MQ-7 is a Carbon Monoxide (CO) sensor, suitable for sensing Carbon Monoxide concentrations(PPM) in the air. The MQ-7 sensor can meassure CO concentrations ranging from 20 to 2000ppm. This sensor has a high sensitivity and fast response time. The sensor’s output is an analog resistance. The drive circuit is very simple , just a voltage devider; all you need to do is power the heater coil with 5V DC or AC , add a load resistance, and connect the output to an ADC or a simple OPAMP comparator.