by Maurizio @ dev.emcelettronica.com:
Although a remote control is not so difficult to design and build, there is an even simpler option: the IR barrier. This consists of a constant IR signal permanently going from an emitter to a receiver, both of them being in two different places (a few meters apart). When an obstacle comes in between the emitter and the receiver, the IR signal is blocked and the receiver senses that it is missing, flagging the event in an appropriate manner (figure 1).
The principle of operation is simple enough, and with most components taken off the shelf, it is easy to design the schematic. The IR element in the emitter is a simple IR LED. This behaves absolutely like any other LED, with the sole exception that it emits light in the infrared spectrum. The emitted light is thus invisible to human eye, but it is meant to excite the infrared receiving element on the other side of the barrier. One important element of this LED is the wavelength of the emitted light, of which we already know that is in the infrared spectrum.
Simple Infrared Barrier – [Link]