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 source for devices for automotive use was found in a 1985 documentation. The power...
This circuit trips when the voltage set in the pot is exceeded. The tone depends on C3 and...
The circuit shown is from the 1977 GE semiconductor manual. The circuit uses a controlled silicon...