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A makeshift piezo earphone arrangement comprises a stethoscope and a piezo transducer.

Stethoscope

The sound emanating from the transducer is captured by the bell of the stethoscope held against its face.

Piezo Transducer

A Germanium transistor may be configured as a diode, having its forward voltage as low as 0.1 V, by just interconnecting its base and emitter.

Basic Shunt-fed Crystal Radio

It makes for an ideal detector in a shunt-fed crystal radio.

I was a SWL in the mid-1970s and the antenna I used with a Philips transistor portable receiver was a long wire at a height of about 50 feet. One summer afternoon, the receiver front end (AF117) blew right after a loud crackle of static. 

Then again, my first tube homebrew CW rig had a 3-pin mains supply plug. The antenna was a straight dipole 50 feet high. Operating on a summer afternoon, I received a jolt through my Junker CW Key and survived to hear the crash of thunder from a nearby lightning strike. I immediately yanked the twin-line feeder and threw it on the floor (upper floor of my 2 storey house). After a few moments I was surprised to see the arc from the banana plugs to the cement floor.

Those days, in our sparsely populated area, my antenna was way above other surrounding structures. During thunderstorms…

A medium wave broadcast station antenna system generally comprises a ¼ λ vertical element

and earth.

It would not be wrong to infer that such an antenna would be ideal for a crystal radio.

Crystal Radio Antenna Arrangement

However, practical reasons necessitate use of a much shorter element, with a suitable loading

In a ham shack, either half of a 40m inverted ' V ' dipole antenna would come in quite handy

as a usable crystal radio antenna element.

Handy Crystal Radio Antenna

However, a suitable loading coil would be required to make it resonant at the desired frequency.

These are test results of a couple of low pass filters designated as 0 - 30 MHz and inexpensively available on AliExpress.

There are apparently at least two types - one is labelled 30M on the back and the other has a Rev 1 silk screened on it. The latter appears to be the better performer.

For testing I used a Red Pitaya StemLab 14 and set one of the outputs to sweep 0 - 50 Mhz. I set one of the FFT channels to look at 0 - 63 Mhz. I connected the generator output directly to the filter and connected a 30 dB 50 Ohm attenuator between the filter and the Red Pitaya input to simulate a 50 ohm dummy load. There is probably still some mismatch at the filter input, but I only had the one attenuator. Probably need to build another .....

Anyway, here are the…

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:      

1. Parallel-tuned, series-fed configuration:

Parallel-tuned, series-fed Crystal Radio circuit

The diode-clipped parallel-resonance voltage is output to the phones. 

High-impedance diodes and phones are required to match the high impedance of the

parallel-resonant circuit.