Modulation and LFOs
One of the most common ways to add to the basic synthesizer circuit is to add modulation, in this case an LFO (low frequency oscillator). Modulation is a general term describing a changing the property of sound over time. Modulation requires a source signal called a modulator that controls another signal called a carrier. Modulating sounds can add a sense of motion, dimension, and depth.
The simplest way to add modulation to our hex schmitt circuits is to use one voice to modulate the pitch of another voice, producing a changing pitch rather than a static pitch. Here is how that looks.
In the diagram above the voice that uses pins 5 & 6 is modulating the voice that uses pins 1 & 2. The result is a changing pitch for which you can change the base pitch of the tones and/or the speed of the oscillation.