Room air sensor for fan control, measuring CO2 concentration, relative humidity, and temperature. Features a visual display via an LED traffic light system with adjustable threshold values. It includes two CO2-controlled switching outputs for two-stage control, with both manual and automatic operation and a switching status display. There are three 0-10 V outputs for CO2, temperature, and relative humidity. Simple retrofitting of uncontrolled ventilation systems is possible. The green display indicates the first threshold value adjustable from 500 to 900 ppm, the yellow display indicates the second threshold value adjustable from 1100 to 1500 ppm, and the red display indicates levels above the second threshold value. A suitable power supply unit for flush-mounted installation is optionally available. This sensor is suitable for schools, kindergartens, offices, conference or meeting rooms, and living rooms.