a simple but efficient way of creating a ramp up and down from a triangular waveform. Really handy if your LFO is a schmitt trigger as you already have the square wave to control the transistor. This isn’t from me it’s from an old Elektor magazine dealing with DIY synth (the formant which I encourage you to read ). However, the simplicity is rather interesting . It can also be used with simple power supply (V+/0V) to fit a small tremolo pedal project for instance. To do so, just replace ground with a proper bias (V+/2)
the limited number of parts can be ideal to add those waveforms in a small circuit.