This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
This circuit locks after a short touch on the sensor. The circuit can be powered by voltages from 5...
May 2017 - Torex's XBP14E5UFN, an array of TVS circuit protection diodes with only 0.8 pF of...
This circuit triggers oscillator 555 when the supply voltage drops below the value set in the pot....