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Battery Over Charge Protected Emergency Lamp Circuit

Battery Over Charge Protected Emergency Lamp Circuit

The following LED emergency light with battery over charge protection feature circuit was designed by me in response to the request sent by PP.

Main Features

The article describes an LED emergency light circuit with advanced features such as,

  1. over charge battery cut off,
  2. day time auto-disable,
  3. and need less to say that the circuit switches ON the LEDs automatically when AC mains fails and reverts to charging mode when power is restored.
  4. The good thing about this circuit is that it incorporates ordinary, cheap components which can be easily procured from the local market.

Circuit Operation

Let's try to understand the circuit functioning with the help of the following points:

IC1 which is our very own IC555 has been set as a comparator. During day time, the light over LDR keeps the LDR resistance low such that the potential at pin #2 of the IC is kept well over 1/3Vcc. This situation ensures that the output of the IC at pin #3 stays at logic high.

The logic high at pin#3 of the IC keeps T1 switched ON, which consequently keeps T2 switched OFF.

With T2  switched OFF, the LED array remains inhibited from the ground connection and therefore the whole white LED array also stays shut off.

Another factor that keeps T1 switched ON and T2 switched OFF, is the voltage from the transformer power supply stage.

This function is implemented via the resistor R9. This also means that as long as mains AC is available, T2 is restricted from conducting and therefore the LEDs cannot light up.

Now suppose the mains power to the transformer fails, and assume that this happens during night or complete darkness, pin#3 of IC555 reverts to zero and also there's no voltage from the power supply, means T1 has absolutely no base bias and therefore has to switch OFF.

This instantly prompts T2 to switch ON and consequently the entire LED array also switches ON, providing the required emergency illumination to the surrounding.


The battery charging section consists of T3, T4 and the associated parts. P1 is set such that it switches ON T3 when the battery voltage reaches just above 14 volts.

The moment this happens, T4 switches OFF, cutting of the negative supply to the battery and restricting any further charging of the battery.

Diode D2 ensures that the battery receives the negative supply during the charging process only through T4 and also provides a normal negative path to to T2 and the LED array when they conduct.

The left side LED indicates, mains power ON or presence of day light.

The LED at the right side indicates, battery is charging.

Parts List

  • R1 = 2M2
  • R2 = 1M
  • R3, R4, R5, R9, R6, R7, R8 = 4K7
  • D1, D2, D3 = 1N4007
  • D4----D7 = 1N5402
  • C1 = 1000uF/25V
  • C2 = 1uF/25V
  • T1, T3 = BC547
  • T4, T2 = BD139
  • Z1, Z2 = 3V/400mW
  • P1 = 10K PRESET
  • IC1 = IC 555

Using a Single PNP BJT

The above circuit can be much simplified by eliminating the IC555, and by using just a  single PNP transistor instead of two NPN in the battery auto-battery cut of section.

P1 is used for adjusting the ambient light threshold at which the LEDs stop illuminating.

P2 is set such that at 14.6V (across the battery terminals) the base LED becomes very dim, hardly visible, and at 12.5V it's brightly lit.

Adding a Solar Panel

The above circuit can be also coupled with a solar panel for getting an automatic charging facility from both the sources that is from the panel during day time and from mains after the sun sets.

Parts List

R1,R2,R3, R4, R5 = 1K
P1 = 470K
P2 = 1K
C1 = 1000uF/25V
D1---D5 = 1N4007
T1 = BC547
T2 = 8050
T3 = TIP127


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!

124 thoughts on “Battery Over Charge Protected Emergency Lamp Circuit”

  1. I have a 3.6V recheargeable battery powering an SMD array, is it possible to use a lower V transformer (5 v) and keep same circuit?

  2. Hello Swagatam,

    I built the 2nd circuit you gave and checked a couple of times to make sure everything is connected to each other well. I got some issues though. First of all, I would like to use a led strip (rated 12V – 4,8W per meter), but I don't know how much length I need yet. I also discarded the LDR section with the LDR, P1 and R1. A couple of times I tried, it worked as expected. Later on, it didn't work. Here are the issues:

    1- While it was working fine, when A/C mains was cut off, it took some 7-8 seconds for the leds to light up 🙂

    2- As you mentioned in the description and in some of the answers you gave, I adjusted P2 to get 14V from battery terminals and I got a dim red light, but when it went down to 12V the light got dimmer, almost not visible.

    3- With the use of strip leds, I guess I fried T2 (S8550), because after a few tries turning A/C power on and off, the leds starting lighting up with the A/C power on turning off when the power goes off 🙂 Also, the transistor was pretty hot.

    • Hello Alihan,

      the issues which you have mentioned can never take place because there's no possibility of those happening.

      there's no capacitor associated with the T1/T2 so the delay effect cannot happen.

      I hope you have only removed the LDR, R1 and P1 in your circuit and kept rest as shown in the diagram with R2 connection intact with the base of T1?

      T2 can only become hot if the LEDs current is much higher than its tolerance level.

      The charging LED will become dim as the battery voltage reaches 14V and vice versa so that's fine.

      when mains is present T1 will be triggered ON through R2, which will keep T2 switched OFF, keeping the LED bank switched OFF, so the LEDs can never light up when mains is present unless one of the transistors has gone faulty.

      by the way this a very basic and crude design so you cannot expect high precision from it.

      for a high precision you can probably refer to the following design.


    • Thanks for your reply, you've been pretty helpful..

      The only direct connection between T1/T2 and the capacitor is the negative pole, which I guess not very important.

      As I presumed, I had fried T2 and when I replaced it, it worked fine again. Then it got fried again as I forgotten to remove some part of led strip before plugging it back :))

      I realize that it is a crude design, still it is important for me to start building something small and make it useful at home..I have seen the other project you have mentioned and I will surely have some questions for that one too 🙂

      If I change T2 to a more powerful transistor, would I be able to use it with a load of 2-2,5A? Or I need to upgrade the whole circuit for that? For T2, I found BDX53C, which seems to be pretty powerful, still not sure if I can use it for the same purpose, or do you have another suggestion?

      Thanks in advance..have a great week

    • yes obviously T2 must be dimensioned as per the LED current otherwise it will keep blowing of.

      Only T2 and R3 will need to be upgraded as per the load current rest will as is unless any other parameter such as battery is upgraded too.

      R3 = Batt supply – LED operating voltage / LED operating current

  3. Hi Sir. We want to make a request. Is it possible to add a feature on this circuit? We would like to add a fan in this circuit so we can use it when there is power failure. Thank you.

    • Hi Mayu, you can connect the fan exactly where the LEDs/resistor network is connected that is across the positive of battery and the collector of T2

    • Varun, you can use a 0-12V 2amp transformer for it, connect a bridge-rectifier with a 6800uF/25V to getting the required 14V 1 amp charging output for the battery.

  4. Hi am Akhil

    In the first circuit circuit using IC555,where should the earth sign indicated point be connected?
    to the battery -ve or bridge circuit's -ve?

  5. what changes should be made to the circuit if a large number of LEDs (say White coloured,75 or more in number) should be included??

  6. Sir can i ask where is the positive and negative in solar panel . the positive side is connected in the diode right ?

  7. Hi Swagatam..
    I just constructed the second circuit using 6 volt battery, with 24 Led, single series with R combined 3 diodes. For the emergency light, it is functioning well. However, for the charging and cut off section, i set up the LED with the fully charged battery (6.9volt) by triming the pre set of TIP 127, to get very dim LED. When the battery discharged to 4.4 volt, the LED is still dim, then i detached the Battery and measure the collector voltage of TIP 127, the output is zero. I trimed the VR in such away to get the output of the collector to 8.5 volt, the LED became very brigth. I put on the battery and let it get charge. After the whole day, i still saw the LED bright. Then i detached the battery and measured the voltage of battery, it was 6.9 volt. i put on the battery again and trimmed down the LED until dim. After that, i detached the battery and measure the output of TIP 127, it was zero volt. Could you please advise me, what is my fault. I use 7.5 volt transformer of 1 amp and 6 volt 4.5AH batery.
    Many thanks

    • Hi Kanta,

      using a single transistor as a sensor can be very crude and produce less accurate results…still you can try attaching a 3V zener diode in series with the base of the transistor (cathode towards base) and check the results, or you can opt for the next design using two transistors.

    i implemented your circiut with ic lm555 ,the current with leg 3 of ic is fllowing in every condition of LDR if we opened the leg 3 of ic the emergency light works fine plz help me to make LDR operational in this circiut

    • Dear Farhan,

      in the absence of light on the LDR, pin3 will stop producing a voltage…..so make sure the LDR is held in sufficient darkness for this to happen…..or reduce the value of R2 until pin3 becomes zero volts at the desired darkness level

  9. dear sir i implemented your fiirst circiut with ic lm 555 i am having an issue emergency leds are not glowing becasue in every case current is flowing from leg 3 of ic if we opened leg 3 of ic the emergency light works fine plz help me and make circiut operational with LDR
    image url is

  10. Hi sir
    I am so glad to see ur interest towards electronic circuitry ..
    So eagerly waiting for circuit which will be having following (few or all) features
    -Low Battery Cut-off
    – Overloading protection
    – Short Circuit protection
    – Reverse current protection
    – Reverse polarity protection
    – Thunder protection
    – Over discharge protection
    – Auto battery shut-off at Low voltage detection
    – Overcharge protection
    – Auto charge stop/ High Volt Detection
    – Battery capacity level display(SOC)

    Making this circuit for underprivileged location as donation for poor via charity
    So hope I can have one ckt diagram with some or all features mentioned above or lts link..
    Looking for ur reply ..
    With full excitement

    Thank you

    If successful I am in plan to put ur & website name on my device 🙂
    As part of tribute to you sir

    Email: Lokeshelectronic@gmail.com
    Plz WhatsApp me sir if possible to

  11. Sri. Majumdar,
    You say that the IC555 in your first emergency light ckt. can be dropped and have given the modified ckt. without the use of IC555. Does the second ckt.retain all functions of the first ckt?

  12. hello sir swagatam.
    sir i want to create your wonderful circuit above but the problem is., i never have bc547 if i substitute this by 2n5551 what is the difference and what transistor that can be substitute in bd139 if i never have tip122.,., i wait for your answer sir.,., thanks.,.,

    • hello eroll

      although not very much suitable, 2N5551 can be used in place of BC547, for BD139 you can use any other equivalent rated to handle at least 2 amp at 30V

  13. hello Sir,

    i build your second cct ,its working fine ,except the charging part…….. you are right single transistor cct is crude………could you please integrate the charging cutoff section in first cct in second one…………..I tried some thing …………kindly make necessary ammendments………..

    i have sent u this modified cct at homemadecircuits@gmai.com ,please check and reply accordingly.

    Thanks in advance for your kind response//

  14. Hello Sir,
    Firstly I would like to appreciate your effort !!! G
    Now my question is …can I use a 6V battery instead of 12 V…I would like to avoid LDR….Is there any modification neeeded?

    • Thanks Rahul!

      Just remove P1, R1 and the LDR if don't want the dark activation section.

      yes 6V can also be used, the LED series resistors will not be required with a 6V supply

  15. Hi Swagatam, GN,

    Your circuit is awesome and Good. Could u plz clarify some doubt! Thanks in advance.

    I am using 12v 7.5AH rechargable battery, i am planning to connect 6 1 Watt LED to the battery. Can this circuit be used or do i need to modify or add any resistance to the circuit.

    • Thanks kishore, you can try the above circuits, but T2 will need to be changed to TIP122 for driving 6 watt LEDs safely. Use large heatsink for T2

  16. which simulation software do you use sir, am using proteus and i have problem editing its transformer to give a desire voltage. so i'd appreciate if u can share me the one you think will help me the best or suggesting me the link to downloading it. moreso am new to designing but love to design

    • simulators are like blind persons with a stick, do you want to depend on a blind man's judgement??

      Therefore i never depend on simulators and have never used any, rather I design the circuits by simulating them in my mind…I know it may be difficult for a new hobbyist to follow this but it also may not be a wise idea to depend on something which itself is so unreliable.

      the best way (although hard) is to develop knowledge and understanding regarding the various concepts and begin simulating them with the help of the acquired knowledge

  17. hello there sir, thanks for your hard working. its really useful here.
    may i request an led indicator for battery level like low, med and full when charging and using battery power 🙂
    thanks for your answer sir

  18. dhanish, the center lead or the slider lead of the preset must go to R4.

    The LED will become dim only if the supply voltage is 15V or equal to the battery full charge level.

    Are you supplying 14 to 15V to the circuit? If not pls do it.

    A single transistor charger is always very crude so you canot get accurate resuts from it….if you want better results make the first circuit using two transistors, it will be more responsive.

    and if you are looking for even better and satisfactory results you can try the following design which is by far the most reliable of the designs.


  19. sir some doubts again
    1)for connecting p2 preset there are 3pins two pins give fixed resistance,two pins give variable resistance, i connected the variable pin to r4 resistance, is it correct?

    2)As you said i adjusted the voltage level to 14.5 without connecting battery but the t3 base led is not that dim.And when i connected the battery which have a voltage level of 12.6v the base led again becomes dim.Is that any problem in my wiring ,but i made the circuit 2nd time pls help me sir?

    3)but with all these i connected battery it charges to13.06v and stops further charging.the base led is glowing still the battery voltage not increasing.why?

  20. dhanish, adjust it to get 14.5V and the LED should glow dimly at this voltage.
    after this setting this done, you can connect the discharged battery which should force the voltage to drop to the level of the battery and render a higher brightness on the LED, as the battery charges the LED brightness will reduce and at full charge of 14.4V the LED will glow very dimly.

    another point is, your panel voltage must be first regulated to 15V and then applied to the circuit.
    use an LM317 circuit for this.

  21. sir now ok your 2nd circuit is working but 3rd circuit i have problem.i want to use switch instead of ldr so i didnt use ldr,r1,p1.

    and i didnt connected the white leds and the remaining circuit is completed without giving solar panel input.And when i give transformer input it gives only 400mA output without connecting battery terminals.

  22. sir now its ok.
    sir how long it take to charge the battery current charge is 12.68v?

    sir how can i check my circuit that everything is working well?when i checked the output given to battery terminal without connecting battery it shows 13.00v and 400mA and the t3 base led lights up is it correct?

  23. sir i made the circuit but i have problem with the preset.pls help me to how to adjust it.
    when i started adjusting a spark is coming from the 1k preset. so i stopped adjusting is that due to any wiring mistake .

    please reply fast sir.

  24. 1) it will stop overcharging if the preset is correctly adjusted, but it's a crude circuit.

    2) solar panel should be 15V/2amp for 7ah batery, the base led will indicate charging state of he batt.

    3) yes can be done

  25. Sir I finally decided to make your circuit, i need your help during my assembling if any error arises. sir please answer each question.
    Please reply as early as possible.
    some doubts are

    1) In the 2nd circuit does it have overcharge protection?If yes can you just elaborate please?

    2) Sir please specify the rating of solar panel (max current and max voltage) in 3rd circuit?And how to show a charging indication for solar panel?

    3) Sir i know you said that my solar panel(17.5-19v and 500mA) not capable of charging this battery eventhough if i am connecting that solar panel, can i connect it as shown in your circuit?

    • Friend i am little bit confused thats why im asking to many questions dont feel irritated.please tell me that if i want to connect the 1w led direclty to 12 7ah battery then how many can be connected to the battery so that i will get a back up of 6-8hrs

    • friend, make strings of 3 leds in series with a 6 ohm 1 watt resistor.
      make 2 such strings and connect them in parallel (join their positive/negative ends together)
      and connect the network to the battery.

      2 strings in parallel having 3 leds in each string gives 3×2 = 6 leds
      this would approximately run for 7 to 8 hours.

    • sir pls reply,
      in the above comment you said to remove t1 t2 stage,then should i have to remove r1,r2,r3,p1 with that.
      if yes then where i have to connect the led +ve and -ve.

      I want to charge my 12v 7ah battery with this circuit pls tell me how.

    • how many 1w led bulb can be connected to 12v, 7ah battery (pls give the resistor value also) which give max brightness and backup hrs.

    • i am answering this for the second time, pls don't repeat your questions.

      connect 3 leds in series with 6 ohm 1 watt resistor

      make two such strings and connect their common ends with the battery

      backup will be upto 3 to 4 hours

  26. when i measure the voltage and current of solar panel with multimeter i got 19.2 approx 20v and
    current within .5A.

    I dnt know the rating of that bulb

    • the solar panel can be used with the above circuit but after stepping down through a LM317 voltage regulator.

      the OSRAM bulb tat you have mentioned is a fluorescent tube so it cannot be used directly with the above circuit.

  27. hi
    my battery is 12v and 7amp,so is there any problem. And i have Osram Dulux S/E 7W 4 Pin CFL – 827 so is it possible to connect that in this circuit.

  28. Sir, I want to build the second circuit (without 555), but want to additionally incorporate a small solar panel for charging battery (first priority solar charging, second priority mains). Will be obliged if you please suggest a modification in circuit. safdar

    • I'm very delighted and thankful for your prompt response. I will look forward that u have time to advise a good circuit. To elaborate requirement further, the battery should normally be charged by solar panel, and if not sufficient, like cloudy days, it should auto switch to charging from mains till battery is fully charged (of course with overcharge protection). Thanks once again.

  29. sir can I use same circuit for using 12v 7w led and battery of 12v 2.5Ah and how much time transformer needs to charge battery of 2.5 ah fully

  30. sir can I use same circuit for using 12v 7w led and battery of 12v 2.5Ah and how much time transformer needs to charge battery of 2.5 ah fully

    • 12V/2.5 ah will not support the led for very long, may be only for 30 minutes.

      lead acid battery needs 10 hours to get charged correctly.

  31. Good day sir!

    I have some questions.

    1.Can I replace the 12-0-12v 1A transformer with 15-0-15v 1A transformer then just use voltage regulator like LM7812 to regulate to 12v? Doesn't it affect the current?

  32. hello sir
    i gopal malviya from m.p. and i have just started my cfl repairing and making business but i don't have any good electronic parts supplier
    so sir please suggest me any parts supplier with your reference
    i'll thankfull for your

    Thanks for Your Kind Support
    Gopal Malviya

  33. hello abhishek sir
    i gopal malviya from m.p. and i have just started by cfl repair and making
    business but i dont have any good reasonable electronics parts supplier so please suggest me parts supplier vendor contact detail with your reference

    Thanks for kind support
    Gopal Malviya

  34. for lower voltages how LDR's can be configured
    ie which transistor should i use???

    waiting for ur reply…………

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