There are two “extra” elements that I highly recommend adding to your circuit. The first is a power switch, which will let you turn on and off your synth without disconnecting the battery or opening your enclosure. The second addition is a simple volume control, which will let you change the overall volume of your synth.
Adding a power switch to your synthesizer is pretty straightforward. You will want to use a toggle switch or a latching pushbutton. “Latching” means that each press toggles between on and off, which is different from a momentary pushbutton that only makes a connection while you are holding the button down. While momentary pushbuttons are great for playing your voices like keys on a keyboard, they are not as usefull as on/off switches. Most power switches are toggle switches, which come in a few different forms…
A toggle switch simply makes or breaks a connection, letting you prevent the battery’s current from getting to the power bus, effectively turning your circuit off. Adding an LED that indicates whether the synth is on or off is also recommended. The diagram below shows how to wire the power switch and indicator LED. Using the same method, you can add multiple LEDs that you can use illuminate your synth from within, or provide light sources for your LDR (Light Dependant Resistors).
A logarithmic potentiometer (A10K) works well as a volume control. Logarithmic potentiometers have a marking that begins with an “A”, where linear potentiometers are marked with a “B”. Connect the 10k resistor from the synth chip to one of the outer legs of the potentiometer and connect the other outer leg to ground (-). The middle leg of the potentiometer will pass the attenuated signal — connect it to the signal leg of your output jack.