Receiver monitor

The alarm plugs into the earphone jack on a receiver. Then when a signal (normally fed to the headphones) is detected and applied to the gate of SCR1, it conducts, sounding whatever alarm is connected to SOI. The signaling device can be an audible alarm or a lamp.
Receiver monitor  - schematic

Variable resistor Rl functions as a sensitivity control so that background noises won't trigger the alarm.

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