This circuit was found in the September 1971 Radio Electronics magazine. The buzzer can be replaced by a relay according to the supply voltage. R1 adjusts the sensitivity.
Found in Popular Electronics from the 1980s, this circuit can use more modern transistors such as...
This circuit is from a Radio Electronics from February 1970. The integrated circuit is an...
This is stage is transformer coupled and was found in na old publication of the year 1958. The...