IR Receiver Sensors are photocells tuned to receive IR frequencies. The IR frequency is not visible to the human eye, but can be picked up by a webcam or cell phone. 

Try pressing a button on the Mini Remote Control or a TV/DVD-player remote and shining it at your webcam. The light you see emitted by the remote is Infrared light. These Infrared Signals are PWM signals carrying pulses (marks) and intervals (spaces) carrying 32 bits of data. 

The Experimenters Guide uses IRLib, an easy-to-understand Arduino library that de-mystifies the infrared light receiver. In the code section, we will go over installing this library and use it with this circuit.