VCO by nor gate
Electronic
74HC02

Now, we're going to assemble a tone generator that emits a tone in varying height of pitch as you turn the control volume. Tone generators of this type are used as the sound source of music synthesizers and electronic organs.

This tone generator uses the VCO made up by NOR gate. The oscillation frequency is determined by the value "C1 + (R + control volume)".

So, the oscillation frequency varies when the control volume is turned. Now, let's see how this VCO works.

Turn power ON first, put the earphone in you ear, and turn the control volume. You'll note that the tone becomes lower when you turn the control volume clockwise, and higher when you turn the control volume counterclockwise.