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## 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 unit.

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.

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.

HAVE ANY DOUBTS? FEEL FREE TO COMMENT AND INTERACT.

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.

Using Transformerless power supply

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

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

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

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.

Circuit modifications for operating 150 LEDs, requested by SATY:

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.

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.

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:

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• #### 328 comments:

1. Hi friends.....

1. 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

2. I use corel-draw to draw these diagrams.

3. 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

4. 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.

2. 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.

Regards

Sam

3. 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.

4. 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

5. 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.

Regards.

6. 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?

1. 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.

7. Can you add an LDR also, so that during daylight when mains supply is lost the LEDs don't light up and waste energy.

1. Great idea....actually I already did this in the built prototype, I'll include it here as well.

Thanks!

8. 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.

1. 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.

9. 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,

Arif

1. 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.

10. 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!

11. 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...

Thanks!

1. 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?

Thanks.

12. Absolutely correct. The transformer output is rated at 12V 120mA.

(Sorry for not getting back to you sooner.)

1. What will be the load current?.... it will be required for selecting the correct transistor.
Regards.

13. 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?

1. I have posted the required circuit above, please check it out.

Thanks

14. 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

1. Hi,
I'll design the circuit and post it here for you very soon.

15. This comment has been removed by a blog administrator.

16. can this circuit will be able to provide the same brightness as normal incandescent emergency lamp?

1. It will provide more light than incandescent types of emergency lamps...

17. Hi,
I have BD 140 transistor and i dont know is it PNP or NPN.Is that ok?Sorry for the noob question:)

1. Hi,

A BD140 is a PNP transistor, whenever in doubt, check the datasheet of the particular device.

Regards.

2. Thank u for the reply

3. You can Check for its pins, using a multimeter, Just type in Google About finding the pins of the transistor,I have also done the same thing. BD140 is a PNP transistor.
Tanmoy

18. 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

1. 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.

Regards.

2. 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!!!

3. 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.

19. 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?

1. Hi,
Yes you may do it, but the LEDs will remain switched ON all the time.

2. hi,
i can add a switch for the LED's right?What about the diode value?

3. Yes, A switch will be handy, you may use 1N4007 diode.

4. Thank u for the reply

20. can i use 4.5 v 300 mA(output is around 6-7 volts) transformer ??

1. A 4.5 v transformer will not provide enough charging voltage to a 6 volt battery, so cannot be used.

2. sir ,can i get a transformer less emergency which charges battery when there is supply and lights when power fails.the circuit also should cutoff when battery is fully charged

3. Hi Arun,

I'll publish the required circuit and give you the link... probably by tomorrow...

4. Hi Arun,

Check it out here:

Regards.

21. Hi,
how much back up time (approximate)will be there from the 150 led circuit?

1. Hi,

It will depend on the AH of the battery being used.

2. 4.5 Ah battery

3. Not more than 2 hours with a 4.5AH, fully charged battery.

4. For the circuit with 24 LED's how many more LED's can i add without losing too much back up time from the battery.Battery 6v 4.5 Ah? Please help!!!

5. Around 60 LEDs would make the battery last for more than 7 hours. The efficiency increases proportionately as the load decreases.

6. can i increase ah capacity of battery or quantity of led.
ex- can i take 6v,14ah battery from market instead of 6v,4.5ah for more back up. reply me soon.

7. yes you can do it.

8. i am making the first circuit in that 24 leds are used so if i use 6v,14ah battery then there is harm to led or not.reply me soon ( i am using eon rechargeable battery it is good or not).

22. How many hours the LED glows after AC switched off......?

1. Up to 6 hours..

2. Hi Swagatam.... :)
thanks...
i am doing 2nd circuit above.... that is With LDR.... i can put 45 to 50 LED's for 6v 4ah battery....? and how many hours it will glow....?

3. OK you may do it, but please note that I haven't tested it practically, though I believe that it should work as expected.

Yes you may connect 50 LEDs, just connect a 10 Ohm 1/4watt resistor with each LED for better uniformity of light.

According to me it should stay ON for around an hour

4. Thank u...
Which circuit u did practically and working good....?

for 30LED's what is the backup time, for 6v 4ah battery.

5. I tested the above circuits without the LDR, and with less number of LEDs.

For 30 LEDs back up time will be around 1 and 1/2 hours or so.

6. ok thanks.... :)

23. hi what is mean by switch with three change over contacts.

1. 1 pole three contacts, the pole can be shifted to select any of the three contacts.

24. pls give a ckt diagram of IR light use in CCTV night cameras .

25. 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?

Regards,

Donovan

1. 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.

Regards.

26. hi
can u give the specification of capacitors and resistor o be used for 12v transformer input?
and specif the diodes?

1. 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.

2. 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?

3. 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.

27. 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?

1. 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.

Regards.

28. 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.

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.

Regards.

29. 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.
Regards,

1. 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.

Regards.

30. hi sir,
i am kiran.i had lot of difficulty in giving numbering to ic.can you say how give numbering to an ic

1. 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.

Regards.

31. 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?

1. 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.
Regards.

32. 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.

1. 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..

33. 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.

1. Hi Masterfiles,

I have explained the power supply working in the article, you can find the entire explanation just below the first diagram.

Regards.

34. 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.

1. Hi Masterfiles,

Please check the last circuit diagram, it's exactly what you are looking for....

35. 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

1. Hi Masterfiles,

I have updated the diagram with the required explanation, you can check it just above the last diagram.

2. Thanks Swagatam. That was what i wanted so i can compare with my own explanations. I really appreciate. Keep up the good work

3. My pleasure...

36. Hi,
i like to make one ultra high bright led emergency light. fully automatic circuit diagram,with battery low cut off and battery full cut off and over load cut off and i like to use solar panel also pls give me a detailed circuit diagram pls pls...

Thank you
N.Muthukrishnan
krishna198023@gmail.com

1. Hi,
The below given link will hopefully provide you with the required design

37. Hi :)

Do you have a mini project that use 3 ICs on its circuit?
Thanks.

1. Hi,
Can you specify the project subject?

38. 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. :D

btw, I am beginner.

Thanks for your concern Mr. Swagatam

1. OK.... please check the below given circuit, i hope this will do?

39. Thanks Mr. Swagatam :)

40. hi Mr.swagatham i have just seen your blog while searching solar equipments, actually i like to manufacture low cost solar products if u r interested / u know any person having same idea please mail me nazugatz@gmail.com

1. Hi Nazugatz,

Thanks for contacting me!
I am interested if it's a long term work, and if the transactions are within India.

My email is hitman2008@live.in

41. 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?

1. Hi Ranju,

You can remove them if you want, they are not critical.

3904 is NPN so cannot be used here.

42. Thanks Swagatham.
Purpose of 100K Pot is to control luminosity. is n't it?

1. Hi Ranju,

It's there to select the ambient light cut off point.

43. 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.

1. 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.

44. can i use bc 548 instead of bd 140

1. no, you will have to use BD140 or 2n2907 or similar

45. Hi swagatham,when i connect power from transfomer to the base of (TC BD140 B7CW) transistor n from base to led.the led illuminate.is the transistor npn.pls help

1. Hi muanthang,

The transistor is PNP.....but I did not understand your question properly....."from base to LED" meaning???

46. 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)
Regards

47. 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

1. Are you referring to "mains input" in the diagram? it's taken from the AC mains outlet...220V or 120V AC

48. Good morning Mr. Swagatan,
Sir can you pls. tell me if i want to used a 12v car batt, what components will change? Thank you.

1. Good morning,

with a 12V supply, there should be at least two LEDs in series on each string with individual 100 ohms resistors.

2. Good Morning,

Thanks you very much for the reply, so the only the resistors will change, and the rest will be the same? Thank you.

49. I can't find AD149 in my town. Can I replace it with 2N3055?

1. No, you cannot.... AD149 is PNP, 2N3055 is NPN.

Try TIP42

50. 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.

1. 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.

51. 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....

1. 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.

52. 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.

1. 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.

53. Just to confirm the T1. I'm using TIP42 in place of AD149 as you suggested.

1. 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.

54. Hi swagatam,

why there is no bridge rectifier on the first circuit...? and can we use IN4007 instead of 1N5408 for D3---D6 ?

Bibin Edmond

1. Hi Bibin,

A 9V transformer has been selected for TR1, a bridge would enhance the voltage to 12V which wouldn't be appropriate for the battery, therefore only a single diode rectifier is used to make the output inefficient and lower.

In4007 will be lower in current so not suitable for the application.

2. oh... ok thanks bro... !

3. swagatam,
can we use 6a4 ( 6amp diodes) instead of 1N5408 ?

4. yes can be used, but it will look quite bulky....

5. sir can u plzz make the first circuit with and npn transistor? plz plz can u make it fast becuz i have my porject coming up. and can u simplify it with a bridge rectifier plz

6. I'll do it soon....

7. it's posted, please check it.

55. What is purpose of the diodes in series and the selector switch

1. for limiting the battery voltage to 3.3V without dissipating any heat and increasing the efficiency of the system.

56. 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.

1. 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.

57. sir,
i want to know that how to attache cfl lamp replacing led. please reply me.

1. I haven't tried it yet, so presently I cannot say about the procedures...

58. for more back up time of light can i replace the battery 6v,4.5ah by 6v,14ah or increase the quantity of led connected

1. increase batt AH

2. Sir,i want to know what is the total watts of brightness of 150 led?

3. around 15 watts.

59. 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!

1. Hi Thanks,

You may refer this article, see how R3---R14 are arranged you will have to wire the LEDs in this way, IC1 circuit is not required, you may connect the strings directly to a 12V battery (100 ah).

You will have to use 50 to 60 such strings together.

2. 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)

3. 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.

4. 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?

5. 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

6. 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!

7. 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....

8. Thanks sir! I learned a lot from you!

60. Hi Bibin,

You can try a LM317 circuit, nothing can be better than that.

61. Hi swagatam.. Good morning.. !

can this lm317 can control both current and voltage at the same time ? if so can u send me a circuit for that. ?

Regards,
BIBIN EDMOND

1. Hi Bibin,

Yes it can control both, you can make the following circuit, you may remove T1, R1, and replace R2 with a link.

2. Hi swagatam,

Thank u very much yaar... let me try that circuit... !

Regards,

Bibin Edmond

62. 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

1. I didn't understand your question...

63. i have made the first circuit but when i check the charging voltage comes at emitter of transistor and negative of transformer is 10v, can the circuits run for charging 6v battery.

1. 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

64. 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

1. Check out the last circuit shown in the above article.

remove all the 4 horizontal diodes, and the switch.

connect the switch point to battery positive.

2. make sure you connect a 50 ohm resistor with each led

65. thank u very much sir sry for the late reply

66. 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 .

thanks

1. you may try the last circuit shown here:

Eliminate T1 and the relay section entirely, connect your leds directly across the free end of R2 and ground.

67. 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 ?

1. For battery back up you may refer to the second circuit from top given in this article.

include only one LED string, as per your requirement

2. I am curious if you could substitute the LEDs with a 12 volt relay in order to use higher current lighting on a separate (or same) battery? If so, would part of the circuit need "beefing up" to drive the relay?

I am thinking of using a car or deep cycle marine battery with it's own regulated charging device and then placing LEDs all over the house in discrete locations for when the power goes out. I'll make sure everything is fused for safety.

Thanks!

3. Yes that's quite possible without much complications, I have already discussed it in this article, please refer to it:

68. That circuit will blow all leds of some one by mistake keeps switch at wrong position

69. 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
Scientist-C
Ministry of Communications & IT
Govt of India
Mokokchung, Nagaland.

1. 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.

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

Tanmoy.

1. Hi Tanmoy,

yes you may connect a bridge rectifier for better response. I connected a single diode to keep it simple.

71. 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....

1. you can increase the battery ah it won't damage the LED, but the voltage should not increase

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

73. 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
Jon
PS Great Site.

74. 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
Jon

1. Thanks Jon,

check out the second circuit in this link:

connect its supply terminals across your emergency lights transistor output and ground, it would start working as per your requirement...

75. 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.

Thanks & Regards
Faisal Mateen

1. Dear Faisal Mateen,

You can try the following circuit:

http://1.bp.blogspot.com/-I2zlapFq6RM/UML_NxUJy2I/AAAAAAAAB3w/27ZLDjK9C9g/s1600/simple%20modem%20UPS%20circuit.png

Use R1 = 100 ohms, 1 watt and the adapter can be a compact 12V/2amp smps
Make 2 strings of 3 LEds each with individual 6 ohm/1 watt resistors.
connect these strings in parallel to the output of the above circuit.

2. ...D1, D2 = iN5402

76. Is S1 a automatic switch? If not then how it to be used manually? Thanks.

1. 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.

77. sir i want to know that y u used a resister of 10k ohm in parallel in the 1st circuit

1. For providing the required negative base biasing so that it is able to conduct when mains supply fails.

78. 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?

1. 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.

79. 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??

1. 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.....

2. ...do not remove D1, D2, they are required.

3. thanks ... plz also answer my 2nd question i have 12V 1500ma adapter and i wish to charge my 6V 4.5AH bettery with this adapter ,, what should i do??

80. 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..
Regards,
Ranju

1. 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.

81. 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?

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