Found in documentation two years 80, this toy instrument uses an integrated circuit little commonly. As notes are given the values of key series resistors on the keyboard.
The circuit below shows how to use an LM139 comparator or series equivalent as an amplifier. The...
We found this receiver circuit for modulated light in a 90s Popular Electronics. Equivalent audio...
The circuit was found in the February 1992 Radio Electronics magazine. The scale can be changed by...