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With most pieces of consumer electronics, from camcorders to stereo equipment, an infrared remote control is usually always included. Video and audio apparatus, computers and also lighting installations nowadays often operate on infra-red remote control.The carrier frequency of such infra-red signals is typically in the order of around 36 kHz. The control codes are sent in serial format modulated to that 36 kHz carrier frequecy (usally by turning the carrier on and off). There are many different coding systems in use, and generally different manufacturers use different codes and different data rates for transmission.
"IR" stands for infrared. Infrared light is invisible since its frequency is below that of visible red. Otherwise, it is like any other light source, operating under the same laws of physics. In most cases, the IR signals are produced by an LED source.
TV remotes send commands only one way, in a low-speed burst for distances of up to 30 feet. They use directed IR with LEDs that have a moderate cone angle to improve ease-of-use characteristics. The IR signal sent out by those devices is generally modulated to around 38 kHz carrier using amplitude shift keying (carrier on or off). The data rate send is generally in ra range of 100-2000 bps. Please note that there are some IR systems which use other frequencies and other modulation systems.
IR transmit and receive systems are inexpensive and are generally reliable. However, interference from other IR sources can be a minor issue. Interference can come from IR remote controls, IR audio systems (these broadcast an IR signal continuously) or other IR sources. Interference can also be caused by other light sources such as fluorescent lights (the ballast can cause IR interference). Sometimes some electronic ballasts powered light can cause interference problems. In order to avoid any interference with this kind of equipment, the operating frequency of all electronic ballasts has to be chosen so that problems in the 36 kHz frequency area are out of the question.
Many existing IR systems modulate the IR light at around 36-40 kHz (this is the frequency of the IR carrier and should not to be confused with the actual frequency of the IR light itself). The possibility of interference is more likely around the 40 kHz frequency. One way to limit interference is to use higher IR carrier frequencies. Some IR systems now use carrier frequencies into the megahertz region.
Generally infrared remote controls are a 32-40 kHz modulated square wave for communication. This square wave is then send to IR transmitter (IR LED). The carried frequency is amplitude modulated by the data, usually full on/off type modulation. The data rate is typically in 50-1000 bit/s range depending on the system used. Usually the tranmitter part is constructed so that the transmitter oscillator which is driving the infrared transmitter LED can be turned on/off by applying a TTL voltage on the modulation control input (the signal that goes here is usually serial data from remote control keyboard decoding IC). On the receiver side a photodiode takes up the signal. The integrated circuit inside a typical receiving chip is sensitive only around a specific frequency in the 32-40 kHz range. The output is the demodulated digital input, just what was used to drive the transmitter. Usually this kind of receivers work os that when IR the carrier is present, this output is high. When no carrier is detected, the output is low. This type of circuits can usually transmit a 1-3 kHz digital signal through infra light. When trying to receive IR signals, leave demodulation to one of the special IC's/modules ment for this and deal with the data only.
The free air IR data transmission, IR remote control as well as the most optoelectronic sensors and light barrier systems work with a wavelength between 870nm and 950nm.
The system described above is not the only one IR remote system in use, it is just the most commonly used one. Asystem that use unmodalated signals of a one kHz or 100 kHz (and several other frequencies) exist as well.
IR repeater circuits are remote control signal extenders. They can be used for example to pick up remotecontrol's signals and send them trough a long cable to a IR-transmitter in the other end of the cable, that would send the remotecontrol's signals to the IR signal recieving equipment that is placed in another room.
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