Household gas alarm circuit composed of gas sensor

Household gas alarm circuit composed of gas sensor

Shown is a household gas alarm circuit composed of a gas-sensitive sensor. In this circuit, QM-N10 is a gas-sensitive resistor .
The 220 V mains power is reduced to about 5.5 V through the power transformer Tl, which is used as the heating voltage of the gas-sensitive resistor QM-NIO. The resistance value of the gas-sensitive resistor QM-
NIO in clean air is about tens of kC1. When exposed to harmful gases, the resistance value drops sharply. It is connected in the circuit to increase the output voltage of the gas-sensitive resistor. A voltage is applied to the NAND gate. The NAND gates ICIA and ICIB form a gate control circuit, and ICIC and ICID form a multivibrator. When the QM-N10 gas sensor is not exposed to harmful gases, its resistance value is high and the output voltage is low, so that the ICIA pin ② is at a low potential and the ICIA pin ① is at a high potential, so the ICIA pin @ is at a high potential. After ICIB is inverted, its ④ pin is at low potential , the multivibrator does not start oscillating, and the transistor Vr2 is in a cut-off state, so the alarm circuit does not sound. Once QM-N10 is sensitive to harmful gases, the resistance value drops sharply. The voltage drop across resistors R2 and R3 causes ICIA pin ② to be at a high potential. At this time, ICIA pin ③ becomes low level and is inverted by ICIB. Then it becomes high level, the multivibrator starts to oscillate, and the transistor VT2 is turned on and off periodically, so the positive feedback oscillator composed of VT1, 12, C4, HTD, etc. works intermittently and sounds an alarm. At the same time, the light-emitting diode LED1 flashes, thereby achieving the purpose of warning of harmful gas leakage.