May 26, 2013

Low Battery Indicator Circuit Using Two Transistors Only

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.

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.

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.

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 triggers the right hand side transistor, switching ON the LED.

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

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


video



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122 comments:

  1. Can i use this circuit for indicating 3v ni-cad battery

    ReplyDelete
    Replies
    1. yes can be done by slightly modifying the resistor values.

      Delete
  2. Thank u sir, i will make this circuit and send feedback soon.

    ReplyDelete
  3. Hi Swagatam

    This is agood circuit for warning of minimum DC.

    Is it possible to Drive a Relay instead of a LED for monitoring and centralized Alarm

    thank you very much

    ReplyDelete
    Replies
    1. Thanks Sayad!

      For driving a relay you will have to introduce another transistor stage, using a PNP BC557 transistor.

      Delete
  4. hi sir way i have a simple charge control circuit for charging 50ah battery within a few hours by using 30w solar panel? regard

    ReplyDelete
    Replies
    1. try this circuit:

      http://homemadecircuitsandschematics.blogspot.in/2012/04/how-to-make-solar-battery-charger.html

      Delete
  5. thanks am going to try it how long the battery will be full charged?

    ReplyDelete
    Replies
    1. battery charging has no connection with the above circuit, the above circuit is just an indicator device.

      the battery charging parameters will depend on the rating of the transformer used.

      Delete
  6. This is superb because simple,what will be the operating battery volt&AH in between,Preset 47K threshold control process,Thanks Swagatam

    ReplyDelete
    Replies
    1. thanks, the above circuit is just an indicator which will read the battery voltage status...it will work with all types batteries.

      Delete
  7. Hi how do I do the LED flashes when the battery is low ... Thank you very much. Luis Argentina

    ReplyDelete
    Replies
    1. let it flash..... it will happen for a few minutes then stabilize to a constant glow.

      Delete
  8. Hello sir my name is faysal,im a student of eee.
    Im not experienced.
    Sir,im not understand where to connect the base of left side transistor, after 47k resistor or before?
    Plz help me.

    ReplyDelete
    Replies
    1. Helo Faisal,

      47k is a preset, or a variable resistor. it will have three leads, the center will go to the base of the transistor while the other two leads will go to the respective points of the circuit as shown.

      Delete
  9. hi what I want is that the flashing or flasher LED when the battery is low.
    thank you very much
    Luis from Argentina.

    ReplyDelete
    Replies
    1. you can try using a flashing multicolor LED in place of the ordinary LED.

      Delete
  10. Sir,if i use a normal resistor after setting the value, using variable resistor then how to connect ?
    Can u plz help me sir?

    ReplyDelete
    Replies
    1. after setting up the preset, measure the resistance between the center and the upper arm of the preset and similarly across the center and the lower arm of the preset.

      then solder these two value resistance across the transistor base and 33k.... and across base to ground of the circuit

      Delete
  11. Sir,im faysal.
    Thank u for giving me reply.
    Sir i want to know another thing that
    if i want to use a normal resistor in the place of variable, then how to connect them?

    ReplyDelete
  12. thank u for your replay sir(faysal)

    ReplyDelete
  13. First of all I salute to your hard work and quick response to your subscribers. You have now a new subscriber..! <3 <3 <3 <3 <3 <3 for your website... Best website i've ever found

    Please tell how this circuit can be adjusted to indicate for 12V Battery with a threshold of about 10.5 DC Volts and can we use beeper instead of LED, if so, how??

    ReplyDelete
    Replies
    1. The preset can be adjusted right from 1.5V to 12V....so it can be set for 10.5V also.

      you can replace the LED with a piezo buzzer directly for getting audio indication.

      Delete
    2. Thanks for your swift response...!

      Delete
  14. Sir thanks very much for this wonderfuk circuit, I appreciate. Pls i don't knnw how to set the low battery refrence voltage. I mean the voltage at which the LED will glow, indicating low battery. Could you pls tell me how? I want to use the circuit to monitor a 12volts battery i am using for my inverter to avoid over discharging the battery. Thanks a lot. Awaiting your reply.

    ReplyDelete
    Replies
    1. You will need a variable power supply unit for setting up the circuit.

      adjust the variable supply to 11.2V and connect it to the above circuit.

      Then carefully adjust the preset such that the LED just illuminates.

      The circuit is all set.

      Now you can remove the power supply and connect the battery supply to it, when the battery voltage reaches around 11.2V the LED will light up.

      Delete
  15. Hi Swagatam. Thank you for this circuit. I have built it for use in paranormal investigations. A YouTube video of the completed unit is here:

    http://youtu.be/Nuv6iuhyReY

    I have credited you in the video description and posted your url.

    Thank you very much for sharing your knowledge.

    Allan

    ReplyDelete
    Replies
    1. Hi Alan,

      That's great! I am really pleased to hear this.

      Thanks!

      Delete
  16. Sir, what happens if i use BC 557 instead of BC 547 in the above circuit? Whether it works as indicator for battery high indicator? Or what else alteration should i do , to make it as battery high voltage indicator?

    ReplyDelete
    Replies
    1. yes that's correct! it'll become a full charge indicator.

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

    ReplyDelete
    Replies
    1. yes it can be used with 12V supply also.

      Delete
  18. 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

    ReplyDelete
    Replies
    1. 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.

      Delete
  19. 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.

    ReplyDelete
  20. if a bc557 is used instead of the bc547 it becomes a full charge indicator but my question is the bc557 is a pnp so will the emmiter of bc557 be connected to ground just like the bc547 is grounded.

    ReplyDelete
    Replies
    1. 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.

      Delete
  21. so that means that the emiiter of bc557 will now recieve a postive suppky while negative supply goes to the resistors .

    ReplyDelete
  22. Just for reference, I was wondering for which voltage your diagram is, with the 47K resistor?

    ReplyDelete
  23. I think I understand, sorry for my previous question about the 47k resistor, but if I'm not mistaken the 47k is a potentiometer :-)

    So the 33k are not to be changed with other resistors at different voltages?

    ReplyDelete
    Replies
    1. 47K is a preset to be precise,

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

      Delete
  24. 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 ??

    ReplyDelete
    Replies
    1. 10K preset will also work, you can use it.

      Delete
  25. Dear Swagatam, I was wondering if you could draw a simple 'fully charged' circuit based on two transistors, if that is possible. I build a charger for 3xaa batteries and i want to light a green LED when voltage of the combined batteries reach 4.2 volts, without the LED slowly getting brighter but just stays off and then goes on suddenly.

    Thanks for your great circuits!

    John

    ReplyDelete
    Replies
    1. 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.

      Delete
    2. Thanks for your swift answer! Do you have any idea which component(s) to use to not have a dim glow? Some kind of relay of some sort?

      Delete
    3. 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.

      Delete
  26. Yes I understand, but what I meant was, is it somehow possible to skip the dim glowing and go from an OFF led straight to a bright ON led?

    Thanks for your time!

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

      http://homemadecircuitsandschematics.blogspot.in/2011/12/how-to-make-simple-low-battery-voltage.html

      Delete
  27. Hi Swagatham,
    Hats off to your contribution. I tried this circuit http://homemadecircuitsandschematics.blogspot.in/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.

    ReplyDelete
    Replies
    1. 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

      Delete
    2. 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.

      Delete
    3. 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?

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

      http://homemadecircuitsandschematics.blogspot.in/2011/12/how-to-make-efficient-led-emergency.html

      Delete
    5. Sure Swagatham, will try this and give feedback.

      Delete
  28. This comment has been removed by the author.

    ReplyDelete
  29. 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.

    ReplyDelete
    Replies
    1. Hi Sham,

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

      Delete
    2. Yes Swagatham, tried that too before posting. Changed the resistor values to lower ones too.. :-(

      Delete
    3. 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.

      Delete
    4. 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.

      Delete
    5. 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.

      Delete
  30. 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."

    ReplyDelete
    Replies
    1. 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.

      Delete
  31. sir can i use it for 12V supply? what changes do i need to do? thank you :)

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

    ReplyDelete
    Replies
    1. 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.

      Delete
  33. 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..

    ReplyDelete
    Replies
    1. Mujahid, you can try the first circuit shown the following link:

      http://homemadecircuitsandschematics.blogspot.in/2012/03/how-to-make-simple-scr-application.html

      replace the "sensor" points with an LDR

      Delete
  34. 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.

    ReplyDelete
    Replies
    1. 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:

      http://homemadecircuitsandschematics.blogspot.in/2013/02/make-these-simple-cheap-home-burglar.html

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

      http://homemadecircuitsandschematics.blogspot.in/2013/10/accurate-infrared-motion-detector-or.html

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

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

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

    ReplyDelete
    Replies
    1. Dear Mujahid,

      please read this article for more details:

      http://homemadecircuitsandschematics.blogspot.in/2011/12/how-to-understand-ic-4017-pin-outs.html

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

    ReplyDelete
  38. can i use a different resistor than a 33k one??

    ReplyDelete
  39. Can u please send the design of the above circuit.

    ReplyDelete
  40. can u please send me the design of the above circuit

    ReplyDelete
  41. Will this ckt work for 1.5 volt battery ?


    To indicate its low voltage?


    Plz give proper details of ckt if not?

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

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

      Delete
    3. you can use the same set up as given above

      Delete
  42. 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

    ReplyDelete
    Replies
    1. 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.

      Delete
  43. 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.

    ReplyDelete
    Replies
    1. 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:

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

      Delete
    2. correction:

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

      Delete
  44. 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.

    ReplyDelete
    Replies
    1. thank you very much,

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

      http://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.

      Delete
  45. 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.

    ReplyDelete
    Replies
    1. 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

      Delete
    2. 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!

      Delete
    3. 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.

      Delete
    4. 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.

      Delete
    5. Thanks Swagatam, I see what you mean.

      Delete
    6. 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.

      Delete
    7. 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

      Delete
  46. 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!

    ReplyDelete
    Replies
    1. Hi Aminu,

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

      http://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

      Delete
  47. 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.

    ReplyDelete
    Replies
    1. 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

      Delete
    2. 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?)

      Delete
    3. 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

      Delete
  48. 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?

    ReplyDelete
    Replies
    1. 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.

      Delete
  49. 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..?

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

      It should be done as shown in the following article:

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

      Delete
  50. 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.

    ReplyDelete
    Replies
    1. Rohit, you can refer to the last circuit presented in this article:

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

      It works on a similar concept

      Delete
  51. 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??

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    1. 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....

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  52. 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.

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    1. 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.

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