We found this circuit in a 90s Popular Electronics. The circuit uses a pickup coil that consists of 100 to 200 turns of wire 28 around the cable to be monitored.
This circuit is from a 1967 documentation. It consists of a sensor for memories used at the time....
We found this circuit in the November 1992 issue of Radio Electronics magazine. The circuit has...
Found in EEE documentation, this circuit can have the duration and frequency adjusted separately....