Many wireless devices transmit and receive energy at infrared (IR) wavelengths, rather than at radio wavelengths. Infrared energy has a frequency higher than that of radio waves, but lower than that of visible light.

Infrared is sometimes called heat radiation, but this is a misnomer. Some wireless devices transmit and receive their signals in the visible-light range, although these are encountered much less often than IR devices.

The most common IR transmitting transducer is the infrared-emitting diode (IRED). A fluctuating direct current is applied to the IRED. The current causes the device to emit IR rays; the fluctuations in the current constitute the modulation, and produce rapid variations in the intensity of the rays emitted by the semiconductor junction.

The modulation contains information, such as which channel your television set should seek, or whether the volume is to be raised or lowered. Infrared energy is not visible, but at some wavelengths it can be focused by ordinary optical lenses and reflected by ordinary optical mirrors.

This makes it possible to collimate IR rays (make them essentially parallel) so they can be transmitted for distances up to several hundred feet. Infrared receiving transducers resemble photodiodes or photovoltaic cells.

The only real difference is that the diodes are maximally sensitive in the IR, rather than in the visible, part of the electromagnetic spectrum. The fluctuating IR energy from the transmitter strikes the P/N junction of the receiving diode.

If the receiving device is a photodiode, a current is applied to it, and this current varies rapidly in accordance with the signal waveform on the IR beam from the transmitter. If the receiving device is a photovoltaic cell, it produces the fluctuating current all by itself, without the need for an external power supply.

In either case, the current fluctuations are weak, and must be amplified before they are delivered to whatever equipment (television set, garage door, oven, security system, etc.) is controlled by the wireless system.

Infrared wireless devices work best on a line of sight, that is, when the transmitting and receiving transducers are located so the rays can travel without encountering any obstructions. You have probably noticed this when using television remote control boxes, most of which work at IR wavelengths.

Sometimes enough energy will bounce off the walls or ceiling of a room to let you change the channel when the remote box is not on a direct line of sight with the television set. But the best range is obtained by making sure you and the television set can “see” each other.

You cannot put an IR control box in your pants pocket and expect it to work. Radio and IR control boxes are often mistaken for one another because they look alike to the casual observer.

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