Viking Horns VPS-150/3 Air Pressure Switch, Rated 120/150, for Train Horns - Three unit Deal Ideal for air horns that require high air pressure Pressure Switch Ratings: on at 120 PSI, off at 150 PSI With 1/8" NPT male thread, and 1/4" male electric spades, This offer is for a 3 unit pack of air pressure switches For 12 volt power source