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.
The simple optical intercom we show is from a 1995 documentation. The LEDs can be infrared or...
With this circuit it is possible to monitor a signal that is being recorded on an old tape...
An oscillator that can be frequency controlled by a resistive sensor is shown in the figure. The...