Back when I was an undergraduate at MIT eight link under controlled conditions, there are several years ago, my fraternity had a loft in every room. reasons why it won’t work at home. Many house-This suited me fine, but there was no way to turn hold light sources emit energy at wavelengths that the light switch off from up there when it was time excite the IR detectors typically used in home elec-for some sleep, and I certainly did not want to trun- tronics. This would make it difficult or impossible dle across a floor land-mined with DIP ICs, papers, for the receiving device to distinguish intentional IR and tools, then climb up the precarious ladder to signals from stray noise in the red/infrared range my roost without the benefit of sight. — for example, sunlight reflecting off a Coke can.
Meanwhile, I also had a Sony stereo system with This problem is known as spurious pulse rejection.
a remote control that sported a TV power button,
but I had no TV. And so, like peanut butter and chocolate, two worlds came together. I decided it “I decided it was time
was time to put that derelict TV button to good use: switching the lights off at night. Doing this meant to put that derelict TV
making a light switch that could understand signals from an infrared remote control — specifically, one button to good use.”
that used Sony’s IR protocol, since different manu-
facturers do infrared in different ways. Another problem is that remote controls need to
work in a wide range of ambient light conditions,
Illustration by Dustin Amery Hostetler/ UPSO.org
How IR Remotes Work between bright sunlight and pitch dark. Without At a physical level, an IR link consists of an infrared some encoding scheme for the signal, it’s difficult LED on the remote, which typically emits light to build a receiver that can filter out such a large at a wavelength of around 940-980nm, and an possible range of ambient light and still be sensitive infrared photodiode detector on the main piece of to the data it needs to receive. equipment. The detector is covered by an IR-selec- The solution that most IR remotes use is to tive filter, usually a deep red piece of plastic, that modulate the IR light with a carrier frequency. My helps block stray ambient light. The detector turns Sony remote, for example, used a carrier frequency incident photons into electrons that can be easily of 40kHz, which is fast enough and regular enough sampled and amplified. that it is unlikely to be confused with random fluc-
A naive implementation of an IR link would be to tuations and reflections. This frequency also allows transmit a beam of infrared light to represent a “ 1” noise from ambient light hitting the IR photodiode or On condition, and use the absence of signal to to be largely filtered out by a relatively small cou-indicate “0” or Off. While this can yield a functional pling capacitor.
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