Generating sawtooth wave signal can be implemented using several ways, and one of the very popular method is by using constant current source to charge a capacitor. If you’ve seen many method that use an SCR or its bipolar transistor equivalent circuit to discharge the capacitor, here you can see an alternative method using 555 IC. Here is the schematic diagram of the circuit:
The output pin (pin 3) is connected to pin 5 through a diode, and this configuration make one input the internal comparator is forced down to almost zero level at the discharging cycle, forcing the capacitor to discharge below this level to switch the state to the charging cycle. The period of this sawtooth wave signal is equal to the value of C capacitor and the value of R resistor. You should keep the current through R fairly low, since the current will be shorted to ground at the discharging cycle of the capacitor. The current can be computeb by the formula: i=(Vz-Vbe)/R. Where Vz is the zener diode voltage (2.7V) and Vbe is the forward bias base-emitter voltage (0.7V) of the transistor. Keep the current below 20 mA for the best result.
