Infrared Controls
Infrared controls transmit light within the frequencies of 30 KHz and 40 KHz. This is just below the red part of the visible light spectrum. These high frequencies cause infrared light to be invisible to the eye and prevent other light frequencies from interfering.
Signals are sent by a binary code, a series of logical zeros and ones. The controller sends different combinations of zeros and ones to tell the receiver what to do. There are three different ways manufacturers code signals. The coding is based on varying the length of pulses, the length of spaces between pulses, or changing the order of spaces and pulses.
Pulse-Width Coded Signals- These vary the length of the pulses.
short pulse width = about 550µs = logical 0 = a low
long pulse width = about 2200µs = logical 1 = a high
Space-Width Coded Signals- These vary the length of the spaces between pulses.
short space width = about 550µs = logical 0 = a low
long space width = about 1650µs = logical 1 = a high
Shift-Coded Signals- These vary the order of pulse space.
short space width (about 550µs) + long pulse width (about 1100µs) = logical 1 = a high
long space width + short pulse width = logical 0 = a low
These codes hold information like the address to the machine and what the machine must do. When you press a button on a remote it sends a series of signals. The first thing the remote sends to the receiver is called the header. The header is a burst of highs that alerts all of the infrared receivers in the area. After the header comes a code including the address to the specific machine you want to operate. Then comes the command. As long as you hold down a button the command will continue to occur. When you release the button, the remote sends the machine a string of code that tells it to stop. When signals are received in the machine, they first enter a photodetector. The photodetector determines what type of signal is being sent, and sends it to the appropriate locations so the command can be executed.
How a remote control sends commands to a television set.
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