Knowing whether a room is occupied or not is crucial for improving electrical energy efficiency. For instance, if a given room is empty there is no need for the lights to be turned on. Usually in small rooms such as elevator halls, a PIR sensor is used together with the lighting, but as it lacks of precision, people often are left in the dark after a few minutes. In the present work a device capable of counting people in a room is implemented and analyzed. It is also compared with other solutions.