Using the LM3914 this unusual digital clock uses a Bargraph LED indicator. The LEDs are the segments of a 7-segment display. We found it in February 1980 Radio Electronics magazine.
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...