An air pressure-controlled electrical switch responsive to a change of pressure either above or below a predetermined desired pressure in a vehicle tire, to actuate a magnetically activated system for signaling such tire pressure change. The switch has piston means subject to one end to air pressure in a tire. The piston means is movable from an intermediate position where it normally is maintained by spring pressure and an opposed force imparted by the predetermined desired internal tire pressure which act i opposition on the piston means. The switch is actuated by movement of the piston means in one direction or the other from the intermediate position. The switch is normally closed when the piston means is in the intermediate position. The switch opens when the piston moves in one direction or the other from the intermediate position upon increase or reduction of the tire pressure from the predetermined desired value. The switch is connected electrically with a magnetically activated electronic signaling system.