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.
This circuit was found in an application note of the LTC1164 from Linear Technology of the 1990s....
This micropower inverter serves to obtain high voltages with very low current from direct current...
This circuit shows how to get 5 V under low current from an operating amplifier 741. I got this...