We found this circuit for musical instruments in the March 1992 Radio Electronics magazine. The circuit has outputs according to the desired scale from a crystal oscillator at the indicated frequency.
This circuit simulates a resistor controlled by digital signals. We found it in American...
Found in Popular Electronics this circuit will trigger a buzzer if the ignition key is removed...
We find this circuit in documentation from the 1980s. The triac and diac are of conventional types...