Image Number 2 for United States Patent #4402382.
A robust air gun fire time sensor consists of a housing to which is secured a protruding nonmagnetic internal barrel having a bore. The bore is closed at one end by a magnetic end cap defining an air chamber and the bore is in fluid communication with the firing chamber of an air gun. A generator coil surrounds the barrel. A bar magnet is floatingly mounted inside the bore and is urged against a shoulder formed in the end cap. When the air gun is charged with compressed air, some of the air becomes entrapped in the air chamber. When the air gun is fired, the entrapped compressed air in the air chamber pushes the bar magnet away from the shoulder. The collapsing magnetic field creates a transient electric pulse in the generator coil.