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 monitors extra-low frequency (ELF) fields. We found it in an American documentation from the...
We can use any type of resistive transducer with central resistance in the range of 10 k ohms to 1...
This circuit controls two lamps so that when one increases in brightness the other decreases. The...