Transformer T1 supplies power to the heater of the sensor. Since the sensor is fairly sensitive to heater voltage, diodes Da, D4, and D5 regulate the heater voltage. Tl, together with D1 and CZ, forms a de power supply, whose current is regulated by Q1 and adjusted by Rl. The constant current from Ql feeds a variable resistance, consisting of thermistor R7 and the parallel combination of R5 and the sensor resistance.
When a hazard causes the voltage at A-A to drop, the net voltage at the SCR gate turns positive, triggering the SCR on and operating the alarm. The alarm draws a small amount of current, so the battery will last a long time. Switch S1 turns off the alarm and resets the SCR.