This circuit that generates signals between 1.5 and 2 kHz was found in a documentation from the 90's. The capacitor C1 can be changed to obtain other frequencies.
This circuit is from a Radio Electronics from March 1970. The crystal depends on the frequency at which...
This circuit detects magnetic fields through a sensor coil, providing a signal in its function. The...
Described in an article that taught how to assemble the complete intercom in Radio Electronics magazine...