• 3 digital pitch reference tones for tuning analog synths, generated by an Arduino nano. (pretty stable)
• 5V reference for having constant current
• 2 passive attenuators
• 1-3 multiple

The Arduino code:

void setup() {
}
void loop() {

noTone(5);
tone(5, 440, 117);
delay(117);

noTone(6);
tone(6, 880, 117);
delay(117);

noTone(9);
tone(9, 1760, 117);
delay(117);

noTone(11);
tone(9, 1760, 117);
delay(117);
}