4、oncetheconcentrationexceedsthealarmlimitvaluecanbeaudibleandvisualalarmsignal,openventilationdevices,andcanalarmvolumeincreasesastheconcentrationincreases,whileremotealarm,intimetoplayasecurityrole.Keywords:SIM900A; MCU; Gas detection; Alarm;Display;目录1.绪论11.1研究的