I've found the circuit below in "The Art of Electronics" book.
This circuit generates sawtooth signal. This is some of what is written about it:
By using a current source to charge the timing capacitor, you can make a ramp (or "sawtooth-wave") generator. Figure 5.35 shows how, using a simple pnp current source. The ramp charges to (2/3)Vcc, then discharges rapidly (through the 555's npn discharge transistor, pin 7) to (1/3)Vcc, beginning the ramp cycle anew. Note that the ramp waveform appears on the capacitor terminal and must be buffered with an op- amp since it is at high impedance.
I have simulated the circuit and it works well giving a sawtooth output between (1/3)Vcc and (2/3)Vcc.
I have two important questions about this circuit:
- How to control the frequency of this circuit? What is the formula that can be used for this?
- Is there a way to make the sawtooth signal starts from zero (or somewhere else) rather than (1/3)Vcc?