Gas alarm circuit (2)

Circuit working principle: This alarm circuit
consists of four parts: power supply, sensing, delay and alarm circuits. When the gas sensor tube comes into contact with gas containing a certain concentration,
the internal resistance at both ends of the measuring electrode AB decreases, and the voltage at both ends of Rp1 increases. This voltage controls the conduction of VT after being delayed by R2, R4,
VD7 and C3.