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.
This circuit is from the National Semiconductor Audio Handbook. The amplifier operates with 6 V...
This switched filter was found in the 1969 documentation. The FET should be current-matched and eventually...
This independently charged circuit was found in a 1969 EEE. The operational amplifier supports a...