This crystal-based circuit can generate accurate 50 or 60 Hz signals. The circuit is from a November 1998 Electronics Now magazine. The output signal consists of rectangular pulses.
General Electric suggests this circuit in a 1980s documentation. The operational of the time is...
This circuit, from a 1973 publication, locks the relay when the light that falls on the LDR is cut...
This circuit can be adapted to an FM radio or other receiver that does not have a tuning indicator LED....