Harmful gas alarm controller is a gas leak detection alarm instrument. When the flammable or toxic gas leaks in the environment, when the gas alarm detects that the gas concentration reaches the critical point set by the explosion or poisoning alarm, the flammable gas alarm will send out an alarm signal to remind the staff to take safety measures and drive the exhaust. Wind, cutting, and spraying systems can prevent explosions, fires, and poisoning accidents, thereby ensuring safe production. In the design of this suite, the knowledge of the two courses of "Analog Electronic Technology" and "Digital Electronic Technology" are used to comprehensively complete the course design. The circuit is shown as the toxic and harmful gas alarm circuit. It is composed of gas sensor, trigger and alarm sound circuit. The semiconductor gas sensor adopts the QM-2 type, which only needs a stable heating voltage, which is about 5V. This voltage value can be achieved by adjusting W1 and R1. In addition, the gas sensor can also be replaced with a sensor that is particularly sensitive to a certain harmful gas, and the heating voltage of different types of sensors is different. There is a warm-up time of about 1 minute during debugging. When the indoor air is normal, the corresponding resistance value of the gas sensor QM-2 is large, the voltage divided by this resistance and R2, W2 makes 555 set, 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, liquefied petroleum gas, gasoline, alcohol, smoke, etc., its internal resistance decreases, and the voltage divided by the resistance and R2, W2 increases, so that the 555 is reset, and the ③ pin The output is low level, and the relay J is pulled in accordingly, the alarm power circuit is connected, and an alarm signal is issued. LED2 (red light) flashes when alarming. The potentiometer W2 in the circuit is used to adjust the trigger level of pin ② of the trigger terminal of 555, which is about 3.5V under normal conditions. The relays in this circuit can also control external alarms or exhaust fans and other facilities