Simple LED Emergency Light Project

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 unit.

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

What an LED Requires to Illuminate Optimally

The concept: We know that LEDs require a certain fixed forward voltage drop 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.

Increasing LED Efficiency

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.

Circuit Diagram

simplest LED emergency lamp circuit using a single transistor

How this white LED Emergency Light Circuit Works

Referring to the circuit diagram, 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

R1 = 10K,

R2 = 470 ohms

C1 = 100uF/25V,

Bridge diodes and D1, D2 = 1N4007,

D3---D5 = 1N5408,

T1 = BD140

Tr1 = 0-6V, 500mA,

LEDs = white, hi-efficiency, 5mm,

S1 = switch with three changeover contacts. Using Transformerless power supply

The design presented above can be also made using a transformerless supply as shown below:

Here we will discuss how an emergency lamp can be build without a transformer using a some LEDs and a handful of ordinary components.

The main features of the proposed automatic transformerless emergency light circuit is though very identical to the earlier designs, the elimination of the transformer makes the design pretty handy.
Because now the circuit becomes very compact, low cost and easy to build.

However, the circuit being completely and directly linked with the AC mains is hugely dangerous to touch in an uncovered position, so it is obvious that the constructor implements all the due safety measures while making it.

How it Works

Coming back to the circuit idea, transistor T1 being an PNP transistor tends to remains in switched OFF condition as long as AC mains is present across its base emitter.

Actually here the transformer is replaced by the configuration consisting of C1, R1, Z1, D1 and C2.
The above parts constitute a nice little compact transformerless power supply, capable of keeping the transistor switched OFF during mains presence and also trickle charges the associated battery.

The transistor reverts to a biased condition with the help of R2 the moment AC power fails.

The battery power now passes through T1 and lights up the connected LEDs.

The circuit shows a 9 volts battery, however a 6 volt battery may also be incorporated, but then D3 and D4 will need to be completely removed from their positions and replaced by a wire link so that the battery power is able to flow directly through the transistor and the LEDs.

Automatic Emergency Light Circuit Diagram

9V simplest 10 LED emergency lamp without using transformer


Video Clip:

Parts List

R1 = 1M,

R2 = 10K,

R3 = 50 ohm 1/2 watt,

C1 = 1uF/400V PPC,

C2 = 470uF/25V,

D1, D2 = 1N4007,

D3, D4 = 1N5402,

Z1 = 12 V/ 1Watt,

T1 =BD140,

LEDs, White, High Efficiency, 5mm

component layout for simplest emergency lamp circuit

PCB Layout for the above circuit (Track side view, Actual Size)

PCB track layout for emergency light circuit

Pats List

R1 = 1M
R2 = 10 ohm 1 watt
R3 = 1K
R4 = 33 ohm 1 watt
D1---D5 = 1N4007
T1 = 8550
C1 = 474/400V PPC
C2 = 10uF/25V
Z1 = 4.7V
LEDs = 20ma/5mm
MOV = any standard for 220V application

For High Watt Application 

5 watt simple transformerless surge free emergency lamp circuit

Emergency Lamp with Automatic Day Night LDR Activation

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.

day and night automatic LDR controlled emergency lamp

Circuit modifications for operating 150 LEDs, requested by SATY:

customized 100 to 150 LED emergency lamp circuit

Parts List for the 150 LED 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

Emergency Lamp Circuit with Low Battery Cut-off

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 lamp circuit with low battery sense and cut off

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.

DC emergency lamp using only diodes

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:

adjustable emergency lamp circuit using NPN transistors

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)


mnasim31 said…
Is S1 a automatic switch? If not then how it to be used manually? Thanks.
Swagatam said…
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.
aimal daud khan said…
sir i want to know that y u used a resister of 10k ohm in parallel in the 1st circuit
Swagatam said…
For providing the required negative base biasing so that it is able to conduct when mains supply fails.
Swagatam said…
The LED will be OK but will be dim due to low voltage, I am assuming it to be a white LED which needs 3V minimum for optimal glow.
Swagatam said…
use TIP127 for the transistor, use 10 ohm, 1 watt resistor for each LED (parallel connection)

remove all the diodes, they won't be required.....
Swagatam said… not remove D1, D2, they are required.
Ranju said…
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..
Swagatam said…
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.
achilles hector said…
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?
Swagatam said…
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.
achilles hector said…
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... :-)
achilles hector said…
I mean for the first group of question which is #3 is: not a diode a LED. thanks a lot again SIR!!!
Ranju said…
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..
Swagatam said…
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.
Swagatam said…
You can use two 1N4007 in parallel for each 1N5408, and TIP127 in place of BD140
Divyansh Gehlot said…
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.
Swagatam said…
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.
Jinnamuru Xunte said…
can i use 12v battery here?
Swagatam said…
yes, but include 3 LEDs in each string
Jinnamuru Xunte said…
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.....
Swagatam said…
You can try this circuit:

backup time will depend on battery AH and no of LEDs used.
sat said…
Sir, What type capacitor is used PPC / others (C1 = 100uF/25V circuit for SATY for 150 LEDs)
achilles hector said…
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.
Swagatam said…
1) a 1amp trafo will also do, volts = 14V
2) resistor for what??
3)use 1N5402 not 1n4007
achilles hector said…
the resistor above is for what?
saty said…
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
Swagatam said…
Hi Saty,

The above circuits are not so advanced, and does not have proper protections, if you would like to have a perfect emergency light circuit then I would suggest you to make the following design:
deepti sharma said…
need ur help as fast u can.......totally confused for my project......which transistor is better a common base....aur ....common emitter........
sat said…
what is the equivalent for AD149 ( see circuit -150 LEDs requested by SATY).
Y0f4N said…
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
Swagatam said…
TIP2955 can be used
Swagatam said…
it will be posted soon in the new article
Ram said…
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
Swagatam said…
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.
Arun Dev said…
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 ???
Swagatam said…
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.
Arun Dev said…
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.
Swagatam said…
Arun, the linked circuit could be a bit complex to set up, moreover adding another high voltage cut off stage could make it even more's better to go for the following design:

the day/night feature is not there, but can be easily included.
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 ?
Swagatam said…
Dear Subhagata,

you can use TIP127 instead of AD149, or any other PNP equivalent will also do.

Hi Swagatam, is it okay for me to use a 6v 1 amp transformer in the modified circuit? and is it okay for me to only use 10 LEDs? thanks
Swagatam said…
Hi John, yes you can use a 6V 1 amp transformer and also 10 LEDs, arranged as given in the above explained circuits.
john ocharive said…
i love these circuits but please can you post a simple 9v rechargeable battery circuit diagram for me, I need it for my project urgently pleeeeease. this is my mail address:
Swagatam said…
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.
Vinay Rao said…
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
Swagatam said…
Thank you Vinay,

The above circuit cannot be used with an over charge cut off properly, you'll have to incorporate relays (5V) for your proposed requirement, as shown in the folowing link:

the zener at the transistor emitter can be removed for a 6V battery.

For indications use LED from pin6 of the IC: one from positive to pin6 and another from pin6 to ground, both these LEds must be accompanied with individual 4k7 resistors.
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
Swagatam said…
Thanks Fredrik,

You would require a just two diodes for implementing it.

I'll post the design in a new article and let you know soon.
Vinay Rao said…
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

Swagatam said…
Hi Vinay,

I have already suggested you an appropriate design in your similar previous comments, this circuit will fulfill all your requested specifications, here is the link once again:
john ocharive said…
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.
john ocharive said…
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.
Swagatam said…
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
gogutu said…
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 mutumesc,cu stima
Swagatam said…
you can try the following design:
Swagatam said…
you can use but a capacitor will not give you any backup, it will keep the LED illuminated may be just for a few seconds....
Raj said…
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?
Swagatam said…
Hello Raj, that looks impossible, the AA maximum delivering capacity can be expected to be 1 amp x 1.5 x 6 = 9watts, at full thrust.

and 10 watt x 4 = 40 watts,,,,, that's nowhere near to the batteries capacity.
Raj said…
Please look at this video, "". This guy made 20 10W LEDs light up with 6 AA batteries.
Swagatam said…
as I told you before, 9 watt source will NOT support 40 watts, in the video the LEDs are not even mounted on heatsinks that means those are not loaded for emitting 40 watts, may be just around 5 watts....

if you are looking for a voltage booster you can try this:
stuart jones said…
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
Swagatam said…
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!
Anirudh Kundu said…
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.
Swagatam said…
Hello Anirudh,

Please let me know about the LED and the battery current rating that you intend to use.
Anirudh Kundu said…
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.
Swagatam said…
OK, you can try the last circuit shown in the above article with a 5V power bank
Anirudh Kundu said…
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.
Swagatam said…
Anirudh, you can try the following circuit for illuminating 64 leds from a 5/7V input..... use a 47K pot in place of R2 for getting the dimming feature:
Anirudh Kundu said…
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 .
Swagatam said…
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.
chinmoy1955 said…
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.
Swagatam said…
Thanks so much dear Chinmoy, You are most welcome to post your opinions and comments in my blog, it'll be a pleasure to answer them....

Anirudh Kundu said…
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.


Swagatam said…
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.
Anirudh Kundu said…
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)


Swagatam said…
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.
Anirudh Kundu said…
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)
Swagatam said…
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
Pritam Bhowmik said…
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?
Swagatam said…
where did you connect the LED as the charging indicator
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.
Swagatam said…
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
Unknown said…
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?
Swagatam said…
You can eliminate the switch, it's not compulsory...

the LED quantity will depend on the the battery AH
mani said…
Sri. Majumdar,
I have a suggestion for you; please give serial numbers to your circuits, so that you indicate the of the ckt. in your replies not say the first ckt, the second ckt. etc.
Mohammad Saif said…
Hey plz show me diagram of charging that look simple its too hard diagram
gela Valenzuel said…
can i use 9v battery instead of 6v? anyways thankyou sir
Unknown said…
Sir tell me how many number of led used in circuit
Hi Sir,

I am looking for a circuit, 6V 4.5AH battery charger with out of 2VDC 50mA. Please help me.
Swagatam said…
Hi Rajeesh,

you can use the following circuit and adjust the pot and the Rc for acquiring the required V and I specs at the output

Thank you for reply,
I am not able to view the circuit diagram please post the circuit.

I have seen Many circuit of your are very good.
My application is to drive the Fish tank small motor (2V 25mA) and due to frequent power failure i need power backup up to 3 hours kindly provided the circuit.
I am awaiting for your reply.
Swagatam said…
I can see the circuit clearly in the linked article, please refresh it again and check.
Swagatam said…
For your fish tank, you can use the first circuit from the following article:

just replace the 14V zener diode with a 2.7V zener

the adapter may be replaced with your existing 3v ac/dc adapter
Thank your very much for your reply,
1, From the circuit output is 12V socket but i need to connect the small motor to pump (2V 25mA motor) what will be the resistor in can add?
2, 2.7V zener with watts?

Swagatam said…
Rajeesh, A resistor will not work, you can use a 7805 IC and then further drop the voltage to 2V by adding 4nos of 1N4007 diodes in series with the positive line and feed it to the circuit suggested can eliminate the zener diode at the base of the transistor if you employ this method.
Thank for your reply,
Based on your instruction please find the circuit i prepared please correct if any
Swagatam said…
That's not correct, the 7805 and the 4 series diodes should come immediately after the bridge rectifier and before the TIP122....reaming circuit on the right side of the TIP122 will be exactly as shown in my article.

and the TIP122 base should NOT be connected to ground, only with the 220 ohm resistor.
Swagatam said…
...and the battery is supposed to be a 3V battery not 6V
Thank for your reply and i'll follow it.
faisal khan said…
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.
Swagatam said…
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 will take time though.
javed iqbal said…
Any one help me I need 105 Led -718 A remote control emergencylight with 8th battry connection and circuit
MovieS-PoinT said…
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
Swagatam said…
Dear Irfan, please search for "smart emergency light circuit", you'll get the required circuit...
Unknown said…
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.
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
Swagatam said…
You can try this circuit:
Swagatam said…
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.
Swagatam said…
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.
Sir is there anyway to illuminate 50 white LEDs using a 4.7 V li ion rechargeable battery with maximum brightness output ?
Swagatam said…
50 LEDs in parallel will consume 1 amp minimum, not possible, unless the batt is 5 AH rated
Swagatam said…
it's just to keep the circuit can use a bridge if you wish to...
Swagatam said…
yes you can use it
Kanta said…
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 ?
Kanta said…
Hi Swa,
I forgot to mention that the emergency bulb can stand for about 4 hours.
Swagatam said…
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.
Swagatam said…
It could be having a small li-ion cell and hi efficiency LEDs
Kanta said…
Hi Swa,
I may buy the bulb and open it to take a look inside. The price is cheap it is about USD 3.
I will update you once i got it.
Kesava Raj said…
Hai sir
Few doubts in this topic ..Power Supply Circuit with Emergency Backup..

I tried this circuit working well..
Today i saw in one ic datasheet..
BACKUP power supply they are using condenser and resitor, they not mention the value...
I need the value of R & C
using for 5v and 12v
Pls explain abt this type of backup circuit and how long it will hold the backup and how to calculate the values R & C.

Swagatam said…
Kesava, if the back-up is for an LED then it won't work, unless the capacitor value is huge and if it's a super-capacitor.

Possibly I'll try to discuss this topic through a new article soon
Shibananda Mohapatra said…
Design rechargeable emergency light using 6V battery and led? pls reply to my email id on please...Thank you Sir,
babai said…
Sir I am a fan of u.I wish i could meet u.sir i have a problem ....when I connect a 4 volt smd led strip containing 34 leds to 6 volt 5 amp rechargeable battery the aluminium strip of smd led is getting heated up.please tell me why this is happening and what should I do to overcome this heating issue.
Swagatam said…
Thanks Babai, do you have resistors for the LeD in the strip??

If not make sure to connect a resistor with each LED in the strip

Sir good evening
IAM using 3w leds but light is very dim
So what I want to do.
I want LED luminate brighter
Swagatam said…
Ho David, what is your battery rating?, the battery rating must be able to support the LED power, and the transistor must be rated accordingly to handle this power....
Amey said…
Hello Sir,
I am trying to built a simple emergency light having 2 individual set of 4x1W LED for 2 different rooms that are powered using a single 4V 1Ah (or 2 batteries connected in parallel) Sealed Lead acid battery. The circuit should be designed with minimum components.
circuit is always connected on mains to automatically switch to backup on power failure. also it should charge the battery when the mains is On and show an indication when battery is fully charged . Both LED and buzzer indication is required.
also the battery can be charged through a DC source(Mobile Charger or 6V ) not AC
Do you have any schematic for such design..?

and also how to calculate the backup time of this circuit...?
waiting for your reply.
thank You
Swag said…
Amey, you can try any one of the concepts presented in the following article

estimating back up can be difficult, you will have to check it practically.
Paul said…
Hi. I am very happy with your circuits an explanations. My problem is, I am not understanding the part of diodes used since 3 diodes are in series and each has a voltage drop of 0.6 V so I thought all will drop 1.8 V and not 2.5 V. please explain to me here.
Also explain to me the connection of S1, how is that part connected in ideal circuit
Swag said…
Hi, Im glad you liked them, yes it should be 1.8V as per the calculation, but 2.5V is the recommended value, so you can add one more diode in series to enable this drop in voltage.

S1 can be a 1 pole 4 contact rotary switch configured as per the shown wiring.
Paul said…
Thank you so much. I am new in circuit design. Please guide me on how to choose the correct value of the components used in your design and also their work. please.
Swag said…
I appreciate your enthusiasm, however the parts list is given in the article, you just to copy it an show it to the parts dealer, the resistors are all 1/4 watt and the rest are standard type.

the working is also already explained in the article.
Paul said…
Hi Swag. Please assist me with an LED circuit that is on when water flows and off when water stops to flow in a pipe
Swag said…
Hi Paul, you can achieve it in the following way:

make a Darlington pair transistor using two BC547.

connect a 1uF/25V capacitor across its base/emitter, and connect two copper probes, one terminating from base of the transistor, and another from the positive line through a 1K resistor, these probes can be used for sensing water.

the LEd can be connected between positive supply line and collector of the BJT via a 1K resistor.
Paul said…
thanks so much. You are great.
Swag said…
you are welcome!!
Paul said…
Hi Swag, please give me your email because i am really learning a lot from your circuits
Swag said…
Hi Paul, I am glad you are learning a a lot here, however the comment section is the best place to communicate and interact because other readers also get the opportunity to learn from our discussion, so please feel free to comment in this forum and learn as much as you wish :)
Paul said…
Hi Swag. In your circuits, immediately after transformer, you have a bridge for rectification and capacitor for for smoothening, other circuits have been checking, if you take one side of transformer as positive and the other negative, on the positive the circuit has a capacitor and resistor in parallel before bridge and on the negative side, there is a resistor before bridge, then the others follow after bridge. I don't understand this. please explain to me
Swag said…
Hi Paul, sorry I could not understand your question, which circuit are you referring to?

the transformer circuit does not have a bridge, it has a half wave rectifier using a single diode?
Paul said…
How can I send you the circuit am talking about?
Swag said…
If it is from some other site then I won't be able to suggest much, if it is from this website then you can show me the link here
Paul said…
Ok. I understand now what you meant. You have given the values of the components used in your circuit, but my question is how do you calculate to get those values? please explain to me the calculation.
Swag said…
the component values are actually not too critical, I have approximate values set in my mind and I use these approximations while designing circuits, for example here the base resistor can be anywhere from 1K to 10k etc, having said this, formulas are definitely there but it is not crucial to go by the formulas always, you just have to know the part specs to configure all of them together.

I am able to do it quickly due to my vast experience in this field.
Paul said…
Hi Swag. Please guide me on how to calculate the output voltage of a droping capacitor in a transformer less power source.
Swag said…
Hi Paul, output voltage will not be dropped by the capacitor in a capacitive power supply, the current will be dropped, as explained here:
Paul said…
Thanks. How also do you determine the best resistor to use in parallel with a dropping capacitor and also in a smoothing capacitor in a transform less power supply
Swag said…
any resistor above 100K and below 1M will work.....

for capacitor you can read this article:
Paul said…
Thank you for your good help and guidance.
Paul said…
Hi Swag. I need to power an LED with operating current of 300 mA and 8 V forward voltage to obtain an output power of 30mW. What value of components do I need with a 4.5V 200mAh rechargeable battery in a transformer less circuit? please help
Swag said…
Hi Paul,

a 300mA 8V LED will require 0.3 x 8 = 2.4 watts, so it won't run on 30mW.

also a 4.5V battery cannot be used for operating a 8V LED
Paul said…
Thanks. I have LEd rated operating current of 100-300mA and forward voltage of 5-8V, what rechargeable battery and value of components to use in a transformer less power supply please?
Swag said…
you will need a 9V battery, at 3AH may be. do not use the diodes at the collector side of the transistor.

for R2 use 220 ohm 1/4 watt, and make sure to connect a 10V zener across the battery terminal

rest can be as is.
Rajkumar said…
My Chinese emergency light transistor was burnt so I need it instead of the other one transister. Here is the mentioned in D2470. Can you tell something in equivalent transister?
It has a 4v 4Ah battery.

Thanks in advance
Swag said…
Hi Rajkumar,

you can try D1351, D880, or TIP31C
Rajkumar said…
Thank you very much for your reply
अमन said…
हाई सर प्लीज़ अंडरस्टैंड। मैं इंग्लिस में कमजोर हु इसीलिए हिंदी में लिख रहा हु। किया मुझे इसी सर्किट की जान कारी हिंदी भाषा मे मिल सकती है? मुझे 7वाट AC/DC बल्ब का सर्किट डायग्राम ओर कॉस्टिंग चाहिए। जैसे ( सिस्का ) या कोइ ओर कंपनी के जैसा। जिसे होल्डर में लगाने के बाद AC पे चलता रहे। ओर बैटरी भी चार्ज होती रहे। AC POWER कट होते ही बैटरी पे चले। बिना कोई और स्वीच ऑन करे। हा अगर जो होल्डर का बटन जो कि बोर्ड में लगा होता है। उसे अगर बंद कर दिया जाये तो। जो अब बैटरी पे चल रहा है बंद हो जाये। आप की वेब साइट को google translator पे translate कर के देख चुका हूं लेकिन। ग्राममर में बोहत गड़बड़ हो जाती जिसे समजना बोहत मुश्किल होता है। में उम्मीद करता हु के आप मेरे comment का जवाब जरूर दोगे। धन्यवाद।
Swag said…
Hello Aman, tum doosra ya teesra wala circuit bana sakte ho. PCB design bhi diya hua hai. kuchh technical calculation karne honge, mai jaldi hi upadte kar dunga.
Total cost hoga = 50 se 80 rupay tak bina battery ke
7 watt pane ke liye 1 watt ke 7 LED parallel lagana hoga
Aman said…
Thank you sir for you qiuk reply, I will wait for update.
Swag said…
I have updated the diagram, see under the PCB
अमन said…
Sir koi best software bataye cercuit simulation k liye with PCB 3D diagram. Jis me circuit ko run karke dekha jasake jis me total electronic part ho.
अमन said…
Or ha ye jo led circuit ka diagram he, is me battery over charge protection nahi he shayad?
अमन said…
Or ha sir agar parts ki detail mill jaye jayse capacitor , Polaris he ya non Polaris to bohat achchha hoga. Thanks in advance.
Swag said…
sorry uske bare me mujhe nakhi pata.
Swag said…
over charge protection nahi isme, lekin agar full charge level ko thoda kam rakha jaye to over charge ka khatra nahi rahega. Agar 3.7V ka battrey hai, to charge volateg ko 4V par simit rakhna hai. 7 diodes ho kam jada karke is voltage ko adjust kiya ka sakta hai
Swag said…
All resistor 1/4 watt 5% except R3 which is 1 watt
C1 = 5uF/400V PPC or metallized polyester
C2 = 100uF/16V electrolytic
अमन said…
Thank you very much sir for all reply
Ken said…
Sir does this circuit has a protection for overcharging the battery?thanks sir
Swag said…
Hi Ken, A special cut off circuit has not been included for the sake of simplicity, however R2 in the first design and R3 in the transformerless designs are used for limiting charging current to the battery ensuring that once the battery is fully charge current drops almost to zero and these resistors help to prevent the battery from getting overcharged or damaged.
Ken said…
Thanks sir,another question sir,.can i use 750mA transformer instead of 500mA?,.
Ken said…
Sir how many LEDs can i put in this circuits?,what is the maximum no. Of LEDs,....and i can use 750mA transformer instead of 500mA?,thanks sir
Swag said…
Hi Ken, No problems it will do, just dimension the mentioned resistor accordingly to ensure overcharging does not happen for the battery
Swag said…
The number of LEDs will depend on the Ah value of the battery, preferably the total LED current must not exceed 30% of the battery's Ah rating
Ken said…
Sir can i charge 6v 7Ah to a 6v 2.8Ah charger?thanks sir
Swag said…
Hi Ken, yes you can charge a 7Ah battery with a charger meant for 2.8Ah battery, but the charging rate may be very slow.

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