This circuit was found in the Electronics Experimenter Handbook from 1991. The sensor is a diode and the circuit has a relay according to the voltage used in the supply.
The presented differential amplifier, indicated for instrumentation, was obtained in the 1994...
This circuit provides a signal when dialing occurs. The circuit is powered by the telephone line...
This circuit uses two TTL trigger inverters to generate signals that can be adjusted at R1. The...