This circuit was found in the September 1984 issue of Radio Electronics magazine. The circuit is based on a 555 and a MOSFET consisting of a flasher that controls a buzzer.
With the shadow over the sensor, it is possible to interrupt the sound of your amplifier using this...
This circuit provides a signal when dialing occurs. The circuit is powered by the telephone line...
We found this circuit in a 1995 documentation. It can measure frequencies from 100 kHz to over 100 MHz...