This circuit with TTL compatible output was found in a 1968 US documentation. The circuit is powered by 5 V and the frequency can be changed by changing the crystal.
This ultra-simple meter uses an output transformer and a small speaker as a microphone. The...
We found this circuit in a 1970’s Philips documentation. The circuit uses components from that time....
Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine....