Abstract: A pneumatic switch located within a tire emits a pulse of air from the tire when the tire pressure drops a preset amount below the normal operating pressure. A bellows in the switch is charged to normal operating pressure and when tire pressure drops, the bellows extends to open a valve to emit the air pulse to actuate a warning system. A valve in the bellows is opened manually to establish the pressure charge within the bellows at the level of the tire pressure when the tire is inflated. The same valve in the bellows is automatically opened during tire deflation after the air pulse is emitted to equalize the bellows charge with the lower tire pressure in order to avoid a pressure differential across the bellows sufficient to damage the bellows.

Abstract: An air switch senses abnormally low pressure in a tire and provides a pneumatic signal to an actuator which moves a magnet from a normal position to a warning position. An electrical detection system senses that the magnet is in a warning position and provides a low pressure warning signal to the vehicle operator. The actuator contains a mechanical latch to assure that the magnet remains in a warning position even if the pneumatic signal should cease. A reset feature allows the latch to be disengaged to return the actuating mechanism to normal condition when proper tire inflation pressure has been restored.