Heat Detector and Siren Circuit Electronic.
Two transistors BC147 (T3) and BEL187 (T4) are joined in Darlington configuration to boost the sound from UM3561. Resistor R4 in series with a 3V zener is utilized to deliver the 3V supply to UM3561 when the relay is in activated state. LED1, joined in series with 68-ohm resistor R1 across resistor R4, glows once the siren/alarm is activated.