Generating a Low frequency triangle wave
Commonly used is this circuit: -



The integrator ramps up and down. When ramping up, the output voltage reaches the upper switching threshold of the comparator (using positive feedback to create hysteresis) and the comparator then changes state forcing the integrator to ramp down until the output reaches the lower switching threshold and the process repeats.

R1 and R2 dictate the hysteresis levels and therefore the amplitude of the triangle wave but the slope of the triangle wave is controlled by Rt and C.

If you increase the hysteresis, the frequency falls unless you re-adjust Rt (or C) to make the ramping rate faster.

It's cheap and cheerful but produces a very nicely shaped triangle waveform.