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 found in a French documentation from 1982 can be applied to any operator. The...
In this circuit of the 1980s, the T1 transformer has 30 turns of 28 primary wire and the secondary...
Found in American documentation from the 90s, this circuit triggers a buzzer with a Sonalert-type...