As shown in the figure, the toxic and harmful gas alarm circuit. The circuit consists of a step-down rectification and voltage regulator circuit, a gas sensor, a trigger and an alarm sound circuit. The step-down rectification and voltage stabilization circuit is composed of a transformer B, a rectifier circuit D1~D4, an integrated voltage regulator block 7812 and the like. The semiconductor gas sensor uses the QM211 type, which requires only a stable heating voltage of about 5 V. This voltage value can be realized by adjusting W1 and R1. In addition, the gas sensor can also be replaced with a sensor that is sensitive to certain harmful gases, and the heating voltages of different types of sensors are different. There is a warm-up time of about 1 minute during commissioning.
When the indoor air is normal, the corresponding resistance value of the gas sensor QM211 is large, the voltage after the voltage is divided by R2 and W2 sets 555, the 3 pin outputs a high level, the alarm circuit is disconnected, and no alarm occurs. When the gas sensor detects toxic and harmful gases such as gas, petroleum liquefied petroleum gas, gasoline, alcohol, smoke, etc., its internal resistance decreases, and the voltage after the voltage is divided by R2 and W2 increases, thereby resetting 555, 3 feet The output is low level, and accordingly, the relay J is pulled in, the alarm power supply circuit is turned on, and an alarm signal is issued. LED2 (red light) flashes when an alarm occurs. The potentiometer W2 in the circuit is used to adjust the trigger level of the trigger pin 2 of the 555, which is about 3.5V under normal conditions.
In the figure, A and B are connected to the alarm circuit. The circuit can be equipped with an alarm composed of 555 circuits, and a dedicated audio integrated circuit such as KD9561 can also be selected. The relays in this circuit can also control external alarms or exhaust fans.
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