The cross-beam photogate add-on converts either a transistor photogate (TPG) or a Schmitt trigger photogate (SPG1) to a crossed-beam photogate. The crossed-beam photogate is a pair of infrared photogates oriented at right angles to each other.  Each photogate contains a phototransistor (PT) and light-emitting diode (LED) in separate housings.  This type of photogate can be used to automatically discharge a flash unit or actuate a camera shutter when an object passes through the beam intersection.  This makes it useful for photographing objects moving toward or away from a stationary camera.  If a camera is centered and focused at the beam crossing, the flash unit will be triggered only when the object moves through the point of focus.

When ordering the CBP, be sure to also order a TPG or a SPG1 if you don't already have one of these. Note that the SPG2 variety of photogate is not designed for crossed-beam use.

The CBP includes a PT-LED pair, a 6 foot length of 3-conductor cable, several pieces of hookup wire, and wiring instructions to complement those for the TPG or SPG1.