This circuit works with the previous two by demodulating the signals received modulated in FM. The circuit is from the Radio Electronics magazine of December 1992.
In this circuit, obtained from a 1979 publication, IC1 is a 555 and IC2 is a 4020. The transistor is the...
The presented circuit uses a window comparator to keep a relay activated only when the sensor is...
Similar circuits can be found on this site. This is from an 80's American documentation. The...