This two-part device has one infrared sending unit and one receiver. Place them up to 4" 10cm apart (10" 25cm when used in conjunction with 189-5961 and 5963 timers). When a train interrupts the invisible infrared beam they transmit a signal which can be used to activate relays, timers and signal circuits. 3/4 x 3-5/8 x 3/4" 1.8 x 9 x 2cm