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.
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit has a...
This simple circuit indicates if the polarity of a load is reversed when it is connected to a...