This conventional relay light detector circuit uses a 741 as a voltage comparator. The sensitivity adjustment is made in R1. The supply voltage depends on the relay used.
Found in the 1965 Radio Shack documentation, this short-range AM circuit can be built with...
This circuit was found in a 1980s documentation. It is intended for remote vibration monitoring...
This circuit can be found on this website in similar versions. The power supply is given in...