The classic waveform generator made with OpAmps.
The first block is a Schmitt trigger which generates a square wave with its positive feedback; this signal enters the second stage, an integrator made with a capacitor as a negative feedback, which integers the square wave and generates a triangular wave. By changing the 1.3k resistor you regulate the gain, while the DC voltage source is used to regulate the output offset.
I've insert a two positions switch to try the noise injiection into the loop with a smartphone shake, suggested by mr. Selman (see comments below).
