A device for detection of a defined relative position of a first, movable means, relatively a second, fixed means and including a sensor (1) arranged at least at one of the first and second means which sensor is provided with a pressure sensing means (CC.sub.M). The sensor (1) gives rise to an electric detector signal, which indicates whether the defined relative position has been taken. The sensor (1) is also provided with a closed cavity (13) which communicates with a pressure sensing means (C.sub.M) and with means (14/36/45) to momentarily limit the volume of the cavity and thereby create a pressure pulse adapted to activate the pressure sensing means of the sensor. The detector device includes means to convert the pressure pulse to a detection signal to detect the defined relative position.