This simple electronic music box circuit was found in the April 1985 issue of Radio Electronics. The components are common, and the note adjustments are made with trimpots.
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...