Here is a schematic of a 555 setup to create a single shot voltage sawtooth pulse across the capacitor C2. It also outputs a square wave pulse on pin 3. A (reversed biased) Zener diode is used together with a another forward biased diode and a resistor to set the bias voltage for the external transistor, Q. My question is why is D1 needed? You can argue the added voltage drop is just enough to be added to the Zener voltage to create the required bias voltage, however a different Zener would achieve the same thing. I'm guessing it is compensating for something, but not sure what. Will be glad to hear suggestions.