This project show you how to do a flashlight using a cell phone battery

Step 1: Part List

For this Project you will need :


High Brightness LED (1W) 1

Universal Board (5x5cm) 1

Cell phone Battery Push button switch 1


Single Row Headers female 1

Single Row Headers male 1

Soldering kit.

Step 2: Circuit

The resistor is determined by ohm's law, this mean that your resistor depend on of the power of your led. e.g ,

for a led of 1W of power with a cell phone battery of 3.7V (Nominal),Your minimum value of resistor will be :

V=I*R then P=(V^2)/R

where P = 1W and V= 3.7V (note: batteries have a maximum voltage of 4.2 volts be careful)



R=((3.7V)^2)/ 1W=13.69 ohm

Then the current will be :

I=V/R , I= 3.7 volts/13.69 ohms =270mA;

In my case I use a 3W Led

R=(3.7^2)/3W =4.56 ohm, I=811mA,

I recommended to use a resistor with some more ohms because the battery usually have 4.2 volts then I use a 6.2 ohm resistor with power dissipation of 2W.

Step 3: Soldering the Circuit

The push button is toggle push button
Cut the female headers and solder it to the battery

The board is designed to connect directly to the battery

Step 4: Testing

After a few minutes of working I noticed that the led need a dissipator

note: the battery has to give more of current that the circuit requires, this battery give around 2500mAh, to know more about battery parameters see this video https://www.youtube.com/watch?v=cxkVxi9P0EA

I expect your feedback.



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    8 Discussions


    4 years ago

    Hi, that battery you are using is dangerous when used incorrectly. The current draw from the 3 Watt led is in excess of the maximum recommended current draw for lipo batteries such as yours. Risk of thermal runaway and fire. Be careful.

    6 replies

    yes most definitely. The larger the battery the better but i generally avoid non-protected lipo and li-ion batteries since any misuse and they easily experience thermal runaway i.e. catch fire. I have largely adopted lithium iron phosphate batteries since they have excellent safety characteristics. You will see two of my instructables highlighting this battery chemistry.

    Also, applying heatsinks to leds is great if you want long lifespan. I sometimes ignore this requirement depending on what i'm building. I recently completed an led floodlamp instructables you can take a look at. Nevertheless, you made a great DIY instructable so I congratulate your effort!


    4 years ago

    Great idea for old n*kia battery's ...