My parents moved to NC and bought an old southern house that had this awesome old doorbell– but it didn’t work. They installed a new-fangled one but let me take the old one back to MIT to see if I could fix it. It was deceptively easy to fix; it was just missing one wire in the back. It’s a pretty neat, simple device. It takes a DC voltage and powers some electromagnets, which attract the mallet to hit the bell. But when the mallet touches the bell the circuit is opened, the electromagnets turn off, and the mallet returns to it’s original position: which turns on electromagnets again. An oscillator!