A piezo disk transducer is a handy part for reproducing beeps and boops, and can also function as a rudimentary microphone. Being a piezoelectric element, it can also generate usable power. Enough to run a radio transmitter? [b.kainka] is here to find out, with what may be the simplest possible transmitter circuit.
The active element in the circuit, such as it is, comes from a crystal. This functions as an extremely stable and high Q tuned circuit. When excited by a pulse of electricity, the circuit will carry oscillations in a similar manner to a bell ringing until the pulse is exhausted. A small lever fashioned from a piece of wire supplies the voltage by flexing the piezo disk and a contact, a diode discharges the reverse voltage as the disk returns to shape, and a small capacitor provides an AC path to ground. It works, if a small pulse of very low-power RF near the crystal’s frequency can be described as working.
It may not be the most practical transmitter, but it’s certainly something we’ve not seen before. It’s part of our 2025 Component Abuse Challenge, for which you still have time to make an entry yourself if you have one.
This is basically a spy bug circuit. Commonly used in different variations during the cold war.
But not that simple ;-) No modulation, not even a continuous signal, and extremely weak power on the verge of non-existence – what good would that do? Conventional bugs could at least modulate a continuous signal with audio from a microphone for eavesdropping.
And they drew a picture about miniaturization..
In West Germany, these circuits were nicknamed “Minispione” (mini spies).
There had been whole book series, sold freely in book stores and also available in public libraries.
The fascination about miniaturization and radio technology was the important bit.
Only a few citizens were so crazy to actually use them.
In practice, if at all, these little transmitter circuits were rather used on the ham bands, for fox hunting (foxoring, directional finding) or beacon purposes.
Or wireless thermometers, small 2m band walki talkies (in a matchbox)..
Such things, you know. Again, only the crazy, brave or dumb ones used them for intended purposes. Many years of eventually being “locked away” for spying wasn’t a pretty sight back then.
But I’m speaking under correction here. I heard stories on CB radio mainly, which isn’t exactly an reliable source. ;)