The Neon Bulb

The new circuit element we will add to our neon oscillator circuit is the common neon bulb which glows orange and is often used as a night light. The bulb, which we will designate by the symbol  is simply a small glass tube with neon gas inside and two wires as shown in Figure (1). The bulb turns on when there is a large enough voltage difference between the wires that the neon gas becomes ionized and starts to glow. For typical neon bulbs, the glow starts when the voltage reaches approximately 100 volts. When the bulb is glowing the neon gas is a good conductor and the bulb is like a closed switch. When the bulb is not glowing, the gas is inert and the bulb is like an open switch.
A given neon bulb has a rather consistent voltage V_f (firing voltage) at which it turns on, and voltage V_q (quenching voltage) at which it shuts off. In a typical bulb Vf may be 100 volts and V_q equal to 40 volts. These numbers will, however, vary from bulb to bulb. When a neon bulb is included in a circuit, it acts like an automatic switch, closing (turning on) when the voltage across it reaches V_f and opening (shutting off) when the voltage drops to V_q.

Figure 1: A neon bulb. When the voltage across the wires reaches a threshold value, typically around 100 volts, the neon gas starts to glow, and the gas suddenly changes from an insulator to a conductor.