A simple LED flashlight made from a few basic components housed in an inkjet printer cartridge

Step 1: Parts List/preparing Cartridge

Here's what's needed:
Empty inket printer cartridge (the slim, single-color type)
Superbright LED
220-ohm resistor (may vary, depending on battery type)
Switch
Button-battery holder (several sizes are available, it doesnt matter which one you use)
2 3V button batteries to fit holder
Jumper wires
Solder
2 small pieces of stripboard (5x5 holes, 9x4 holes)

Pry/cut off the top of the cartridge. Its glued, not snapped on; use a thin but strong metal blade to pry under a part of it, then work it around the whole thing. Take out the ink sponge, peel off the small plastic square from the ink nozzle, and remove the small rubber gasket; thoroughly rinse out the cartridge. Peel off the label from the top. There is a small plastic thing inside the ink nozzle; pry this out.

There is a small piece of foam on the inside of the cartridge that needs to be removed. Use a small screwdriver to push down through the ink nozzle opening—that will punch a hole though it, and loosen it; then use a pair of pliers to remove as much of it as you can. (The LED will be fed though this hole).

Step 2: Attach Switch

There are 2 holes in the cartridge top–the switch can be placed in either one. Use a drill or utility knife (or soldering iron) to enlarge the hole enough to fit the switch. Insert the switch, place the washer and nut on the outside, and tighten.

Step 3: Start Soldering

Insert the LED through the inkjet nozzle hole, with the negative lead on the top, as far as it will go. On the inside, slide a small piece of stripboard down the leads, to touch the plastic thing on the inside. Make sure the stripboard is aligned such that the leads are not in the same copper strip. Bend the leads slightly to hold the LED in place and solder. For added stability, the piece of sripboard can then be glued in place.

Take the resistor and solder a piece of jumper wire to one end. Bend the other end into an L shape and slide it through the hole next to the positive LED lead (the longer one–it should be on the bottom).

Solder a jumper wire in the hole next to the negative lead.

Step 4: Finish Soldering

Solder the battery holder to a small piece of stripboard. Solder the wire from the negative LED lead directly to the switch; connect the battery holder to the other switch terminal in line with the positive LED lead.

I used 2 (3V) button batteries–the batteries are thin enough to accomodate 2 in the holder. You could of course use just one and a lower-ohm resistor.

Step 5: Finish

Carefully tuck the battery case and wires into the inkjet cartridge. Because it's a tight fit, some of the solder connections may touch and short out. If so, remove the wire/battery holder and wrap the connections with electrical tape.

Place the lid on top of the case and tape in place. It could be glued, but using tape will allow easier access to change the batteries and make repairs if necessary.

Shown are 2 flashlights I made–one with the switch in the top hole, and one in the second hole. I prefer the one with the switch at the top–a wide band of tape can be wrapped around the middle of the cartridge to hold it closed, which is a little more aesthetically pleasing.