Introduction: USB Rechargeable LED Flash Light

In this instructable I show you how to make your own LED flashlight that can be recharged simply by plugging it into the USB port of your computer. I know that you could find something similar from a store, but all of the ones that I found had only a few LEDs so I made my own with eight LEDs.

Step 1: The Tools and Parts Required

The tools include:

1 hobby knife 2 handsaw
3 soldering iron 4 screw driver
5 scissors 6 wire cutters
7 pliers 8 pen
9 drill and drill bits

parts include

1 Stripboard or circuit board 2 8x LEDs
3. on/off switch 4 3x ni-cd button cells
5 1x 36 ohm or 8x 120 ohm resistor 6 plastic and aluminum sheet
7 electric wirier and steel wirier 8 tape
9 solder
10 USB flash drive (I recommend one without a sliding mechanism)

Step 2: Dissisembly

This step is quite easy. Simply use a flat head screwdriver to open the flash drive. The one I started with was broken so I cut the circuit bored off to save some space and to reveal the positive and negative lines, then I used a volt meter to check the polarity of the usb I suggest that you use a pen to mark the positive and negative, if you have done so then you have completed this step.

If you are doing this with working flash drive and you want to keep the memory function, then you have to add an additional circuit. Start by soldering a wire to the ground or negative wire of the flash drive then cut the positive wire and put a three point toggle switch (as seen on the last image) in place of the positive input of the usb will connect to the middle point of the switch and the two far sides of the switch will connect to the positive end of the flash drive and the positive input of the circuit on the fourth step.Finally put some tape on the flash so it doesn't short out.

Step 3: The Battery Asembly

The hardest part of this whole project is is to find some Nickel-cadmium button cell batteries, I found some by cutting open a rare 4.8 v battery I got from a local electronics store. iI you cant find this battery as will most likely be your case, you could get some small ni-cd batteries by opening a 9v ni-cd battery witch is technically 8.4v that can be found at Either way once you have found some small ni-cd batteries you would want to solder some wires to the batteries and
tape three of the batteries together to get a 3.6v battery pack, this will not recharge If you have four batteries you would find that they will not charge because the voltage is divided between the cells and you need at least 1.6v going through the batteries preferably 1.7v
This step is now complete.

Step 4: Wiring and Housing

In this step the first thing to do is to check if all the the parts fit in the flash, most likely they won't so use a hobby knife to remove some of plastic and make a hole for the on/off switch and the optional switch if you have the memory circuit.

Once you have made some room you would want to start making some connections according to the schematic. Start by connecting positive output to the positive end of the battery and one end of the on/off switch, then connect the negative output to the negative end of the battery and to the resistor or resistors. Then connect the other end of the on/off to all of the positive ends of the LEDS.

Between all of the negative LED ends, the battery and negative output, you could either have one 36 ohm resistor or a 120 ohm resistor on each end of the lLEDs I recommend that you make the resistance higher then it has to be in case the batteries or your computer has an out put higher than 5v, Both methods of orienting the resistors will work but it's better if to have a resistor on each LED in case some of them break and the rest will have too much current and they will burn out.

Once you have the switch, batteries and two wires leading out, this step is complete but most likely the batteries will be too thick so use some hard plastic to make a box to raise the lid a bit.

Step 5: The Led Circuit

In this step you will cut two pieces of strip board and soldier the LEDs to it Remember to have the LEDs in parallel and to have all of the leads in the same direction. Once you have the LEDs soldered in place, cut the leads . Repeat this again and this is complete. If you had 8 resistors then you would have to have the resistor between the LED and the large connecting wire.

Step 6: Finishing

This is the last step, start by checking polarity of the two leading wires then solderi the positive wire to the positive end of the LED ciruits then connect the negative end to the resistors. Once that has been completed, solder two wires on both sides of the LEDs. Then glue the lid back on and make a hinge for the second LED circuit and use some super glue to secure it. Now solder the wires to the second set of LEDs and add a two small magnets to keep the hinge in place.
Often ni-cd batteries need a long charge if they spent a long time with out being charged, so simply plug it in your computer for eighteen hours then it's ready use, each battery cell will have 1.2 v multiply by 3 which would equals 3.6V and to charge them you should supply 1.7v, multiplied by the number of cells so you should get 5.1v which is the output of your computer. With 5.1v the battery should charge in 2 hours with 5v it should take a little longer.

Once it has fully charged the LED will take 80 mA or 0.08 A and the battery I stared with was rated to 60 mA that means that it can supply 60 mA, so divide 60 by 80 and you will get 0.75 which is 0.75 / 1hour.
So you could turn 0.75 of a hour to equal 45 minutes which is the life span of the batteries before it needs a recharge.Rremember that this will be different depending on the batteries and LEDs you use.

some of the formulas
A=amps, B=battery, C=battery cell, H= hours, L=led, LA=LED amps, R= resistance in ohms, V=volts, W=watt, Wi=amp withdraw, x= multiplication, X=an unknown number,


the number of ohms required when in parallel
v1=the power supply voltage v2=LED voltage

charger voltage for ni-cd batteries
V= (1.7xC)

battery life

change decimal of an hour(X) to minutes

(Xx60)=a number of minutes

Step 7: Comparing Light Sources

I gathered several portable light sources to compare them to my new USB flash light, the light sources i chose to compare are: a candle with a reflector, a 6v lantern flash light, a key chain flash light with one LED, and a flash light with 16 high intensity LEDs that would have the same power as a large Maglite.
The first image shows the light sources arranged from dimmest on the left to brightest on the right.
The next 5 images are the lights arranged in the same order, all of them are 10 feet from the camera lens. Compare them by the size of the inner most circle. Although the new USB flash light is in second place of the flash light I tested, consider that it will last 45 min before requiring a 2hour recharge and it can fit inside the brightest flash light (the silver on the right) with the batteries removed.
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