Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine. The circuit must be assembled in a shielded box.
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....