Electric toothbrushes generally fail in one of two ways. Either the battery dies or the electrical parts become corroded due to water infiltration. This instructable will show you how to recycle a broken electric toothbrush into a useful LED flashlight. The picture above shows the main components in a typical toothbrush that has been cut open and disassembled. Note the corrosion on the motor housing. The motor has failed, but the battery, on-off switch, housing and charging base are still useful.
Step 1: Diagnose the Cause of the Failure
Before you get started, you will have to determine what caused the failure. If the unit has not been charged for a while, charge it now.
Turn it on. If the motor turns, but the brush does not oscillate, this means the gearbox has failed, but everything else is OK. If the motor does not turn it could be either the motor or battery that has failed. In this case, you will have to disassemble the unit either by cutting the housing or carefully unscrewing the base from the brush housing. The picture shows how the housing was cut. Note that I am saving the switch assembly.
Carefully cut the outer shell of the housing with a saw and carefully remove the components. First check the battery voltage. If it is less than a volt after being charged, you will have to replace the battery. If the battery shows more than 1.5VDC then the charger and battery are OK. At this point, you should test whatever LED you plan to use and see if it lights up at your battery voltage. Don't forget to use a current limiting resistor. You can check the motor, but it will not be used on this project.
Now is a good time to check the on-off switch to see if it can be used or if you will have to add your own switch as shown in photo 2.
(This is a different unit that I modified previously and shows the mini toggle switch that I added)
Step 2: Adding the Extension Wires
Now that you have determined that you have a good battery and a workable switch, you can now add the extension wiring that goes to the LED. You have several choices here depending on what you have in your junk box. You could use small gauge stranded wire, heavy gage magnet wire, or (as shown in the photo) common solid house wire. Cut the wire a few inches longer than necessary because it is a lot easier to cut off the excess than it is to replace them with a longer piece. The heavy gage wire allow you to bend the LED at any angle to the base.
On the base or battery end of the heavy wire, you should solder some light ( 22-26) gauge stranded wire to the end of the heavy wire then run the light gauge wire to the battery and switch before securing the heavy gage wire to the housing. After preparing the ends of the wire, secure them to the housing as shown in photo 2. I used epoxy, but you could use hot glue or nuts and bolts.
Step 3: Wiring the Complete Circuit
The actual wiring will depend on what switch you are using and the physical configuration of the toothbrush, Basically you want the circuit shown in photo 1. The current limiting resistor(s) can be placed anywhere in the circuit. You may have to arrange resistors in parallel or series depending on the battery voltage and LED that you select. I used two 100 ohm resistors as shown in photo 2. Be sure that your joints are secure and insulated with tape or hot glue.
If you had cut the top off of the unit, you should insert the heavy gauge wires through the shaft opening before soldering the LED. You should now test the circuit before attaching the top or inserting into the housing. I used epoxy to re-attach the top.
Step 4: Ta Da - the Results of Your Work!
If all goes well so far, you will have a handy, always charged LED flash light (or torch as our British cousins would say). As you can see from the photo, I made this one rather long - the better to snoop into dark cracks and crannies. I also used magnet wire on this one and used heat shrink tubing to keep the wires together.
Good luck on building your very own, recycled "TOOTH TORCH" !