Crystal Radios

Here's how to interface a piezo earpiece with a crystal radio.

Parallel-tuned, series-fed radio:

The earpiece is interfaced through a 33 kΩ shunt resistor. The resistor provides a path for the

detector current. The voltage drop across the resistor drives the earpiece.

Series-tuned, shunt-fed radio:      

The earpiece is interfaced through a 2.5 mH RF Choke to prevent its capacitance from

short-circuiting the detector. The voltage drop across the detector drives the earpiece.