Step 1: Parts List/preparing Cartridge
Empty inket printer cartridge (the slim, single-color type)
220-ohm resistor (may vary, depending on battery type)
Button-battery holder (several sizes are available, it doesnt matter which one you use)
2 3V button batteries to fit holder
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
Step 3: Start Soldering
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
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
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.