Step 1: Remove the Case
The two sides of the case are sealed pretty well (it's waterproof, or at least water/toothpaste resistant), so it might take a some effort to get it open. I'd suggest using a slotted screwdriver to carefully pry the case apart at the seam. You can also get a lot of leverage at the top, where the toothbrush head screws down over the bottom part. Try not to crack the whole case, or break it badly, as we'll want to glue it back together at the end. Once you get it open, you should see some coils, a circuit board, and a couple of batteries.
Step 2: Exploration!
For doing such a simple thing, this toothbrush has some fancy technology! As long as we've got the case open, we might as well do some exploring.
This particular toothbrush handle is totally sealed, using magnetic coils to charge the batteries and move the toothbrush head. The coil at the bottom is used to charge the batteries, and there is a similar coil in the charger base station. Removing the circuit board lets us do some experimentation on the coil. If we measure the current through the coil when it's not in the base station, we naturally see no current. When I placed it into the charging base station, my multimeter showed about 30 mA. It would probably be more useful to measure it using an oscilloscope, as it's probably an AC signal. I might do that someday. Quite interesting!
Step 3: External Power?
Since the batteries had effectively died, I unsoldered them from the circuit board, and soldered on some wires of my own. I connected the wires to an LM317 adjustable voltage regulator, so I could experiment to see what voltage worked to run the toothbrush. I had to add some power supply capacitors to help keep a steady output voltage, and decided to try for 2.4 volts output. I computed the proper resistor values to add to the regulator, and got pretty close to 2.4 volts. When I push the toothbrush button, it started buzzing like it normally did.
It looked like I would be able to power the toothbrush from a constant DC voltage source. Hurray!
Step 4: Add DC Power Supply
I used a slotted screwdriver to carefully pop out the bottom (charging) coil and the two battery cells, so there would be room for me to drill a hole for the power cord. Try not to break the outside case while prying out these parts!
Next, I drilled a small hole in the bottom of the case, just large enough for the cord to fit through. I stripped the wire ends and soldered the wires into the circuit board where the batteries used to be connected. Make sure to put the cord through the case before soldering its wires to the circuit board!
After soldering, be sure to test the circuit again to make sure the toothbrush is still working.
Step 5: Re-attach Case
The original waterproofing on the toothbrush handle was very nice, as I could leave it sit by the bathroom sink, and let toothpaste drip down on it, all without having to worry about breaking anything inside. I wanted to make it as waterproof as I easily could, so I applied superglue all along the contact area between the two pieces of the case. I looked to make sure I could see a little glue oozing out all the way around the edge. I used a couple of woodworking clamps to apply pressure on the case overnight. The next day, I was unable to pull the case apart with my hands, and I'm pretty sure it's water resistant.
Step 6: Brush Your Teeth, Brush Your Teeth, Brush Your Teeth!
With the case all glued back together, it's time to do some tooth brushing! Be sure to use the proper amount of toothpaste, and brush all of your teeth. Don't forget about the ones in the way back, or the inside of your teeth!
Hope you enjoyed this Instructable, and I hope it helps keep some toothbrushes out of the landfill when their battery cells wear out!