Home » Emergency Light » Simple LED Emergency Light Circuit

Have a Question?

Please feel free to ask any circuit related question and get a quick solution from me.

Simple LED Emergency Light Circuit

The article describes a very simple homemade emergency light
circuit that can be used during power failures and outdoors where any other
source of power might be unavailable.

The circuit uses LEDs instead of
incandescent lamp, thus making the unit very power efficient and brighter with
its light output. Moreover, the circuit employs a very innovative concept
especially devised by me which further enhances the economical feature of the

Let’s learn the concept and the circuit more closely:

The concept:
We know that LEDs require a certain fixed forward voltagedrop to become illuminated and it is at this rating when the LED is at it’s
best, that is voltages which is around its forward voltage drop facilitates
the device to operate in the most efficient way. 
As this voltage is increased,
the LED starts drawing more current, rather dissipating extra current by
getting heated up itself and also through the resistor which also gets heated
up in the process of limiting the extra current.
If we could maintain a voltage around an LED near to its
rated forward voltage, we could use it more efficiently. That’s exactly what I
have tried to fix in the circuit.
Since the battery used here is a 6 volt battery, means this
source is a bit higher than the forward voltage of the LEDs used here, which
amounts to 3.5 volts. The extra 2.5 volts rise can cause considerable
dissipation and loss of power through heat generation.
Therefore I employed a few diodes in series with the supply
and made sure that initially when the battery is fully charged; three diodes are
effectively switched so as to drop the excess 2.5 volts across the white LEDs (because each diode drop 0.6 volts across itself).
 Now as the voltage of the battery drops, the diodes series are reduced to two and
subsequently to one making sure only the desired amount of voltage reaches
the LED bank.
In this way the proposed simple emergency lamp circuit is made
highly efficient with its current consumption, and it provides backup for a much
longer period of time than what it would do with ordinary connections.
LED Emergency Light Circuit

Efficient, Automatic, White LED Emergency Light Circuit Description:

Referring the shown simple LED emergency light circuit, we see that the circuit is actually very easy to understand, let’s
evaluate it with the following points:
The transformer, bridge and the capacitor forms a standard Power
supply for the circuit. The circuit is basically made up of a single PNP
transistor, which is used as a switch here.
We know that PNP devices are referenced to positive potentials
and it acts like ground to them. So connecting a positive supply to the base of
a PNP device would mean grounding of its base. Here, as long as mains power is
ON, the positive from the supply reaches the base of the transistor, keeping it
switched off. Therefore the voltage from the battery is not able to reach the
LED bank, keeping it switched off.
In the meantime the battery is charged by the power supply
voltage and it’s charged through the system of trickle charging.
However, as soon as the mains power disrupts, the positive
at the base of the transistor disappears and it gets forward biased through the
10K resistor.
The transistor switches ON, instantly illuminating the LEDs.
Initially all the diodes are included in the voltage path,
and are gradually bypassed one by one as the LED gets dimmer.
Parts List for the proposed LED emergency light circuit
R1 = 10K,
C1 = 100uF/25V,
D1, D2 = 1N4007,
D3—D6 = 1N5408,
T1 = BD140
Tr1 = 0-9V, 500mA,
LEDs = white, hi-efficiency, 5mm,
S1 = switch with three changeover contacts.

In response to the suggestion of one of  our avid readers, the above automatic LED emergency light circuit has been modified and improved with a second transistor stage incorporating an LDR trigger system. The stage renders the emergency light action ineffective during day time when ample ambient light is available, thus saving precious battery power by avoiding unnecessary switching of the unit.

LED LDR Emergency Light Circuit
Circuit modifications for operating 150 LEDs, requested by SATY:

6V Emergency Light Circuit

Parts List for the modified emergency light circuit

R1 = 220 Ohms, 1/2 watt
R2 = 100Ohms, 2 watts,
RL = All 22 Ohms, 1/4 watt,
C1 = 100uF/25V,
D1,2,3,4,6,7,8 = 1N5408,
D5 = 1N4007
T1 = AD149, TIP127, TIP2955, TIP32 or similar,
Transformer = 0-6V, 500mA

The following circuit shows how a low voltage cut off circuit can be included in the above design for preventing the battery from getting over discharged.

Emergency Light circit with Charger

Power Supply Circuit with Emergency Backup

The circuit shown below was requested by one of the readers, it is a power supply circuit which trickle charges a battery when AC mains is available, and also feeds the output with the required DC power via D1.

Now, the moment AC mains fails, the battery instantly backs up and the compensates the output failure with its power via D2.

When input Mains is present, the rectified DC passes through R1 and charges the battery with the desired output current,  also, D1 transfers the transformer DC to the output for keeping the load switched on simultaneously.

D2 remains reverse biased and is not able to conduct because of higher positive potential produced at the cathode of D1.

However when mains AC fails, the cathode potential of D1 becomes lower and therefore D2 starts conducting and provides the battery DC back up instantly to the load without any interruptions.

Emergency Light with Charger Circuit

Parts List for an emergency light back up circuit

All Diodes = 1N5402 for battery up to 20 AH, 1N4007, two in parallel for 10-20 AH battery, and 1N4007 for below 10 AH.

R1 = volt/charging current (Ohms)

Transformer Current/Charging current = 1/10 * batt AH

C1 = 100uF/25

Using NPN transistors

The first circuit can be also built using NPN transistors, as shown here:

20 LED Emergency Light circuit


    • sir can u just tell me how u make these circiut diagram(means u use any software or like wise) pls sir help me i have to make a electrical circuit diagram which raw drawing is with me how can i draw it with any way

    • I cant figure out how to use 2nd circuit of ldr with already owned 6b emergency light to replace manual on off switch to automatically off in day

    • Please always comment at the end of the page, it becomes very difficult to find the comment if it's posted at some other place.

      It won't be possible to replace the switch of a ready made emergency light with an LDR.

      You will have to make a relay type of switching circuit for doing that which can be be very complex.

    • Hello sir,
      I am a noob in electronics. I just wanted to light up 64 led's with a 5v or 7.2 volt battery source With a dimmer to dim led's. Can you please tell how to do it.
      Thanks in advance.

    • Sir, i prefer using a set of rechargeable AA batteries/5v power bank/ 7.2volts camcorder battery. And the normal white led's available
      I just want the power source to be rechargeable and as portable as possible. And the light to be as bright as possible and dimmable too.

    • Sri. Majumdar,
      I have a suggestion for you; please give serial numbers to your circuits, so that you indicate the sl.no. of the ckt. in your replies not say the first ckt, the second ckt. etc.

  1. Hi Swagatam

    I have to trace you here from BrightHub. You are a very valuable resource and we can't spare you. I observed though that you were not responding at brighthub and searched through comments of different articles and found out your new home.

    However a little warning that BlogSpot has a bad habbit of deleting complete blogs and I have seen some very nice electronic blogs vanished completely. Please backup every content of your blog. Watson's eblog is not on BlogSpot any more. I used to read it almost daily.

    Nice to have you back.


    Your old friend


  2. Hi Sam,
    Welcome to my blogspot friend, I am so pleased to see you here.
    Yes you are very much right,…. I have all my articles saved in my computers harddisk. You cannot trust anything here, a slightest of ignorance can cause a complete deletion of your work.
    Actually I am already facing problems indexing my article pages here as they are being blocked by robot.txt files by Google, I have no clues at all why this is happening, everything here is very unpredictable.

    Anyway, thanks a lot for your valuable inputs.

    Keep posting.

    Best Regards.

  3. hi swagatam i want to make use of my computer ups as a emergency LED light. so really need ur help for the things i required n what is the circuit diagram for that n all. plz suggest

  4. Hi Vivek,

    Just type LED tube light in the search box provided at the top right corner.
    The article will explain you about a nice little LED lamp circuit that can be operated at 220V mains power available from your UPS.


  5. I am never comfortable with transistors and can never seem to work out which pin/leg is which and which way round it goes. Is there an easy way to tell? – is it measurable with a multimeter?

    • Unfortunately there's no such measuring device which can directly identify transistor pin outs, you will have to refer the particular datasheet and remember it while working with electronic devices.

  6. Hi! I'm planning to build this project. May I know if this ckt. will not overcharge the battery if left connected permanently on AC? As this will be use for emergency cases only when power outtage occurs.

    • Hi,
      This circuit is not overcharge protected.
      You will need to add a 470 Ohms resistor in series with D2 so that the battery is only trickle charged and does not get damaged if kept connected permanently to AC mains.

  7. Hi,

    I'd like to use your design to prevent a device connected via a transformer to the mains going through a startup cycle every time a power outage occurs and power returns. Do you have any idea of the delay between the mains power falling and the power being provided by the battery? Perhaps this circuit would not work for me if the period is long enough for the device's board to sense a momentary drop in voltage and treat that as an outage.

    Also, how would the circuit change if I built an adapter circuit that fitted between my existing transformer output plug, and the device's transformer input socket? I apologise in advance if I'm not providing enough information for you to answer that. I guess the off-the-shelf AC to DC transformer unit will have some components built into in on the DC side.

    'Much appreciated,


    • Hi,

      I couldn't exactly understand your application need, however as far as the change over speed of the circuit is concerned, it's definitely quick, probably within a few milliseconds.

      Thanks and Regards.

  8. If it's a few milliseconds it might just work!

    The application is indeed a strange one. Living in hard water area, we have a water softener unit that has a controller PCB connected to the mains via a plugin transformer. Unfortunately my village suffers from frequent outages when it rains or when the wind blows. Seriously. As soon as the power returns, the softener, by design, recharges the resin with salt which incurs some expense. It doesn't have the ability to keep track of recharges between power outages. That means if we have a pretty bad day, the entire vessel of salt can be emptied without ever drawing softened water from the taps. Hope the application make sense!

  9. It would be good to have an adapter module built that includes the battery and electronics that simply inserts into the existing DC plug and socket so should I need to remove it, the original transformer and PCB device is unaffected, no wires are spliced, and they can be reconnected without any fuss…


    • OK, so basically you need a change-over circuit which is able to switch to battery back-up mode for your circuit each time the power fails and switch back to adapter mode the moment power is restored? Right?
      Can you provide the operating voltage of your circuit?


  10. Not sure. Do you mean peak current when the motor is being driven during the resin re-charge cycle or whilst in dormant mode when just the PCB with water flow-rate detection and clock is running?

  11. Hi…. actually i am an MBA by profession, but i have great interest in Electronic projects and i always make different different projects.
    So after seeing this project i got an idea to do a project with 5 piranha high flux LED, which would be powered from 230volt AC main, but i am getting the circuit diagram, if anybody can help me out in having the complete diagram…please

  12. Hi,
    I completed the circuit,connected to the mains(via 0-9v 500mA tranformer),measured the output at battery out(without battery,led's) and i got 13.xx volts.Is it high?? or when i connect to the battery,it will run fine?Please help

    • Hi,

      Yes it is quite high for a 6 volt battery, either use a 0-6v transformer or connect at least six diodes in series with D1 for dropping the voltage to some extent.


    • Can i use some high watt resistor in series instead of diodes?If yes what will be the value and where to connect it?I used 9v transformer as it is given in the list.Please help!!!

    • If you replace the collector diodes with resistors, power will be unnecessarily wasted….please read the concept in the article.

      You may use a 0-6v transformer for safer results.

  13. Hi,
    For the last circuit(Emergency Backup)can i connect LED's as in the 1st circuit(with 12 LED's)?And the diodes used (D1–D4 , D1,D2)is IN 5408?

  14. 1. What type of battery is being used? Lead acid, gel, etc.?

    2. I thought that when led are placed in parallel, one may hog the current due to variation in characteristics of the leds, and therefore a resistor is normally used to limit the current…?

    3. Would the series diodes (not leds) consume as much power as resistors?

    4. You mentioned that ideally a constant voltage source is being used here. Aren't constant current sources better drivers for leds?



    • 1. Any type of battery may be used, as long as ot's rechargeable type and suits the LEDs, will do.

      2. You are right, a resistor may be introduced with each channel for better efficiency.
      3. No…Diodes have been especially incorporated here to stop dissipation unlike resistors.
      4. If the voltage remains constant, current won't rise, that's why we don't require resistor for testing LEDs with a 3 volt cell.


    • R1 becomes 1k,

      LEDs: two nos in the series (white),

      LED resistors: 50 Ohms 1/4watt

      diodes: 6 nos, all 1N5402,

      battery: 12V

      rest everything remains the same.

    • sorry …i am asking about the battery backup circut when main power fails….what is the transformer ,resistor,capacitance specification should be used for 12 volts battery….and is it posible to stop battery charging when battery is complectly filed,with zener diode?

    • The transformer will be a 0-12V, current BattAH/5….. rest will be as explained in the above comment by me.

      The battery will be charged through trickle charging method, for fast charging and auto cut OFF you will have to make more complicated circuit, I have explained many automatic charger circuits in this blog, you may refer them.

  15. Hi Swagatam
    I am trying to build an always ON LED light.
    It will work from an AC to DC adapter (3.3V) during normal operation.
    I want to connect a 3V battery along with LED in parallel to this adapter.
    Will this work?
    Please help!

    • Hi Kdude,

      If you are using a rechargeable battery then you may incorporate the last circuit shown in the above article. If will work nicely.


  16. Hey Swagatam,

    Nice to find you here. I have come to you with a query.

    I have designed a LED lamp operating on 230 v ac, using a X capacitor to drop the voltage and driving 4 strings of 35 white LED's = 140 LED's. The circuit is working well and I am aware that the circuit is not isolated.

    What I would like to know, is how exactly is why the X capacitor makes a humming noise? What parameters should I keep in mind, when opeating at low temperatures?

    Thanks for the help.


    • Hi Madhav,

      Thanks! Welcome to my blog.

      If the capacitor is making noise, means it's not a good one, replace it with a correctly rated, good quality capacitor, for example you may replace it with a 105/400V PPC capacitor.

      Low temperatures won't affect the circuit in any way, because the parts used are not critical and has wide temperature tolerances.


  17. Hello Swagatam..

    Part list for Power Supply Circuit with Emergency Backup is NOT published???
    what are values of D1..D4, D1, D2, R1 and C1 and also about the transformer..??
    Does it protect Batt. form over-charging.

    • Hi PP,
      Now it's published, thanks!

      If the resistor is chosen for trickle charging then it might not damage the battery, but at higher currents this circuit will not protect the battery from over charging.


    • Hi Kiran,

      Hold the printed side of the IC toward you, then the pin at the left of the slot or the half-notch on the IC, is the pin#1 and the pin which lies at the right of the given notch, is the last pin of the IC, I hope you got it.


  18. Hi, Swagatam,

    I appreciate your generosity of posting your innovative ideas in electronics. I was looking your schematic diagram of LED emergency light circuit. I guess LEDs will light when AC power is used.
    If we would see it on the principles of biasing transistor. Am I right?
    Is the switching on of transistor does not affect the current flow to LEDs when battery supply is used?

    • Hi friend, thanks!

      The transistor used is not an NPN, rather a PNP, that means a positive biasing will keep the transistor switched OFF.

      Thus, in the presence of AC mains the LEDs remain switched OFF, and illuminate the moment AC fails.

      I hope you got it.

  19. Hi Swagatam. Am building this circuit correctly for the second time. In my last construction, i added a red Led in series with a 1k resistor after the capacitor for AC on indication. However, the red LED lights even when battery is used and no AC. I suspected diode D2 and tested it only to find it is ok. I therefore believe there is leakage current problem which dissipates power even when battery is not in use. Please i want your view and suggestions on this. Thank you.

    • Hi Masterfiles,

      Connect the LED/1K series directly across the transformer secondary winding. This will give the correct required indications and will keep the LED switched OFF in the absence of AC power..

  20. Hi swagatam. Thanks for your prompt response and am writing to acknowledge that the red LED is now indicating as expected. Please kindly share more knowledge with me about the working of the power supply circuit with emergency backup. Thanks once more as i expect your usual prompt response.

  21. Good day swagatam. Please the circuit i need you to explain further is that of the "power supply circuit which trickle charges a battery when AC mains is available, and also feeds the output with the required DC power" I want to see if your explanation of the circuit's workings will match what i have in my head.

  22. Thanks Swagatam for taking the pain to answer my questions even when they seem the answer is so bvious. However, the truth is that am actually asking for the working principle of the circuit. In other words, i need the 'electronics' of the circuit. Thanks

  23. Thanks for your quick reply 🙂

    It is electronics. We have to find a circuit which contain at least 3 IC on its circuit. It doesnt matter whether the circuit is complex or simple. But of course i'm looking for a simple one. 😀

    btw, I am beginner.

    Thanks for your concern Mr. Swagatam

  24. Hi, Swagatham,
    Doubt about Circuit(2) with LDR.
    1.Is it necessary to add 100K pot &1K to LDR? If necessary why is it?
    2.Can i use 2N3904 instead of BC557?

  25. Hi swagatam,thank you four ur post.i had connected the circuit as the diagram but the led in illuminate when the power is on,and off pls help.its my mini project i need to submit this month.

    • Hi Muanthang,

      You might have connected the transistor wrongly, because if the transistor is connected as shown in the diagram it will never conduct in the presence of power supply from transformer and the LEDs will stay shut OFF when mains is present.

      Please check the connections, or replace the transistor with a new one.

  26. Can anyone from Delhi help me to find shop for buy these electronics chips. I tried to find in Lajpat Nagar Marker and Gaffar Marker but no one telling about specific shop. Its looks stupid but I can;t find a single shop to buy even a LED since years. I usually telling my brother to buy and ship to delhi. (from Darbhnaga, Bihar)

  27. Goodevening Mr. Swagatan sir.
    ahhmm i would just like to ask if the voltage input at the very first circuit design is DC? sorry for the nood question, I am just wondering
    thank you

  28. Hi Swagatam Majumdar. This is mahendra singh. B.Tech. in Electronics and communication Engineering in 2007. Right now i am working in a telecom company but i want to switch in Electronic Circuit designing so please suggest me is it possible and how?

    Awaiting for your response.

    • Hi Mahendra,

      You may switch to electronic circuit designing field but it will require a lot of hard work to master the subject, the results will depend will depend on your level of interest and patience.

  29. I've successfully made the circuit with 150 LEDs, but I used only 24 8mm straw-hat typed LEDs and I found a problem with the result. The LEDs are still glowing even it's connected to the mains. I checked the output voltage to the LED (mains supply is present) and it gave 0.6 vdc. Could you tell me what's wrong with the circuit and how I can solve that kind of problem. Thanks….

    • That's quite impossible, if the positive from D1 is connected to the base of the transistor, the transistor would never conduct.

      please check your transistor, either it's blown-of or connected wrongly or may be its not a PNP transistor.

  30. What I mean is the circuit no. 3, the one requested by Saty. But I used only 24 LEDs. What do you recommend me to fix it? Thanks for quick reply.

    • It's one and the same, when mains is on, the voltage from the bridge will hit the base of the transistor directly, and this will keep the transistor switched off at any cost.

      anyway, you may increase the value of the base resistor to 1k or more and check the results.

      Number of leds will not make any difference in the functioning of the circuit.

    • Hi Swagatam,

      I want to thank you for the circuit which you have provided to me (3rd circuit) it is working fine.
      This is saty here after long time, i thing u remember, according to the 3rd circuit for 150 LED with same setup i have built it.
      1. want overload protect to the battery, since it is conneted to the mains always (470 ohms should be connected series to the D2, pls let me know is that right)
      2. can i use 12v 7AH battery, if so please advice what are the components to changed or represent with clear diagram so that i can try to use the 12v battery with 3rd circuit so that i can have long backup hrs if absence of mains. Thanks in advance

    • yes TIP42 will do….

      keeping the printed side facing you (pins downward), the right end pin is the emitter, the left pin is the base, the center is the collector.

  31. Hi Sir, I finally mode the 1st circuit and it also work good with 6v 5 w solar panel(used 6v, 4ah battery). Can I safely use the solar panel and set it permanent on my roof. I only tested for 15-20 minutes not continues hours. D1 already there so not added any extra diode for blocking. However autocut needs more light to switch led off. But its ok if remain on up to morning 8am.
    And what will be total AH of Battery if I use 4 X 1.2v 2500mah AA battery in series to make it 6v.

    When I will go to my home town I will set this to roof so i light in night and light also bright your name for all your help.

    • Hi Sir,

      You cannot connect the circuit permanently with the solar panel, because the solar panel voltage will vary with sunlight which may not be suitable for the circuit and also for the battery….and the circuit does not include an auto cut off stage so that's another issue.

  32. Sir, i like your all articles here specially in led lighting because its more lifewise…Sir, im in indiefilm for the first time with low budget we are still in planning we would like to use light thats not using AC volts so its mean DC volts…Sir,do you have or can you create schematic or diagram for 12 volts DC that can lights the leds at least 150-200 watts brightness with power charger?….I hope you can help us! Thanks in advance!

    • Sir, please don't post my previous comment.. i think 50 to 60 leds all…in your circuit 12 strings and 3 leds for each string…so if i have 60 leds it will be 5 leds every string? correct me if im wrong, sir…

      Parts list:
      R3—-R14 = 10 Ohms, 2watt
      LEDs = 1 watt, 350 mA, cool white
      battery : 12V battery (100 ah)

    • a 12v supply will hold only 3 white leds in series comfortably.

      so dividing 60 by 3 gives 20…..you would have to make 20 strings of 3 leds with a 6 to 10 Ohms series resistors with each each string.

    • Sir, thanks for fast reply again…now i understand!

      but what is the total power watts consumption and total watts brightness for this 60 leds and battery hours for continues using?

      and if i ever want to add more leds to increase more brightness for this circuit using 12V battery (100 ah),how many leds can i add?

    • Multiplying the optimal current with voltage of the LEDs will provide the consumption wattage of the LEDs.

      100 ah means if the battery is loaded with 100 watts, it would last for 1 hour ideally, atcually it would last only for 15 minutes.

      so keep the wattage well below the 100 ah mark

    • Im so proud of you sir! You gave me a big knowledge about this circuit..

      If ever i convert this 12 volts DC to AC 12 volts adapter is (for the future purposes if we have a lot of budget in our indiefilm production) is it ok?

      what parts need to change or adding components in this circuits? and how many leds can i adds? or any suggestions sir if ever i can use AC volts…

      I hope you can help me again sir! thanks in advance!

    • Thansk!

      As long as the power is a 12V DC, you don't have to change anything in the circuit, just make sure the adapter current is sufficiently rated for driving the connected LEDs brightly….

  33. hi,
    i have completed the circuit but in that circuit the charging voltage for the battery comes to 10v, i have used transformer whose output voltage is 0-7v and the supply to led comes to 9v what shoul i do.reply me soon

    • use a transformer whose current is 1/10th that of the battery ah…. the voltage will automatically adjust after the battery is connected.

      Also, you should use a 0-6V transformer

  34. sir can u make the circuit very simple cuz i am a beginner but this is for my project sir. is it possible to do it with a bc54 7 transistor if so plz post me a schematic. i want to be done with a bridge rectifier and capactitors, transistor *3(max), a transformer,etc and no diodes my friend had bought one from the shop but dint show me properly. thnx in advance

    soo sry to trouble u sir

  35. hello . I would like a circuit for a wall plug nightlight that has 3 common straw hat leds ,a battery backup . i would like the leds off during daylight .


  36. hello . I would like a circuit for a wall plug nightlight that has 3 common straw hat leds ,a battery backup . i would like the leds off during daylight

    Thankyou for the quick reply ,

    the circuit you refer is the one with the 555 timer ?

    I do not see a battery backup in that circuit ?

    do i use the circuit from this thread and apply output d1,d2 to the 555 circuit ?

  37. Sir,

    I really enjoy reading your articles, your talent is that, you make everything so simple. The best best part is, unlike other Circuit designers that we see in the internet, you are always there to explain or reply a reader's problem.

    The only problem is, most of the people don't get spares specially for people living in North East India.

    So here is a request/suggestion, why don't you start a enterprise to sell assembled kits & electronics parts of your wonderful designs? This can be sold at a higher price than market but since you will be supplying all the parts, a hobbyist won't mind buying it from one source instead of roaming the marking searching for spares.

    You can send the items through postal and courier service to any part of India, like "Electronics for you Magazine, Kits&Spares"?

    I am sure, this will not bring you only profit but widely popularize your designs as your circuit will be used by many hobbyist in the country.

    Yours truly,
    Chuba Ao
    Ministry of Communications & IT
    Govt of India
    Mokokchung, Nagaland.

    • Thank you very much Cedar! I appreciate your interesting suggestion.

      I indeed would want to implement your suggestion, but the problem is that maintaining electronic kits in a professional manner can be difficult, and providing full range is another difficult part, next comes investment which can be a bit risky. Being a sole manager of the blog all these can be difficult to handle and therefore I haven't yet started selling anything here.

      However, I am studying readers interests, if things looks conducive it would't be long for me to implement the option that you have provided.

      Thanks and Regards.

  38. Hello Swagatam Da,
    In The first circuit, do we need not to connect a bridge rectifier circuit after the transformer.


  39. Would increase in Battery Ah means More Backup time or it will damage the LEDs
    if it will damage the LEDs than how much battery current we must use….

  40. there must be a limit, as we cannot connect 200Ah Ups battery?
    I want to know that limit and how to calculate that?

  41. Hi Swagatam,
    I am a newbie to electronics but am enjoying making bits to use around my house, I have built your Power Supply Circuit with Emergency Backup which works very well on some 12v LED Strip lighting in my shed. But I would like to conserve the battery by adding a 12v PIR to switch it on when the mains supply is off. Any suggestions on how I could add this to the existing circuit.
    Any help would be great Thanks
    PS Great Site.

  42. Hi Swagatam,
    I am a newbie to electronics but love to make stuff to use around my house, I have built your Power Supply Circuit with Emergency Backup with a meter length strip of 12v LED's which works very well in my shed. I would like to add a 12v PIR to the existing circuit to switch the lights on when the mains is disconnected. Please could you advise me on how to do this so I do not damage the existing circuit.
    Thank you

  43. Dear Swagatam

    I need a LED 1watt with a minimum quantity of 9 LED's to lit up without transformer but with a battery backup system.
    It should lit up the led's in the presence of Electricity 220v ac.
    But when electricity goes away then also the LED lights should remain ON with a battery backup system for mimimum 2 to 3 hours or more.
    Please advice me the Best circuit for this purpose.
    Waiting your quick reply.

    Thanks & Regards
    Faisal Mateen

    • No, it's manual, initially it must be kept at the topmost point, then gradually brought to the lower points for optimizing the brightness as the LED gets dimmer.

  44. IT is just a inquiry.
    Can we connect a led to 2V 1400Ah hi power battery, will it glow or the excessive current will blow up the led?

  45. hi swagatam..
    plz tell me what changes should i make in above mantioned circuit to connect only 5 1 watt leds ??
    and i have 12V 1500ma adapter and i wish to charge my 6V 4.5AH bettery with this adapter ,, what should i do??

  46. Hi Swagatham,
    I made this circuit.. Its working properly.. Thanks to you..
    I wish to incorporate a Charging LED to the first circuit.. Please suggest a way..

    • Hi Ranju,

      Connect low value resistor in series with D2 and connect an LED parallel to this resistor with its cathode towards the battery.

      The resistor value may be calculated with the help of the following formula:

      R = 3/I, where I is selected 1/10 of battery AH.

    • Thanks for the reply Swahatham..
      It's perfectly done and working fine..
      Please suggest a way to add an LED for full Charge indication for battery..

  47. Good day Sir!
    A question for the circuit: Using NPN transistor
    1. Can I replace BC547 and TIP122 with a 2 TIP31C?
    2. Can I omit the 4 DIODES but instead use a a SPST switch?
    3. Can I use 1 watt LED accompanied by 100 ohms resistor instead of 24 5mm diodes?

    • Good day Achilles,

      Although it doesn't look a good replacement but you may try it, might just work.

      If you want to use a resistor in place of diodes, yes you can do it.

      For 1 watt LED you will need to calculate the resistor value correctly as per the voltage specs.

  48. good day sir!
    a question for circuit using NPN transistor:
    1. If I have 6V 4Ah battery how long will it take to charge?
    2. I have a problem: my transformer generates a little heat, (but no so dangerous), after plugging the circuit for about 12 hours. Is that okay?
    thanks a lot for the answer.
    I need it ASAP. that is before my circuit fails… 🙂

    • with a 6v/500ma transformer, the battery should get charged within 8 hours of time.

      you can use only two or three 1 watt leds not more than that.

      use two in series without any resistor and no diodes at the transistor collector.

  49. Hey Sam,
    I am a class 12th student, could I make this one easily for my board project and can you please tell me basically what are the steps that I should take to make this one?
    I lvoe this project.

    • Hi Divyansh,

      First procure all the components and also a general purpose PCB and then correctly assemble all the components over it very carefully by soldering them together using wire links.

      Do all the connections correctly as per the given diagram.

  50. how many led in SERIES can I use? with 12v battery

    how many hours will it take to charge the battery?

    can you change the power supply of this emergency light with transformerless power supply? plssssss just requesting…i'll wait here…..

  51. in Power Supply Circuit with Emergency Backup

    Is this correct?
    1. I have 12v/7.5Ah battery that means my transformer should be .75 A and how many volts?

    2.the Resistor is 12v(from battery) divided by .75 A(from transformer) =16 ohms right? or wrong?

    3.diode should be 1n4007.

  52. just few minute ago, post some comment about protect batt from overdischarge. and now I can found preventing the battery from getting over discharged circuit here. sure I need modified for 3.7 Volt battery

  53. BD140 not taking load of many leds. I had to remove it and made it manually by slide switch. I want to drive 48x 5mm led but it working with more then 12 led. What went wrong. I am using 500ma 9 v transformer and 6v 4 ah battery

    • use TIP127 in place BD140, it will surely work…..use a 50 ohm resistor with each LED and eliminate all the diodes at the collector of the transistor.

  54. Interesting… How can i modify this circuit with trickle charging ( as shown in the Figure 5 ) using a 12 V 7Ah battery and all the LEDs replaced by two LED tubes ?? How can the switching diodes be placed as per your first design ???

    • thanks!, R1 may be increased to satisfy a trickle charge response.

      You can connect more number of diodes in series with D2 for getting the required amount of voltage at the different junctions of the diode.

  55. Sir I am planning to build a portable emergency light.
    My intention is that to make the light from the emergency lamp available during power failure at night only…. So the following circuits suits best for me….


    Now can you help me in providing details with how to add a Higher voltage cut off for the battery so that during charging, when the battery tends to overcharge it should stop charging at a suitable voltage level..

    I am also planning to place a transformerless power in place of the charging transformer to make the device very portable and can be hooked easily on the wall.. SO I AM USING A 3.35 uf capacitor in parallel with a 1 M,1/4W in the Phase line and then rectifying and bringing down the voltage to the charging voltage level using a suitable zener..
    So i need your valuable suggestions on these..

    i shall send a model diagram to your hitman inbox. Please check it out soon.

  56. Dear Swagatam,

    my Class 9 son is eagerly ready to construct the 150 LED emergency circuit mentioned above. But I have not find AD149 transistor anywhere in Chandni market Kolkata. Can you kindly suggest something equivalent which is available nowadays in Kolkata ?

    • just connect the battery to a 12V/1amp DC source via a 220 ohm resistor, let it charge for 10 hours, you'll find it almost fully charged.

      for indication you can connect an LED with a 1K series resistor parallel to the 220 ohm resistor, when this LED shuts off completely you can assume the batt to be fully charged.

  57. Hi Swagatam…

    How r u? First of all I Thank you for your great help and valuable suggestions.

    I need help in my new project hope u dont mind. I have a old BPL Emergency lamp with dual Mercury tubes and now it's not in working condition. I just want to convert that unit into LED Emergency lamp. It has got 6V 4.5 Ah Battery. I would like to try the first circuit but with battery over charge protection. I also have a dual color(Red & Green) 3mm LED which I would like to use it as indicator for charging and Battery full.
    Kindly let me know the modification to do in the Circuit.

    Thank you in Anticipation and have great day

    Vinay Rao

  58. Hello good people.

    This blog gave me a lot of interesting information. Especially the power supply circuit with battery backup part.
    The reason for this is that I am working on a Arduino based system for monitoring and controlling heating cables at my summer place. This system will eventually be gsm controlled so I quickly can get an update on for example the temperature in the bathroom. The part that I am stuck on is that I would like to have the Arduino to have a battery backup of some sort so it can still monitor the temperature around wunerable waterpipes, and possible notify me if mains power goes out. I'm thinking of using a car battery so it can last for ages if power goes out.

    What changes will I have to make to the "Power Supply Circuit with Emergency Backup" circuit to make it work with a 12V car battery and still have it trickle charge slowly?

    Thank you in advance for any advice.

    – Fredrik

  59. Hi Swagatam,

    I thank you for your guidance and support. I am sorry for not providing you the complete information about my requirement in my previous question. It was a mistake from my end. Well as I said I have an BPL Emergency light with dual small Mercury Tubes which is a Portable lamp with 6v 4.5ah battery. It has also got the 12v 500 ma separate adapter for charging purpose. This is being used only when there is a regular power cut in our place (Normally for 2 hrs daily). Hence we need to keep this plugged in to A/C mains. We used to charge it when the battery gets completely exhausted. We just switch it on manually when the power goes off (especially in the night).
    Hence in the circuit suggested by you should be always connected to the Ac mains.
    So kindly suggest a circuit with overcharge protection facility with an indication dual colored LED (Red n Green).
    I would like to use the 1W power LED's (warm White) for this Lamp and I guess the maximum No of LED's I can use is 5 if I am not wrong for 6V 4.5 ah Battery.

    Hope you understand my requirement. If u think I can use the same Schematic suggested by u for my previous blog by making some alteration then please suggest me the those alterations to be made. Hope u help me out in this matter.

    Best regards


  60. hi thanks for your circuits they are in did very helpful in developing me but in this circuit instead of the three pin switch can't I use a relay to get just on and off and how do I do away with the third switch pin. thank you.

  61. hi thanks for your circuits they are in did very helpful in developing me but in this circuit instead of the three pin switch can't I use a relay to get just on and off and how do I do away with the third switch pin. thank you.

    • Hi John, the switch is for controlling brightness of the LED via the diodes as the battery voltage drops, in an efficient way. If you don't want this feature you can simple remove the switch and the diodes and connect LED positive line with the transistor via a 30 ohms 1 watt resistor

  62. buna seara domnule.ce modificari trebuie aduse diagramei pentru a folosi un acumulator de 12v/100A ,si cum pot face oprirea autamata a incarcatorului cand bateria esta incarcata complet.va mutumesc,cu stima

  63. Hello Swagatam, can you tell me if I can run 4 10watt LED for few seconds with 6 x AA batteries? I need to make a ring flash for my camera?
    Which will be better? 4-10W LEDs or those smd LED strips?
    Which will give more light output?

  64. yet another fantastic lesson.
    I woild like to install nearly this as an LED light fixture in my wife's horse barn tack room where we currently have no electricity. I'd like to hook up a battery (whatever size) to a solar panel (small for a trickle charge through out the day and be active ON when she enteres the tack room or tied to a limit switch at the door – door open = light on.

    brilliant! another gold star for Swagatam Majumdar

    • Thanks Stuart, I am glad that you are liking my circuits, if you have problems while building it just let me know, I'll guide you through the procedures!

  65. Thanks sir, but the fact is i don't want it to be an emergency light. I want to use it as a video light.
    so i want something very compact without the transformer and stuff.
    i'll be glad if you could guide me directly for something thats easy. Also i want to connect at least 64 led's. And also add a dimmer.

  66. Thanks a lot sir. i was looking for something like this only.
    Few more queries, as what is the value of L1(tried figuring out with the formula, but failed).
    The no. of led's to be attached is 64. And the voltage say is 7.2 volts using 6 AA rechargeable batteries.
    and i'll be glad if u could link me to the pcb layout. The diagram seems to confuse me sir.( i am not into electronics)
    Thanks .

    • L1 is not critical you can begin with a 20 turn coil over a ferrite rod, and measure the voltage across, you can or increase or decrease the no. of turns in order to get higher or lower voltage at the output…the wire can be any thin magnet wire.

      you can keep tweaking the number of turns until you find the LED getting the optimum brightness….more turns will generate more voltage and vice versa.

      I am sorry a PCB design may not be possible at the moment, if possible i'll try to update some other time when I am free.

  67. Dear Swagatam,
    I came across your blog today while searching for a 12V output SMPS circuit. I really appreciate your hard work and dedication to this blog. Your articles are very well written indeed. I especially appreciate your replies to queries from readers, which is a plus point especially for beginners and novices. I am looking forward to more articles from you. I shall be following your blog closely and make suggestions from time to time if you don't have any objection. Thanks a lot.

  68. Hello sir,

    I greatly appreciate your help with the led circuit that i needed to make.
    Now i need another guidance from you.

    Am in a need of getting 5v and 3v from a 9 volt battery (the cheap 9v batteries used in toys/remotes). I need to apply the voltages to a load whose power consumption is 1.65watt (max).
    I seek your help and guidance in designing the same.



    • Hello Anirudh, you can use 6 or 7 series 1N4007 diodes for getting 5V from 9V and 10 diodes for getting 3V, this is the easiest way to implement this, alternatively you can also try a buck converter for the achieving the same, which will be rather complex to build and optimize.

  69. Thanks sir for replying to my query.
    Sir the fact is i want to make a circuit in which i have a single battery source of 9volts and i want a common ground terminal and +5v and +3v.
    I know maybe its a very lame thing am asking but i really seek your guidance. I'll be very thanful if you could help me with a simple drawing of the circuit.( a drawing with a pen on paper will suffice)



    • Hello Anirudh,

      you can use a single battery with a common ground and acquire all desired outputs, please see the following diagram for understanding diode connections:


      9V(-)—————————————————————————-common ground

      the +9V and the -9V are the battery terminals, l>I are the diodes, which are connected in series from the (+) of the battery

      at the 6th diode connection you get the required 5V and the at the end of the 10th diode you get the required 3V….all these are with respect to the common ground or the negative of the battery

      I hope now it's clear to you.

  70. Thanks a lot sir for explaining me the circuit so well. I greatly appreciate it.
    Am using IN4007s diodes. I had to connect 20 of them in series to get +3v at the end of the last diode. And after the 13th diode am getting +5v.

    But the problem arises when i connect any one load say 3v or 5v. The voltage of the other drops.
    How should i tackle this now sir?
    Any refinement needed or conponents to be added? If needed please guide me.
    If needed guide me in making a new circuit. I really need it sir.( as compact as possible)

    • Hello Anirudh, 20 diodes is not correct, you should get the specified voltages with the exact set up that's shown in my previous comment. without any load connected the meter will show incorrect reading.

      Check the voltage with some kind of load connected may be a 1K resistor across the 3V output. then you'll find all the respective voltages falling in place correctly….

      each diode is supposed to drop 0.6V approximately

  71. I have used the 1st ckt. Just conected bd140 via 1k base resistor and add a led as charging indicator. Bt problem is that led is kept on all the time. I nee quick solution. Is my pnp faulty?

  72. Hi Swagatham,

    I want to replace LED's with 1W leds. How many LEDs support good brightness long time and What component will changed by me. and give idea please. I want to connect atleast 10 LEDS.

    • Hi Sivakumar, in order to illuminate 10nos of 1 watt LEDs you will have to upgrade the transistor with a TIP127 and add a good heatsink to it.

      Next, arrange 3 LEDs in series, make 3 such strings and connect these 3 strings in parallel….this final LED assembly can be then used with the circuit in the indicated position….but make sure to include a 6 ohm 1 watt resistor with each LED string

  73. Hi sir. For Circuit modifications for operating 150 LEDs requested by SATY, what type of swith that I should use, can I use single pole double throw or single pole single throw? And How many leds should i use?

  74. Hi Swagatam dear i want the basic of electronics,how can i make the project with my own ideas which are in my mind i mean the calculation from the components ie resister transister etc.

    • Hi Faisal, you will have to first study and grasp how transistors, resistors, capacitors work in electronics circuit then study other parameters like opamps, digital ICs etc…..once you start understanding you'll automatically be able to design your own circuits…it will take time though.

  75. Dear sir,
    I want to make a automaric circuit for 4v 3000mah batery to charging. Max volt 4.5, and min is 3.7 to drive a emergency light of 4watt. Please help me to make it
    Thanks in advance..
    Regards Irfan Ahmad

  76. Sir I would like to make an emergency light using led with 1.5hr backup, and it should contain a charge cutter to stop excess charging of battery.

  77. Sir I would like to make an emergency light using led with 1.5hr backup rechargeable battery and to stop damage of the battery, should have a automatically charge cutting system if the battery is full

  78. Hi,
    Can i skip the diodes and connect LEDs in such a manner directly to the battery;
    2 strings of LEDs in series connected to the battery with 20 LEDs connected parallel in each string. So 40 LEDs in total. Or can you suggest the number of LEDs in each string for good backup time?
    Diodes I'm planning to remove is to make the circuit even more easy.


    • yes you can easily skip the diodes, but make sure the LED string forward voltage is approximately equal to the battery voltage, otherwise the LeDs could get damaged gradually overtime.

  79. Hello,
    I've tried the circuit(first one) but when I use 9v transformer voltage out goes up to 13 v. So shall I use 6v 500mA transformer? Assuming that the normal charging voltage for a 6V 4.5 mA battery is 6.9 to 7.2 volts, and even if I use 6 v transformer charging voltage will be above the specified voltage. Is this ok? Or any other solutions? Sorry if I'm wrong here.

    • you are correct, you must use a 6V/500mA transformer, then rectify it using a bridge diode network and a high value filter capacitor, this will allow you to get the correct charging voltage for your 6V battery.

  80. hello,
    Why there is not the bridge diodes in the first circuit?
    Or why there is the bridge diodes in other circuits???

  81. Hi Swa,
    I saw in the shop a kind of emergency bulb which is put in the common bulb fitting and doesn't require any battery. The bulb also function as a lamp when the main power exist, but when the main power goes off, the bulb is still giving the light as an emergency light. The bulb is actually contains some leds and electronical components. Do you have the circuit for this kind of emergency bulb ?

    • Hi Kanta, no LED or lamp can light up without a power source, the lamp must be having a small Li-ion kind of chargeable battery inside, nowadays batteries have become very compact and efficient and can be adjusted within small spaces.

  82. Hi swagatam,I must confess,you are a blessing to the word of electronics designs. You have frees a lot of us from the shackles of complex electronics problems. I have made a lot of money through your circuit. I say thank you. I have two batteriea, 12v 7AH connected in parallel across a 40amp power supply. In conjunction with other loads,what should be the size of the R1,in series with the charging diode.

    • Thank you Nojeemdeen, I am very glad my designs could help you.
      Since there’s no specific charge controller here, R1 can be calculated simply using Ohms law

      R = V/I = 14/0.8 + 0.8 = 8.75 ohms
      wattage = 14 x 1.6 = approximately 25 watts

      but make sure the input voltage is not above 14V

Leave a comment

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