A measurement circuit in the output circuit of a sensor detects the polarity of a source to which a load is connected. The detection of one of the two polarities establishes one of two states which is stored in a storing element and the correct one of two drivers is switched to the output terminals. This state is maintained for as long as there is no change to the polarity at the output terminals. If the terminals are interchanged, then the measurement circuit correctly restores the stored state, so that the correct driver is associated with the load in each case. If positive instead of negative switching sensors are required, there is no need to replace the equipment. When the voltage is turned on, the equipment is initialized, so that it is set in accordance with the new circumstances.