A CO2 sensor with relay output is a device used to measure the concentration of carbon dioxide in the air. It is typically used in indoor air quality monitoring systems, as well as in industrial and commercial settings. The sensor works by detecting the presence of CO2 molecules in the air and then sending a signal to a relay output. This signal is then used to activate a device, such as a fan or air purifier, to help reduce the amount of CO2 in the air. The sensor is also used to monitor the air quality in areas where CO2 levels are too high, such as in greenhouses or enclosed spaces. By having a CO2 sensor with relay output, businesses and homeowners can ensure that their indoor air quality is safe and healthy.