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.
This circuit for testing old analog lines was published in April 1986 Radio Electronics magazine. The...
We found this circuit in a 1997 publication, but by the components used it can still be assembled today...
For those who want to recover an old multimeter of this type, the circuit presented is worth...