This circuit can be powered by a 5-7.5 volt DC power supply. The switch S1 is a power switch. The resistor R1 and the light-emitting diode D1 are used for power indication. The sensor MQ-3 needs to be heated to a certain temperature during operation, so there is a section of electric heating installed inside it The wire and resistor R4 are the current-limiting resistance of the heating wire. It is normal for the MQ-3 to generate heat after being energized. When MQ-3 does not detect alcohol vapor, its internal resistance is very large, the voltage drop of R3 and RW is very low, the voltage of pin 2 of NE555 is lower than the trigger voltage (VCC/3), its pin 3 outputs high level, D2 goes out , The buzzer does not sound, and the relay does not operate. When MQ-3 detects alcohol vapor, the internal resistance drops, the voltage of R3 and RW rises, the voltage of pin 2 of NE555 is higher than the trigger voltage, pin 3 becomes low level, D2 turns on and emits light, and the buzzer flows The current starts to sound, and the electromagnetic coil in the relay is energized to attract the switching action. In order to realize the detection and alarm function. Adjustable resistor RW is used to adjust the detection sensitivity. This circuit uses the second comparator inside NE555 to detect the voltage change and uses its internal trigger to debounce. Pin 7 and pin 5 of NE555 are not used and are in a floating state. The output terminal J3 of the relay can be used to control other electrical appliances, such as an exhaust fan. When the alcohol concentration exceeds the standard, the exhaust fan is started. After the concentration drops, the exhaust fan is turned off to realize automatic control. The diode D3 is used to eliminate the induced electromotive force generated when the relay coil is turned on and off. The power supply interface can be a DC plug, or you can use a wire directly at the J2 position, pay attention to the positive and negative signs, the DC plug power supply is positive and negative.

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