This listing is for ITF-20001 opto interrupters. On the top side of these sensors are two IR LEDs; one is an emitter, and one is a detector. When an object comes within a close proximity, the light emitted from the emitter is reflected to the detector, which acts as an NPN transistor. When the detector detects the reflected infrared beam, the photo transistor is activated. The “Emitter” is an infrared emitting diode, and the detector is an NPN photo transistor. Use them in security circuits, counter circuits, etc! They’re really easy to use!