This circuit uses a resonant reed relay to recognize the frequency of an input signal. Adaptations can be made to obtain functioning as a shield. The circuit is from 1965 documentation.
The sensor for this circuit, found in a 1992 publication, is a phototransistor. The power is...
This circuit can generate signals of frequencies up to 1 or 2 MHz, tuned in the capacitor. The...
Suggested by Philips documentation from 1968, this circuit uses components from the era that can...