1 Watt LED Emergency Lamp Circuit Using Li-Ion Battery

The post describes a simple 1 watt led emergency lamp circuit using li-ion battery. The design was requested by one of the keen readers of this blog, Mr. Haroon Khurshid.

The Circuit Request

Hello mr swagatam...please can you help me design a circuit to charge a
nokia 3.7 volt battery by using regular nokia cellphone charger circuit and utilize that battery for lighting 1watt leds connected in parallel there should be light indicator and also automatic on of system in case of power failure kindly you consider my idea and design one

kind Regards,

Haroon khurshid

The Circuit Design

The requested 1 watt led emergency lamp circuit using li-ion battery may be easily built with the help of the below given schematic:


1 Watt LED Emergency Lamp Circuit Using Li-Ion Battery

The voltage from the cell phone charger power supply is dropped to around 3.9V by adding diodes in the positive path of the supply. This should be confirmed with a DMM before connecting the cell.

The voltage should be limited to around 4V so that the cell is never allowed to cros the over charge limit.


Although the above voltage will not allow the cell to get charged fully and optimally, it will ensure the cell doesn't get damaged due to over charge.

The PNP transistor is held reversed biased as long as mains AC stays active, while the Li-Ion cell is charged gradually charged.

In case the mains AC fails, the transistor switches ON with the help of the 1K resistor and instantly illuminates the 1 watt LED connected across its collector and ground.

The above design can also be implemented using a transformerless power supply circuit. Let's the learn the complete design:

Before proceeding with the circuit details it should be noted that the following proposed design is not isolated from mains and therefore is extremely dangerous to touch, and it has not been verified practically. Build it only if you  personally feel sure about the design.

Moving on, the given 1 watt LED emergency light circuit using Li-Ion cell looks quite a straightforward design. Let's learn the functioning with the following points.

It's basically a regulated transformerless power supply circuit which can also be used  as a 1 watt LED driver circuit.

The present design perhaps becomes very reliable owing to the fact that the dangers normally associated with transformerless power supplies are effectively tackled here.

The 2uF capacitor along with the 4 in4007 diodes form a standard mains operated capacitive power supply stage.

Adding an Emitter Follower for Voltage Regulation

The preceding stage which consists of an emitter follower stage and the associated passive parts form a standard variable zener diode.

The main function of this emitter follower network is to restrict the available voltage to precise levels set by the preset.

Here it should be set at around 4.5V, which becomes the charging voltage for the Li-ion cell. The final voltage that reaches the cell is around 3.9V due to the presence of the series diode 1N4007.

The transistor 8550 acts like a switch which activates only in the absence of power through the capacitive stage, meaning when AC mains is not present.

During the presence of mains power the transistor is held reverse biased due to the direct positive from the bridge network to the base of the transistor.

Since the charging voltage is restricted at 3.9V keeps the battery just under the full charge limit and therefore the danger of over charging it is never reached.

In the absence of mains power, the transistor conducts and connects the cell voltage with the attached 1 watt LED across the collector and ground of the transistor, the 1watt LED illuminates brightly....when mains power restores, the LED is switched OFF immediately.


1 Watt LED Emergency Lamp Circuit Voltage Regulated

If you have further doubts or queries regarding the above 1 watt led emergency lamp circuit using li-ion battery, feel free to post them through your comments.



122 thoughts on “1 Watt LED Emergency Lamp Circuit Using Li-Ion Battery

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. hello sir, i'm denny

    I had a 3.7V 3800mA battery, can I use first circuit to power led 3 watt?
    if yes, is there any components that should be changed?


    • hello Denny,

      yes you may go ahead. You will have to do the following mods:

      change the upper resistor to 3 ohm/1watt.

      change the transistor to TIP127.

      you may also need to tweak the 1k resistor value, i'll leave that up to you for obtaining optimum brightness from the LED.

    • Dear firiends I want a help from this forum, my 14" crt tv showing EEPROM ERROR what to do ? I replaced the new EEPROM IC also, but still the EEPROM is showing please help me….

  3. Sir,

    I have a 5v 3000 mAh Li-Polymer Battery so i want emit 1wattx4 LED. Do you have any circuit for this & also charging circuit. Please Provide me…


  4. Hi Swagatam,

    Can I can add a diode from base of transistor to LED, so that LED is ON even with power from mains.
    I want to keep LED glowing always, either from mains or battery.
    Please let me know, if that is possible.


  5. Can I use a higher capacity mobile battery (more mAh) for longer backup with the 1watt led? Any modification required ? Gr8 blog . Keep it up.

    • Thanks! yes it can be done, no modifications would be required except the series 10 ohms resistor which will need to be decreased for appropriate charging of the batt

  6. i have build this circuit with three 1W led in parallel with proper heat sink….and it really works good….and happy with this …..thank u Swagatam….i have been waiting for it so long

    • Hi Dear,
      Happy Holi….!
      Hope u'll be fine….
      i have a little problem, i have charged this lamp for over night. now if i am using the lamp, it works good but after 3 to 4 days when i am going to switch on the lamp, i found the battery drainout completely. again i charge this unit fully but i am facing same problem…..hope u understand this…….need solution

    • no…..not at all. after completely charged i had disconnect the unit from charger. i am using nokia's small pin charger and also i have two cellphone batteries connected in parallel ( 800mAh and 1000mAh ).

    • if you disconnect it from the mains the LED would get switched ON, so I can't understand how you could keep it for 3/4 days in that way.

      put a switch in series with the battery…. switch it off when the unit is not connected to mains, this would prevent the batt from getting discharged.

    • hi Swagatam
      i have build this unit properly with brightness control switch, normally s/w is in off positon. and i have uploaded the image on google+, plz check it out….thanks

  7. Hi, I require a circuit that can work from 230Vac input, and the circuit must be able to charge a lithium ion 7.4v 2600mah battery pack as well as drive a micro controller that will power up one 3 watt led.

  8. hi how much(ohms) resistance i have to use to connect 3pcs 1 watt LED with 3.7v mobile phone battery to get optimum brightness? I used 3.3ohms half watt but it becomes hot(resistance+LED)please help me.My Email:saeed_rony16@yahoo.com

    • in your email you notified that you are using 5 batts in parallel, so 3 leds would be ok with them.

      use 3.3 ohms for each led separately…it will not become too hot…just warm.

      actually it should be 1.6 ohms, but 3.3 ohms would be fine.

      use sufficiently large heatsink for the leds.

  9. hi, thanks brother for ur reply.Above 2nd circuit i need auto battery charge cut off system and auto change over ac to dc when main fails.Also i need more simple version of this circuit.is this can be designed?

  10. thanks brother for ur response.For 1st circuit is there is any over charge protection system (auto full charge cutoff) for battery included? if not how can i add it?
    Also can it run by Nokia standard cell phone charger?

  11. Bro thanks for ur reply.Actually im Pharmacist(M.Pharm) & Electronics is my hobby.So i go through ur mentioned link & i dont understand ur suggestion to modify that diagram also about ur mentioned cut off transistor.So would u mind to send me the complete circuit diagram through my email(saeed_rony16@yahoo.com)? My requirement is:(1) Circuit is Operated by Nokia standard cell phone charger (2) Battery nokia 3.7 volt (3) Auto ac to dc changeover system when ac fail (4) Battery overcharge protection system(auto battery full charge cutoff) with LED indicator.I have tried many times to develop such type of circuit but i failed. So Please bro help me urgently. Please design it simple.

  12. it's great idea of circuit and smart cause add 2 diode , thanks for goofle found this link. for above circuit 10 ohm for current control ?
    I just want design some "dream emergency LED" with spect as for battery charger is automatic, when running from battery and battery get low (3.2 Volt) then cut off so LED is off it will prevent damage from battery.so far I just build auto bat charger and LED driver

  13. Bro what about my specified circuit diagram what i am asking u to developed? R u still busy now?My requirement is:(1) Circuit is Operated by Nokia standard cell phone charger (2) Battery nokia 3.7 volt (3) Auto ac to dc changeover system when ac fail (4) Battery overcharge protection system(auto battery full charge cutoff) with LED indicator.I have tried many times to develop such type of circuit but i failed. So Please bro help me urgently. Please design it simple.

    • Ravindra, the right lead is always the gate, the center connects with the mains/load while the left terminal connects with the ground.

      Are you referring to the above circuit? it's a difficult design, you can try other simpler ones.

    • Sir. I have LED driver its output is 6-12v and 300 MA i want to 1 watt led 3 to 6 which resister i use how much watt. I connect 1 WATT 3 LED series with 10 ohm 1 watt resistor but LED & RESISTER going hot

    • Ravindra, 3 LeDs in series is correct (@12V), but use 6 ohm 1 watt resistor for optimum brightness.

      The LED will become HOT, you will need to mount it on a suitable aluminum heatsink otherwise it will get damaged within seconds.

    • Ravindra, use the second circuit from above….add all the 10 leds in series directly across the emitter of BD139 and ground, remove the 8550, 1k, 1N4007 associated parts these are not required.

      now adjust the preset slowly to get full illumination over the connected LEDs.

    • capacitive power supplies become inefficient with their amps when used for low voltage applications, the second circuit will not give more than 100mA output if 2uF/400V is used as the input cap

  14. Hi Sir! I have successfully installed and used this circuit with 5led x 1w, I'm very happy for it. It's all thanks to Sir, I thank you very much Sir! Sir I want to ask is this circuit can be fitted LDR is not? If they must establish how? Sir reply me on my email: memory1gb@yahoo.com. Again thank you very much Sir!

    • Hi Truong, thanks!

      you can add an LDR feature by doing the following steps:

      take a BC557 transistor, connect it's emitter to battery positive, collector to base of 2N2907
      and connect the base of BC557 to LDR's one lead, the other lead of the LDR will connect with the negative of the battery.
      also connect a 1M pot across the base of BC557 and the positive of battery.

      adjust the pot to get the required results.

  15. HI friend,
    I have a issue with this circuit,
    also one friend having the same issue, (@Jignesh)

    My Issue is similar to jignesh issue.
    I build this circuit, added 3.7(18650 2000ma battery Original Samsung).
    when i connect the fully charged battery to this circuit, the led glow much batter and last up to 4 to 6hrs. when battery got low, i connect mobile charger to charge the battery to this circuit as per u show, the mobile charger output is 5v 800ma Nokia, i charged over night (approx 7 to 9 hrs), in morning i checked whether battery is heat or not, or any components are get heat or not, but nothing happen, nothing get heat. alright its fine, i switch on the led but the led glow dimm and last not more then 1hrs, less then 1hr. again i disconnect the battery and charged by other charger (18650 battery charger) and again i connect to this circuit, its last 4 to 6 hrs).
    please solve this.. Thank u in advance 🙂

    • Hi Kamlesh,

      you mean to say that you tried the first circuit, the LED emergency lamp action works fine with a fully charged battery, but once discharged the battery does not get charged fully, right?

      in that case you can try replacing the 10 ohm resistor with a 1 ohm resistor and see if that helps.

    • Ok, I will replace the 10ohm resistor with 1ohm and I will check whether the battery charge full or not, and thank you very much for your reply.
      ok so your 10ohm resistor is 2w, so what watt resistor for 1ohm I need? the same 2w or another?

    • Oh I have to ask you one more thing,
      if input (mobile charger) value is 5v 800ma to 2a, so I can use 1ohm resistor?
      I am mean I told u that I am using now 5v 800ma and u told me to replace 10ohm resistor with 1ohm, if I use 5v 1a or 5v 1.5a or 5v 2a, so I have to use 1ohm resistor for more then 800ma charger(input power from mobile charger)?

    • the watt will need to be judged by touching the resistor, if it gets too hot, then you can think of increasing the wattage.

      use Ohms law for determining the resistor value

      R = (supply voltage – battery voltage)/mAH of battery

  16. .sir can i use this circuit for 240 AC mains?.. Can u m0difY it for me using a relay when the battery is full. The relay is off and therfore the charging stops.im just ensuring the battery life. Thanks in advance

  17. Dear sir,
    I have a two 4 volt 0.5 mAh china Battery connect in parallel . I removed one DIODE of the series, go to the two DIODE. then Instead of using 10 OHMS to 1.3 OHMS 2watt . Well now loading the battery charge. please tell us your advice on this. Two diode in series. Why has been put here for?

    • Dear Rajkumar, normally cellphone charger will show around 7 to 8V when measured without load, therefore to correct this issue I connected the two diodes, but actually it's not necessary since on load the voltage automatically adjusts to the intended correct battery value

  18. Dear sir ,
    Please help me I will have use a 4 V 2000mA or 1500 mA battery, then I use first circuit to power 2 numbers of 1 watt LED . may I use BD138/140 TRANSISTOR
    If it's OK. What changes should I make it?


    • Rajkumar, brightness control can be achieved crudely by inserting a 20 ohm 1 watt pot in series with the LED, or you can use a couple of series diodes with the LED and switch these diodes using a rotary switch for increasing or decreasing brightness.

  19. Hi sir ,

    I have 12v and 7ah battery , I want simple circuit , my requirements is 12v 9 watt DC led bulb like this 3 bulbs must work for 14 hours , or any other option by which I will have bright light in my room each 20 sq meters area (e.g 9 watt everreaday bulb which work on ac that much bright ) but itshould work on battery for 14 hours on 12v 7 ah battery

    • Hi Sidhesh, dividing 9 by 12 gives 750mA, 3 times makes of this gives 2.2 amps, which will not allow more than 2 to 3 hours of back from the battery, 14 hours will not be possible…

  20. Hello sir,That was good,but brightness is dull.And charging battery is below 3.85v,what can i do to achieve more brightness and fully charging battery..

    • Hello sunil, replace the transistors with a TI127 to increase LED brightness, and allow charging of the battery for at least 10 hours.

      make sure to connect a 4V zener diode across the battery terminal.

    • Thank u sir for kind response,
      I’m confused,in the schematic,is that zener diode or led indicator after 10 ohm resistor to the emitter?
      Can i have rough schematic using tip127 plz?

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