Which devices can you use?... Transistors?
Of course, there will be a need for a sort of a bistable oscillator (made by transistors for example) which also generates a triangular or sawtooth (half triangular) signal.
The comparator might be a simple transistor amplifier, better if it is made as a schmitt trigger type (this needs 2 transistors) to get sharp output edges.
For the sawtooth, I found this:
Sawtooth Generator (Oscillator) with Transistors | Simple Circuit Diagram
Added:
Please note that these transistor oscillators suffer from a relatively high sensitivity to temperature.
Kerim