Smoke alarm circuit (1)

Circuit working principle: H21A1 (lC1) is an infrared optical switch, which is actually
a photointerrupter. R1 limits the current through the LED in IC 1 . The transistor part of IC1 is in a conducting state under normal circumstances, so the base of VT1 is at a low potential. The preset potentiometer Rp determines the threshold level for circuit triggering, and R2 provides additional protection functions for VT1 and IC 1.