The voltage required to ignite the neon tube is obtained by using an ordinary filament transformer (240-6 V) in reverse. Battery drain is quite low, around 1 to 2 milliamps for a nine volt battery. The pulses from Ql, unijunction transistor, operated as a relaxation oscillator and are applied to Q2 which in turn drives Q3 into saturation. The sharp rise in current through the 6 V winding of the transformer as Q3 goes into saturation induces a high voltage in the secondary winding causing the neon to flash
The diode D1 protects the transistor from high voltage spikes generated when switching currents in the transformer.