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.
The integrated circuit used in this project is not common today, but it can be implemented with more...
Found in an American publication from 1969, this circuit can be adapted to be used as a shield....
This current meter with many scales was found in a 80s American documentation. The operational amplifier...