In order to provide immunity against vibrations, a pressure sensor for detecting the prevailing manifold suction pressure in an internal combustion engine, for example, has a movable chamber which responds to the varying pressure and thereby changes the distance between a diffuse reflector and an optical detector assembly. The detector assembly includes a light source, a first light-sensitive detector and a second light-sensitive detector. The second detector is connected to a regulator which so controls the electrical power to the light source that the induced photo current in the second detector remains substantially constant independently of the distance between the reflector and the detectors. A number of embodiments is presented.