This simple logic probe circuit was found in the August 1980 issue of Radio Electronics magazine. The circuit works with both TTL and CMOS logic.
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...