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.
Originally this circuit was indicated to be used in a garage door control, but it can be modified for...
This conventional oscillator with the 741 can be precisely adjusted to produce a 400 Hz signal. The...
This current-limited circuit was found in a Popular Electronics from the 90's. ...