Development Of CO/CH₄Gas Alarm

In order to cope with environmental degradation and shortage of fossil fuels,flammable gas, mainly natural gas, is gradually applied in industry and our daily lifeinstead of the traditional fuel widely. The combustible gas has many advantages, such assmall pollution, abundant natural reserves, high combustion value and low price. In aword, it can bring a lot of convenience to people’s lives and production. However thecombustible gas are hard to store and transport, and prone to leak, thereby it lead to fire orexplosion, life and property of people will be threaten. Therefore, combustible gas in theenvironment should always be monitored in real-time, and should be detected andalarmed on time to ensure safe use of combustible gas. Therefore, the gas alarm which isstable and detection is accurate should be manufactured.At present, most of the combustible gas alarm can be detected a single gas on thedomestic market, there are several limitations when they are used. The alarm threshold isgas explosion limitation ignoring the security risks when the gas at low concentration. Forexample, CO can make people coma or even die when it does not reach explosivelimitation. In this paper, CO/CH₄gas alarm can be designed, with the gas sensor andmicrocontroller as the core.CO/CH₄composite alarm controls heating circuit by microcontroller to generate0.2Vand0.9V heater voltage periodically, which is used to drive the combustible gas sensor.Once the gas concentration exceeds the preset threshold, microcontroller would controlalarm unit to make alarm. At the same time, CO/CH₄composite alarm has self-testcapabilities to find the fault which makes the alarm not to work properly. Besides thealarm set up a communication interface which makes upgrading the system convenient.The hardware of CO/CH₄composite alarm consists mainly of the gas sensor, hetemperature sensor, the driving circuit, the A/D conversion circuit, microcontroller, thetest button and the alarm unit and so on.The software of CO/CH₄composite alarm is designed to make the driver modular,including fault self-detection, gas concentration collection, temperature collection, dataprocessing and alarm logic determine. Breakdown self-detection is accomplished by monitoring sensor driving voltage of real-time. Gas concentrations collection unitcontains the driving circuit of the sensor and A/D conversion, the former is accomplishedby the microcontroller to control the drive circuit and the latter accomplished by A/D chip.Data processing contains digital filter for the gas concentration information andtemperature compensation, in order to correcting data value, and make the alarm reliable.If it does exceed the alarm threshold, the microcontroller would make the alarm unitaction. The alarm has two alarm thresholds, the lower alarm threshold is10%LEL（Lower Explosion Limited）, and the higher alarm threshold is20%LEL. In addition, thepaper has involved in the electromagnetic compatibility of the alarm, and analyzed noisewhich may interfere with the alarm and made a solution, in order to improve thereliability and stability of the alarm.The schematic and Printed Circuit Board of the CO/CH₄composite alarm has beenfinished and the experimental prototype has been assembled. By testing the alarm at aCH₄/CO environment, it can be make sure that the CO/CH₄composite alarm basicallymeet the design requirements, and realize the capability of gas detection and alarm.