This circuit was found in the March 1982 Radio Electronics magazine. The component is a special circuit that generates programmed melodies selected by the switches.
In the figure we have the way to use a stepper motor as an encoder. Operational amplifiers can be of...
Placed inside the refrigerator, this circuit triggers an oscillator when the part is opened...
We found this circuit in an old documentation from the 70s, but it can be assembled with more...