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Low Battery Indicator Circuit Using Two Transistors Only

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.

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

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.


About the Author

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!

122 thoughts on “Low Battery Indicator Circuit Using Two Transistors Only”

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

  2. 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

    • 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

  3. 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

  4. 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

    • 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…

  5. 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?

    • 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

  6. 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

      • 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

        • 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.

  7. 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?

    • 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

      • 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.

        • 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?

          • 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.

            • 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.

  8. 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 !

    • 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….??

  9. 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.

    • 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.

    • 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

  10. 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.

    • 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.

  11. 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??

    • 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….

  12. its working .. awesm..

    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.

  13. 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..?

  14. 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?

    • 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.

  15. 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.

    • 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

    • 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?)

      • 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

  16. 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!

  17. 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?

    • 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

      • 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.

        • 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.

          • 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.

            • 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.

            • 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

  18. 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?


  19. 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.

    • 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:


  20. 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?


    ~ Joko

    • 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.

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

  22. 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.

  23. 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..

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

    • 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.

  25. 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."

    • 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.

  26. 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.

    • 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.

    • 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.

    • 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.

  27. Hi Swagatham,
    Hats off to your contribution. I tried this circuit https://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.

    • Hi Sham,

      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

    • 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.

    • 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?

  28. 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.

  29. Dear John,


    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.

  30. 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 ??

  31. 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.

  32. 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.

  33. 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

    • 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.

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