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.
On this website we show how to calculate the components of this circuit for other frequencies. The basic...
We found this circuit in an English publication from 1980. The transistor can be either the BC548...
Found in Popular Electronics from the 90s, this circuit mutes a scanner's speaker in the absence of a...