This circuit can be powered by 5-7.5V DC power supply, switch S1 is the power switch, resistor R1 and LED D1 are used for power indication.
The internal sensor MQ3 needs to be heated to a certain temperature during operation, so its internal installation.
There is a heating wire, the resistor R4 is the current limiting resistor of the heating wire, and the heating of MQ-3 is normal after power-on.
When MQ-3 does not detect alcohol vapor, its internal resistance is very large.
The voltage drop of R3, RW is very low, the voltage of pin 2 of NE555 is lower than the trigger voltage (VCC/3).
Its 3 pin output is high level, D2 is extinguished. The buzzer does not sound and the relay does not move.
When MQ-3 detects alcohol vapor, the internal resistance decreases, the voltage of R3, RW rises;
The voltage of pin 2 of NE555 is higher than the trigger voltage;
the 3 pin becomes low level, D2 turns on the light, and the buzzer flows.
The current starts to sound, and the electromagnetic coil in the relay energizes to attract the switch.
Thereby implementing the detection alarm function.
The adjustable resistor RW is used to adjust the detection sensitivity.
This circuit uses a second comparator inside the NE555 to detect voltage changes, using its internal trigger to debounce.
The 7 and 5 feet of the NE555 are not used and are left floating.
The output terminal J3 of the relay can be used to control other electrical appliances, such as exhaust fans.
When the alcohol concentration exceeds the standard, the exhaust fan is activated.
After the concentration drops, the exhaust fan is turned off to realize automatic control.
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 the line can be directly at the J2 position.
Pay attention to the positive and negative signs, and the DC plug is internally and externally negative.
(Please kindly noted: we have made a mistake of the LEDs between D2 and D1. D2 should be Red LED, D1 should be Green LED. Thanks for Stefano's report.)
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