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 is from a technical documentation from Texas Instruments from 1978. In it we have the...
We found this circuit in an old documentation, but it is still current, being able to feed sets of...
This circuit, found in a 1995 documentation, makes the relay bistable control from pulses of light...