This sensitive bio-monitoring circuit was found in the December 1984 issue of Radio Electronics magazine. The circuit is powered by a 9 V battery and uses a low-impedance headphone output.
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...