This circuit makes use of a specific 80's component and was found in an American manual of the time. A matrix keyboard allows you to select one of several music programs.
July 2017 - Mouser Electronics announced the availability of Littelfuse's GEN2 series Schottky...
This configuration is suggested by Mullard (Philips) in an old manual of 555 integrated circuits....
We found this circuit in a Popular Electronics of the 90s. Its purpose is to detect when it locks...