

    From the 33k ohm resistor tie into the middle pin of potentiometer along with the base of the transistor that was already tied into it, and leave the top pin of potentiometer unhooked. This makes no difference as far as the workings of the circuit but provides a huge benefit I will explain.

    If you do this you will more easily be able to convert this to a more permanent solution with all fixed resistors. If you have any type of device you are hooking this to that can be dropped, shaken, or simply have a flaky pot this can cause problems later and the set point could be off. I would only use the potentiometer as a test to begin with to help find the appropriate resistor value(s) to use so the potentiometer could be replaced with fixed resistors.

    If you leave it as a potentiometer with both sides instead of variable resistor(only one side) then you have two changing values of resistance as you turn the knob, one on top in series with the 33k ohm resistor that is on top and the one below. There is no need for the extra resistance in series with the 33k ohm resistor.

    So instead just take the 33k ohm from top and put it in middle pin of potentiometer leaving everything else the same. Then adjust the knob and voltage until it the LED comes on when you want it to. Then afterwards use a multimeter and check the resistance of the side of the potentiometer that you used(disconnect one or both of the pins used first before checking resistance so you aren't also getting resistance reading from anything else). This will give you the resistance that you need to put in place of the potentiometer. You may not find an exact value but may be able to get close by putting some you have in series or parallel depending on what you have and what value you need but this makes it a lot easier by not using both sides of the potentiometer.

    For example I wanted my LED to come on at 5.2V so I did as I described and the resistance after testing the side of potentiometer with multimeter was 1430 ohms. I wound up replacing potentiometer with a 1.2k ohm and a 220 ohm resistor in series for 1420 ohms and it is working perfectly.

    I also used 10k ohms instead of the 33k and used 330 ohm for LED.

    Reply
    • Swagatam says

      March 20, 2017

      thanks for the interesting suggestions!!

      Reply
    • Charles Wright says

      March 20, 2017

      Your welcome. I was curious though about the resistor showing in series with the LED which is why I chose my own value. I made circuit for use with 4AA batteries in series for 6 volts. And I figured in the voltage drop of LED 2V and and came to the conclusion that with 4volts across the 10k ohm resistor that it would be 0.4mA or 400uA. That was without figuring in any voltage drop across the transistor so it would actually even be less than that amount of current flowing to light the LED. Will an LED even light with that little current, I try to give mine around 15 to 20 milliamps and I came up with only 0.4 milliamps? I'm assuming if it does light with 10k(with 6V as I used) it would be extremely dim. Was that a typo that was supposed to read 1k ohm instead of 10k ohm? Or was this circuit originally designed to test much higher voltage? Either way I used smaller value 330 ohm, I was just wondering out of curiosity.

      Reply
    • Swagatam says

      March 21, 2017

      Thanks again! I appreciate your keen interest!!The 10K is not a typo, yes, the circuit was initially built and tested with a 12V supply, and the 10K was intentionally selected to make sure that the LED did not respond to the transitions, rather lit up only when the threshold was actually close to the preferred point

      Reply
  44. Abdullah bin wahid says

    December 28, 2016

    Sir, i have to present project tomorrow and i have selected this circuit for that purpose. Please tell me that with exactly this circuit at which voltage the bulb will glow off. And at which voltage it will be off. Please give me reply soon.

    Reply
    • Swagatam says

      December 29, 2016

      It will depend on the low voltage level that you may have selected for the circuit.

      This low voltage level trigger must be adjusted with the help of the preset, until the LED just lights up.

      Reply
  45. Rohit Singh says

    August 14, 2016

    Sir,
    I had made a Light sensor lamp that works when it goes dark and turns off when lights falls in it, with using minimum parts to conserve battery power, but the light output is very loq but light sensor works accurate.
    i Had used a smartphone battery that i had an extra peice of 4.2v, a white 1-watt led, 1- 220k resistance, 1-LDR, 1- BC547 Transistor only..
    my problem is when i give power from 4.2v battery directly to 1-watt white led it give maximum huge light, so do i was expecting to give that much light from the light sensor i created but when i connect the led from the light sensor circuit its power out becomes low, but sensor response is awesam it quickly gets on when dark and turns off when lights fall on it.

    i want the same maximum light output from the sensir, is there is anyway to increase the light without increasing the battery power??

    Reply
    • Swagatam says

      August 15, 2016

      Rohit, please show me the schematic that you have built or you can comment under the same article which you are referring to, it will help me to view the diagram and provide an appropriate solution to your problem….

      Reply
  46. Rohit Singh says

    August 13, 2016

    its working .. awesm..
    thanx..

    sir,
    I had saw a screw driver looking type a dc tester, the body of screw driver was made of tranparent plastic just like ac tester, but this tester was dc., inside that i saw a red led with three button cells battery connected with a tiny kit., there were mainy three parts in that kit, two transistors one was BC557 and other was BC547 and resistance, from that tiny kit two wires where comming out ,one wire goes to the top and screwed to a small metal so that we can touch that with out finger and the other wire goes down to the tester connected with the metaltalic type screw.. when ever a person touch the top metal with the finger and touch any electronic compenent part and now touch the bottom part of the tester to the component, then the led glows as to check the continuty of that component is ok.. i saw that many years ago but i cant make that..
    sir can u make that circuit.

    Reply
    • Swagatam says

      August 13, 2016

      Rohit, you can refer to the last circuit presented in this article:

      https://www.homemade-circuits.com/2012/05/make-this-simplest-continuity-tester.html

      It works on a similar concept

      Reply
  47. Rohit Singh says

    August 12, 2016

    sir,
    i had maked battery full circuit by replacing the transistors with BC557 and changing the led and power supply polarity. it works fine !
    but my
    problem is … when ever i connect the battery with the charger when the battery is low, the battery full indicator turns ON… what should i do to make led turn ON, only after battery is full..?

    Reply
    • Swagatam says

      August 13, 2016

      Rohit, please show me the schematic which you have made.

      It should be done as shown in the following article:

      https://www.homemade-circuits.com/2016/08/battery-full-charge-indicator-circuit.html

      Reply
  48. Jason Pierce says

    May 13, 2016

    I am interested in using this circuit with a micro-controller that has a super low power sleep mode around 35uA. While 1.5mA is a small draw, it is large by comparison. I would only need to periodically test the battery level. To conserve power, can the battery indicator circuit be toggled on/off by a digital IO from my micro-controller?

    Reply
    • Swagatam says

      May 14, 2016

      yes that's very much possible, the above low batt circuit can be toggled ON/OFF intermittently for monitoring the batt status whenever required, the circuit does not need to be continuously ON.

      Reply
  49. Unknown says

    April 29, 2016

    Thanks so much for this circuit. I am testing with a bench top power supply and have adjusted the potentiometer so that the LED goes on at around 3.2V. It works wonderfully, except when power is turned on and off at a voltage higher than the 3.2V threshold – the led flashes briefly. Is there a way to prevent this from happening? Thank you.

    Reply
    • Swagatam says

      April 30, 2016

      It could be due to switch ON current surge….you can try adding a capacitor at the base/ground of the right side transistor and see if this checks the issue.

      the capacitor value can be anything between 10uF and 33uF

      Reply
    • Bryan B says

      May 3, 2016

      Thanks for posting. I am also trying to use this circuit as a low voltage indication for 3.2V. I'm trying to work out the resisitance values I will need especially for the 47k adjustable potentiometer. Do you have a formula or do you just adjust the potentiometer? I'm guessing the other resistance values don't have to match yours exactly, but the ones that are the same, need to remain the same? (As in, instead of two 33k's, I could use two 10k's if I had those?)

      Reply
      • Swagatam says

        May 3, 2016

        For a 3.2V input you can use any resistor between 1K and 33K, same goes for the preset…. the values are not critical.

        I can suggest formulas but that might require long explanation and anyway it's not important for the above simple design, so we can skip it.

        the preset needs to be set manually, by applying the intended minimum threshold voltage to the circuit and adjusting the preset until the LED just begins glowing

        Reply
  50. Aminu Yahaya Ibrahim says

    February 10, 2016

    Morning Sir,
    Thank for your effort to help us hobbiests and engeneers.

    Sir, I want use this circuit in my inverter inorder to monitor the battery level.
    1) I want use S8050 inplace of BC546 for the low battery and S8850 instead of BC557 for the battery full, is this possible, and is there any difference in battery power consumption?

    2) My design will include 4 LED look as follow;
    >LED 1 will show fully charged level 100%;
    >LED 2 will show 75%;
    >LED 3 will show 50%
    >LED 4 will show up If the battery level is down to 11v
    Based on this, my questions are:
    a. What level of the 12v battery is 100%, 75%, 50% and Low battery, inorder to save the battery life?
    b. Instead of using VR or Pots can I use zener diod of required volt?

    Thank you once again Sir.
    Am very much proud of you!

    Reply
    • Swagatam says

      February 10, 2016

      Hi Aminu,

      you cannot get all 4 indications with the above circuit, you will need the following circuit to implement this

      https://www.homemade-circuits.com/2013/06/3-step-dc-voltage-level-monitor.html

      14.4V can be considered as the 100% charging level for a 12V battery….just multiply the other percentage levels with it for getting the equivalents in volts.

      11V is the lowest discharge level for 12V batt. that's 77% of 14.4V

      Reply
  51. Alex Baruth says

    February 3, 2016

    Hi, thanks for the schematic. I was looking for a way to have the light turn off when the battery gets to a certain low voltage, or, actually interrupt a portion of a circuit, thereby disabling it. Would I be able to rearrange this circuit to do this?
    Thanks.

    Reply
    • Swagatam says

      February 4, 2016

      Hi, yes it's possible, you will need to add a third BC547 in the circuit with its base connected to the collector of the right side BC547…the collector or the emitter of this transistor can then be appropriately integrated with the intended external circuit, depending on whether the disabling needs a "sink" or a "source" action from this BC547

      Reply
      • Alex Baruth says

        February 8, 2016

        Thanks for your reply Swagatam, you and your contributions have been very helpful. I was not able to get the NPN BC547 to work, but when I thought about it, I realized it was a PNP transistor that was needed. I used a 2N3906, and that seemed to do it. I was also thinking that using other transistors, or another arrangement, a two transistor setup might work. Thanks for the help.
        Cheers!

        Reply
        • Swagatam says

          February 9, 2016

          Both will work actually…BC547 will work better as a "sink" while the BC557 will work better as a "source"

          the emitter/collector can be swapped for achieving the opposite from the both counterparts for triggering the external circuit as may be required.

          Reply
          • Swagatam says

            February 9, 2016

            It can be enhanced to any level, but we want to keep it as compact as possible so probably a single transistor improvement should be just enough.

            Reply
            • Alex Baruth says

              February 18, 2016

              Hi Swagatam, I have been working on this circuit trying to get it to work for some time now without success. I'm not sure what the problem is. I can get the original to work just fine, but I basically want the reverse of it. I've tried the load on either side of the additional BC547 (sink/source)? to no avail. Maybe I'll work on some schematics in case one can spot the fault.

              Reply
              • Swagatam says

                February 19, 2016

                Hi Alex, If you can specify your exact requirement then I can suggest the correct solution, because it depends on the type of the load you are using, if it's a logic input then it should easily work using the earlier idea but if it's a heavy analogue load then the circuit would probably need to be upgraded much more in order to make it work

                Reply
        • Alex Baruth says

          February 11, 2016

          Thanks Swagatam, I see what you mean.

          Reply
      • Lisa Lisa says

        October 11, 2019

        could you use an opto-isolator in place of the led… or the led side of the opto-isolator, in parallel with the led… so you have a visual indicator, and the opto collector/emitter side to do the switching of something external?
        Lis

        Reply
        • Swagatam says

          October 11, 2019

          This can be simply done by replacing the existing RED LED pins with the opto coupler LED pins

          Reply
  52. AngryHelder says

    January 10, 2016

    Great circuit and very useful.

    I have a question though could a second LED be used maybe even one that has 2 colors so it would be one colored when battery is high then switch colors to indicate when the battery is low?

    Maybe something like a reverse able polarity Bi-Color LED or a 3 lead Dual Color LED?

    Thanks.

    Reply
    • Swagatam says

      January 10, 2016

      thank you very much,

      yes it could be done by introducing an additional transistor stage, as shown in the second diagram of this link:

      https://www.homemade-circuits.com/2014/06/flashing-led-battery-status-indicator.html

      the shown connections could be replaced with a single common-anode dual color LED, by connecting its common lead with the positive and the cathodes to the respective collectors of the BJTs via limiting resistors.

      Reply
  53. Jean says

    October 28, 2015

    Hello, I'm going to use your circuit in my self switching off machine, but I have a problem. My machine runs on a 9v pp3 battery and I don't have a 47k pot. If you could give us a formula to calculate the resistance of the preset or give us some values for commonly used voltages, it would be nice. Thank you.

    Reply
    • Swagatam says

      October 29, 2015

      If you are intending to use a relay in place of the LED, and if the relay coil resistance is above 200 ohms, in that case you can simply replace the 33k resistors with 10k, and the presets with 1k presets.

      however a 9v pp3 will never control any relay, so the above assumption cannot be true…but since the resistor values are so critical you can replace it with any nearby value, anything above 4k7 for the fixed resistors and also the presets would do.

      A relevant article can be learned here:

      https://www.homemade-circuits.com/2012/01/how-to-make-relay-driver-stage-in.html

      Reply
    • Swagatam says

      October 29, 2015

      correction:

      …sorry I meant….."since the resistor values are NOT so critical you can replace it with any nearby value…..

      Reply
  54. B. J. Sutrisno says

    March 2, 2015

    Hi there, hope you can answer my question for this old post…

    1) Using your circuit, how to switch on LED if the battery is 3v? My battery's fully charged is around 4.5v…

    2) Is this circuit possible to run without getting off that Red LED. I mean it's still bright until minimum forward volts that LED has?

    Thanks…

    Regard,
    ~ Joko

    Reply
    • Swagatam says

      March 3, 2015

      Hi,
      you'll need to adjust the given 47k preset such that the LED illuminates at around 3V, once it's illuminated it'll stay illuminated until the battery voltage has fallen below the forward voltage spec of the LED.

      Reply
  55. Keyur Patel says

    November 23, 2014

    Will this ckt work for 1.5 volt battery ?

    To indicate its low voltage?

    Plz give proper details of ckt if not?

    Reply
    • Swagatam says

      November 24, 2014

      yes it can be used for 1.5V also….reduce the 33k resistors to 1k that's all.

      Reply
    • Ram kumar says

      November 19, 2015

      Sir please suggest for 14.4 volt lithium ion battery .preset value resistor and normal resistor.please explain

      Reply
    • Swagatam says

      November 20, 2015

      you can use the same set up as given above

      Reply
  56. Adithya K says

    November 10, 2014

    can u please send me the design of the above circuit

    Reply
  57. Adithya K says

    November 10, 2014

    Can u please send the design of the above circuit.

    Reply
  58. monkeypi says

    February 19, 2014

    can i use a different resistor than a 33k one??

    Reply
    • Swagatam says

      February 19, 2014

      yes any nearby value will do +/- 20%

      Reply
    • monkeypi says

      February 19, 2014

      Can i use a 68k one?

      Reply
    • Swagatam says

      February 20, 2014

      yes will do.

      Reply
  59. MUJAHID SHAH says

    February 2, 2014

    Ic 4017 is great but you are greater than it hence you made it easier for us to understand.thank you

    Reply
    • Swagatam says

      February 3, 2014

      You are welcome!

      Reply
  60. MUJAHID SHAH says

    February 1, 2014

    Dear sir You mentioned about the ic 4017 that it produces sequential output what does it mean kindly explain a bit.thank you.

    Reply
    • Swagatam says

      February 2, 2014

      Dear Mujahid,

      please read this article for more details:

      https://www.homemade-circuits.com/2011/12/how-to-understand-ic-4017-pin-outs.html

      Reply
  61. DJ Tube says

    January 30, 2014

    How would i make this work for a 36v battery with an alarm at 30v please?

    Reply
    • Swagatam says

      January 30, 2014

      you can use the same circuit for 36V also, simply adjust the preset for 30v LED illumination

      Reply
  62. MUJAHID SHAH says

    January 29, 2014

    Sir Swagatam circuit you recommended are great. The problem is that I cannot use an Ldr in open surroundings where it can be affected by sun light and other lights as well. I need circuit where there is specific light and its detector so when any object interrupt them means coming between the receiver and detector it should give alarm. I can configure the remaining circuit with scr application circuit.

    Reply
    • Swagatam says

      January 30, 2014

      Mujahid, you can keep the LDR enclosed inside a pipe like enclosure and align the laser beam such that only this beam enters the pipe, no other light is able to enter.

      as shown in the last diagram in this ink:

      https://www.homemade-circuits.com/2013/02/make-these-simple-cheap-home-burglar.html

      otherwise you will have to go for an IR proximity detector circuit as given here

      https://www.homemade-circuits.com/2013/10/accurate-infrared-motion-detector-or.html

      Reply
  63. MUJAHID SHAH says

    January 28, 2014

    Sir I would like to make a circuit which includes a laser light and laser detector where they both can face each other when any human body or any other object comes in between the laser light and detector it should give alarm. I don.t have good idea please help me in making the diagram and description of components needed.thank you..

    Reply
    • Swagatam says

      January 29, 2014

      Mujahid, you can try the first circuit shown the following link:

      https://www.homemade-circuits.com/2012/03/how-to-make-simple-scr-application.html

      replace the "sensor" points with an LDR

      Reply
  64. MUJAHID SHAH says

    January 23, 2014

    I made the above circuit it.s working marvelously now I need a battery full indicator circuit as simple as the above one.

    Reply
    • Swagatam says

      January 25, 2014

      Thanks!

      To make it work as an over charge indicator you will need to alter the polarity of the transistors.

      You will need to replace the transistors with BC557.

      Its orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.

      Reply
  65. ally prieto says

    December 24, 2013

    sir can i use it for 12V supply? what changes do i need to do? thank you ๐Ÿ™‚

    Reply
    • Swagatam says

      December 24, 2013

      yes you can use it for 12V also.

      Reply
  66. achilles hector says

    December 8, 2013

    Good day sir!!! another question from your no. 1 fan!! :-)…

    1.Do you remember the the Ultrasonic weapon circuit of yours? You mentioned about the power supply which is 12v to 18v. Can I use 2 x 6f22 9v battery which would become 18v? And in case yes, I will put the above circuit. What modification will I make so that the led will glow at 12v?

    I will always credit you in my projects…
    My teacher once asked me in one of my project like the transformer less emergency light, she said "who is this Swagatam Majumdar in your acknoledgement" I said "He is an Engineer from India."

    Reply
    • Swagatam says

      December 8, 2013

      Thanks very much Achiless I appreciate your enthusiasm!

      9V PP3 battery will not work for too long, because the ultrasonic weapon circuit could consume considerable amount of power at a go…..therefore you will have to either go for mini SMF batt or arrange many 1.5V pencil cells in series…you would require 12 nos of them in series for optimum results.

      Reply
  67. Sham says

    November 16, 2013

    Hi Swagatam,
    I tried with 4th ckt. I am not getting Battery cutoff. When emitter is connector to base of bd140 indicator is not lighting up.

    Reply
    • Swagatam says

      November 16, 2013

      Hi Sham,

      Remove the LED, connect the collector directly to positive and then check.

      Reply
    • Sham says

      November 16, 2013

      Yes Swagatham, tried that too before posting. Changed the resistor values to lower ones too.. ๐Ÿ™

      Reply
    • Swagatam says

      November 16, 2013

      according to me it should have worked, anyway there's only way left now… revert the connections back to its actual form as shown in the above article.
      Connect an additional BC557 transistor base via a 10k resistor to the collector arm of BC547 where the LED is present.
      connect the emitter of BC557 to positive and collector to base of BD140.
      adjust the 47k preset such that at 5V all the white leds just shut off

      This is 100% work, otherwise the issue could be something else.

      Reply
    • Sham says

      November 20, 2013

      Hi Swagatham,
      I tried the ckt you suggested. But no response. I did some rnd and found that the biasing of ldr ckt is taking over the 557 one.
      When pos supply from transformer is applied to base of bd140, ckt switches off. That means pos supply from battery is not sufficient to turn off bd140. If i connect pos of transformer to 557 emitter, my requirement is not met, bcos in absence of power supply battery may get drained.
      I know its straight forward solution. but its not happening. Hope to get better solution to this.

      Reply
    • Swagatam says

      November 21, 2013

      Hi Sham,

      the suggested circuit is perfect, it's the ultimate way of doing it and should work.

      When the low voltage LED lights up, check the voltage at the junction of collector of the BC557 transistor and base of BD140, it should be equal to the battery voltage.
      check by keeping the meter prods across positive and base of BD140
      with potential at base of BD140 equal to the battery voltage, there's no way it can conduct.
      you may also try increasing the base/ground resistor of BD140 for better and instant response.

      Reply
  68. Sham says

    November 13, 2013

    Hi Swagatham,
    Hats off to your contribution. I tried this circuit https://www.homemade-circuits.com/2011/12/how-to-make-efficient-led-emergency.html, the second one with LDR, through another transistor bd140. It works like a charm. Initially i had set up the ldr ckt without low bat indicator for 440ma. R1 220E and p1 10k. I know one more stage of pnp is required to connect low bat ckt to LDR ckt to cutoff the battery power to LEDs. But i dono what to be used and where to connect to maintain the same 440 ma as output to LEDs. Plz suggest the circuit to be used. Thanks. Sham.

    Reply
    • Swagatam says

      November 13, 2013

      Hi Sham,

      thanks!
      For adding a low voltage cut off in the circuit you can repeat another stage identical to the LDR stage, with the following changes.

      Use a 10k preset instead of 100k, remove the LDR and use a series resistor equal to 10k for the ground link. Adjust the preset such that the LEds just shut off when the battery voltages reaches 5V

      Reply
    • Sham says

      November 14, 2013

      I tried connecting the circuit as per your suggestion, but i could not get the desired result. So changed the values as given. I connected one more bc557 in the same manner as ldr ckt. Emitter to vc+ via 1k pot and other end to base of 2nd trans. Again base to grnd via 1k resis. Collector connected to base of bd140. Now ckt switches off with 5v battery when preset is calibrated but when replaced with 6v battery it continues in the same off state unless i have to recalibrate to new setting. Actually it should be ON since the voltage is more.
      When checked at 2nd trans collector during LED off state at 5v it was reading 3.4 volts. When 6 volts battery was connected, it was showing 4.4 volts which keeps the bd140 base at high and hence it was still off.
      Plz suggest me what to do.

      Reply
    • Sham says

      November 14, 2013

      Ho… BTW addition to my previous reply, in ldr ckt, i hv used 10k pot instead of 100k at emitter and 470E for bd140 base and grnd. I hv connected ldr to grnd without 1k.
      plz suggest where the 2nd bc557 collector to be connected?

      Reply
    • Swagatam says

      November 14, 2013

      Sham, please try the 4rth circuit as shown in this article:

      https://www.homemade-circuits.com/2011/12/how-to-make-efficient-led-emergency.html

      Reply
    • Sham says

      November 15, 2013

      Sure Swagatham, will try this and give feedback.

      Reply
  69. Swagatam says

    August 28, 2013

    Probably not possible through this simple design, you will have use an opamp circuit for that, as shown here:

    https://www.homemade-circuits.com/2011/12/how-to-make-simple-low-battery-voltage.html

    Reply
  70. Swagatam says

    August 27, 2013

    I meant that the LED won't produce sharp turn ONs rather grow brighter slowly as the threshold is reached, but at the actual threshold the brightness would be optimal.

    Reply
  71. Swagatam says

    August 27, 2013

    Dear John,

    Thanks!

    To make it work as an over charge indicator you will need to alter the polarity of the transistors.

    You will need to replace the transistors with BC557.

    Its orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.

    However it won't give a sudden glow, a dim glow could be witnessed when the level is within 0.5V near the set threshold.

    Reply
  72. dheeraj says

    August 13, 2013

    hai sir, i want to use this circuit with 12V 4.5Ah battery but i dnt have 47k preset but am having 10K preset.. what to do now ??

    Reply
    • Swagatam says

      August 14, 2013

      10K preset will also work, you can use it.

      Reply
  73. Swagatam says

    August 11, 2013

    47K is a preset to be precise,

    yes you got it right about the 33k resistors, anyway nothing is crucial here.

    Reply
  74. Swagatam says

    August 10, 2013

    That's correct!

    Reply
  75. Swagatam says

    August 9, 2013

    The transistor orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.

    Reply
  76. Chris Ault says

    August 7, 2013

    This circuit is going on my little single cell micro helicopter to light a super bright led when battery goes from 4.20volts down to 4.0 v. Currently I'm shrinking it down & taking it off the breadboard.

    Reply
    • Swagatam says

      August 7, 2013

      Wow! that's great….go on

      Reply
  77. Yuva Rajan says

    August 1, 2013

    hi sir,
    i am not good in electronics please guide me how second transistor is off while the voltage level is high. And how second transistor is on while low voltage. thanks

    Reply
    • Swagatam says

      August 2, 2013

      At higher voltages the left transistor conducts and grounds the base of the right transistor so that it gets switched OFF, at low voltages the left transistor doesn't get enough voltage at its base to conduct and switches OFF itself which initiates and switches On the right transistor.

      Reply
  78. jonnie walson says

    July 16, 2013

    hello sir can i use this circuit for 12v battery?

    Reply
    • Swagatam says

      July 17, 2013

      yes it can be used with 12V supply also.

      Reply

Primary Sidebar

Calculators

  • 3-Phase Power (15)
  • 324 IC Circuits (19)
  • 4017 IC Circuits (52)
  • 4060 IC Circuits (25)
  • 555 IC Circuits (98)
  • 741 IC Circuits (19)
  • Arduino Engineering Projects (83)
  • Audio and Amplifier Projects (114)
  • Battery Chargers (82)
  • Car and Motorcycle (94)
  • Datasheets (46)
  • Decorative Lighting (Diwali, Christmas) (33)
  • Electronic Components (100)
  • Electronic Devices and Circuit Theory (36)
  • Electronics Tutorial (116)
  • Fish Aquarium (5)
  • Free Energy (34)
  • Fun Projects (13)
  • GSM Projects (9)
  • Health Related (20)
  • Heater Controllers (29)
  • Home Electrical Circuits (102)
  • How to Articles (20)
  • Incubator Related (6)
  • Industrial Electronics (28)
  • Infrared (IR) (40)
  • Inverter Circuits (98)
  • Laser Projects (12)
  • LED and Light Effect (93)
  • LM317/LM338 (21)
  • LM3915 IC (25)
  • Meters and Testers (65)
  • Mini Projects (148)
  • Motor Controller (67)
  • MPPT (7)
  • Oscillator Circuits (26)
  • PIR (Passive Infrared) (8)
  • Power Electronics (34)
  • Power Supply Circuits (77)
  • Radio Circuits (10)
  • Remote Control (48)
  • Security and Alarm (61)
  • Sensors and Detectors (120)
  • SG3525 IC (5)
  • Simple Circuits (75)
  • SMPS (29)
  • Solar Controllers (60)
  • Timer and Delay Relay (53)
  • TL494 IC (5)
  • Transformerless Power Supply (8)
  • Transmitter Circuits (40)
  • Ultrasonic Projects (16)
  • Water Level Controller (45)

Calculators

  • AWG to Millimeter Converter
  • Battery Back up Time Calculator
  • Capacitance Reactance Calculator
  • IC 555 Astable Calculator
  • IC 555 Monostable Calculator
  • Inductance Calculator
  • LC Resonance Calculator
  • LM317, LM338, LM396 Calculator
  • Ohmโ€™s Law Calculator
  • Phase Angle Phase Shift Calculator
  • Power Factor (PF) Calculator
  • Reactance Calculator
  • Small Signal Transistor(BJT) and Diode Quick Datasheet
  • Transistor Astable Calculator
  • Transistor base Resistor Calculator
  • Voltage Divider Calculator
  • Wire Current Calculator
  • Zener Diode Calculator

© 2023 ยท Swagatam Innovations