If you make crystal radios, you’ve probably got a few crystal earpieces. The name similarity is a bit coincidental. The crystal in a crystal radio was a rectifier (most often, these days, a germanium diode, which is, a type of crystal). The crystal in a crystal earpiece is a piezoelectric sound transducer.
Back in the 1960s, these were fairly common in cheap transistor radios and hearing aids. Their sound fidelity isn’t very good, but they are very sensitive and have a fairly high impedance, and that’s why they are good for crystal radios.
[Steve1001] had a few of these inexpensive earpieces that either didn’t work or had low sound output. He found the root cause was usually a simple problem and shares how to fix them without much trouble.
The root cause of failure in these is typically a poor connection on a piece of copper inside the device. [Steve1001] disassembles the unit with a knife, drills a new hole, and adds an additional wire. The process isn’t complicated but seeing how its done might keep you from tearing up one in the disassembly attempt.
Finding a local source for crystal earpieces might be tough, but piezo speakers are common on things that make beeps like motherboards or microwave ovens. Hackaday’s own [Steven Dufresne] shows how to convert one into a workable crystal earpiece in a video below. We’ve seen him make other homemade versions, too. Of course, big time crystal radios have amplifiers and can use conventional speakers and earphones.