The invention relates to a device for detecting the position of a mobile pneumatic piston to which a permanent magnet is attached. The device can unambiguously distinguish the presence of the piston in the final positions of its path. The device in based on the use of amorphous ferromagnetic threads which change their impedance when they are under the influence of a magnetic field. Two sensor elements are placed on the arms of a Wheatstone bridge. The position of the pneumatic piston is determined by comparing the relative phase between the input signal and the signal on the bridge.