The frequency of each note depends on R2, which must be replaced by a keyboard. The circuit is from Radio Electronics from April 1980. The power supply is 9 V for the amplifier used.
In the figure we have an improved method of varying the voltage of a 7812 regulator. The circuit is...
This 5-range current measurement circuit was found in the September 1989 issue of Radio...
Found in an old English publication this circuit uses a dual gate MOSFET. Coils not specified....