To increase operator safety in robotic working cells, a capacitive sensor for the detection of humans has been developed. The sensor consists of two electrically conducting planes, insulated electrically from the ambient. When the dielectric or conductive conditions in the space between the planes are altered, e.g. a person enters, the sensor will indicate the presence of the person. The strength of the electric field caused by the sensor is kept low by lock-in amplifier technique. In the article the basic detection principle, the sensor electronics and the sensor signal handling are described and some practical experiments are presented.