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.
We found this circuit in an American publication from the 90s. The power must be 2 V more than the...
Found in old American documentation, this circuit makes use of operational amplifiers from the 70s...
This linear LED scale circuit was found in the February 1992 issue of Radio Electronics magazine. Note...