The spark developed by the circuit is suitable for a gas ignitor. Capacitor Cl is charged through Rl and Dl toward peak line voltage. C2 is simultaneously being charged at a slower rate through R5. When the charge on C2 is sufficient.to trigger the PUT, the SCR is triggered on, providing a rapid discharge path for Cl through the transformer primary. The SCR is triggered about 20 times per second with the component values shown.
The Ll4G3 serves as a flame sensor. When ignition is achieved and sensed by the photodetector, the low Vc8 prevents further SCR triggering.