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 shield circuit is suggested by Texas Instruments and can employ almost any optical coupler...
In the figure we have a monostable that is activated with switch S1 and pressed for an instant. The...
This circuit for alternating current was found in a 1965 General Electric documentation. The...