While I think over what I might do with parts salvaged from a retired and disassembled Sonicare electric toothbrush, I played with its corresponding charging base.

Since both the toothbrush and the charging base are designed to sit on our bathroom sinks, they are both waterproof and have no exposed electrical contacts: charging is done inductively.

It doesn't transmit very much power, based on the label claim that it only draws up to 1.4W.

After prying off that bottom panel with the information label, I see a solid mass of potting compound putting a quick end to this particular teardown session. What else can I do with this thing?

I hooked up my oscilloscope probes to a coil salvaged from a microwave turntable motor and set it on the charging base.

The oscilloscope reads a ~85kHz sine wave with an amplitude of ~40V peak-to-peak. I played with coil position and was surprised to learn being slightly off-center hardly affected transmitted power.

Removing the alignment post allowed me to go beyond the narrow range of axis alignment, where I confirmed the expected behavior: going too far decreased voltage transmitted. Inside the snapped-off alignment post was filled with a mystery black material. It is a brittle material that does not appear to be electrically conductive. A magnet is attracted to the broken-off post but I can't tell if that's necessarily this black stuff or perhaps there's a piece of steel embedded further inside.

I found some electrical connectors and tested to verify they mate with the microwave motor coil terminals. Unfortunately, these terminals were dependent on the now-absent external enclosure for strength. When I pulled on my connector, the whole terminal came out breaking the wire.

And unfortunately, out of two wires I could have broken, I broke the inside wire. If I had broken the outside wire, it would have been pretty trivial to unwind a single loop to get some extra wire and solder it back on the terminal. But I broke the wire that's buried underneath the entire coil, making this impractical to repair.

I started pulling on wire just for the sake of seeing what it's like. This is extremely thin copper wire and there's a lot of turns in this coil. I ended up with an impressively large hairball of fine copper wire.

It's unfortunate I destroyed a salvaged coil that might have been fun for exploration. As fallback (or maybe they should have been the first option) I also have the Sonicare charging coils that were designed to work with this charging base. I kept it along with the bottom of the Sonicare enclosure to maintain precise alignment, though thanks to this experiment session I now know such alignment may not be strictly necessary.

I thought the fact it was a much smaller coil with far less wire would have meant I have a different voltage transformer. When sitting on the charging base, the oscilloscope reads about 34V peak-to-peak. I had expected more difference in voltage between the two coils.

Some knife work separated the coil assembly from the rest of the toothbrush chassis.

What's the first step when exploring any electronic concept? Make it light some LEDs! Since this is an AC waveform, I soldered two salvaged LEDs side by side with opposite terminals.

When given DC power from my bench power supply, only one of the two LEDs would illuminate. If I reverse the polarity, the other LED would light up.

I didn't bother with electrical connectors this time: the test LED assembly was soldered directly to coil terminals.

I put it on the charging base and both LEDs lit up, the expected response to AC power.

I connected my oscilloscope probes to see how the waveform changed with this load added to the system.

I can see a bump at roughly +/- 3.7V, the voltage drop point for these little blue LEDs.

That wraps a successful first test of using inductive power. Where might things go from here? If I can find the rectifiers I bought for the brushless motor generator experiment, I can get some amount of DC power flowing. It won't be much power, a good chunk less than the 1.4W this charging base drew from the wall socket due to inductive power transmission losses. For comparison, slow USB charging is 5V @ 500mA or 2.5W, and that doesn't even have to deal with inductive transmission losses. So any project idea would either have to be a modestly powered system or incorporate a battery like a Sonicare.

While I wait for a project idea to emerge, I'll set this inductive charging base aside and go back to thinking about the toothbrush actuator and its control electronics.