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Old 10th July 2006, 11:35 PM   #1
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Default LED infrared signal separation

Okay, in my latest remote control scheme, I'd like to make it synchronous, so I'll be needing two LEDs. I know that there are ICs out there that can do this for me, but I'm trying to devise a simple communication scheme using IR myself.

The one IR LED is at 38kHz, 870nm, and the detector is matched to it (38kHz demodulation, tuned to 870nm). I want to use a second LED as the "clock" signal to tell the receiver when to sample the first LED receiver data.

The problem is overlap so that the two do not cross-talk: if I use a 900+nm IR LED at the same modulation frequency (38kHz), there may be bleed over into the 870nm range, causing false readings.

I figure that the second channel should be modulated at 455kHz (because 455kHz is also a standard IR receiver demodulation frequency); if this is so, will that provide enough channel separation using the same wavelength (870nm)?
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Old 11th July 2006, 12:10 AM   #2
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Why not just multiplex the clock and data on a single IR LED? You would include some sort of preamble in your signal to allow the receiver to sync its clock with the IR signal, and it would all be synchronous from there.

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Old 11th July 2006, 01:17 PM   #3
macboy is offline macboy  Canada
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First, the selectivity to wavelength of these receivers is virtually non-existent. Although they may be a little more sensitive to one wavelength (e.g. 870 nm) over another (e.g. 900), any receiver will respond to what comes out of virtually any IR LED.

Using 455 kHz should do the trick. I have a couple of Sony home theatre receivers that have two-way remotes. They use 40 kHz in one direction and 455 kHz in the other.

They problem that you are likely to run into is a difference in the delay through each IR receiver module. These modules need to do many functions to get that digital output signal, including envelope detection. Since the carrier frequencies are more than an order of magnitude appart, they may respond quite differently. You will need to study the datasheets to determine whether there will be a significant difference in the response of the two modules at the data rate that you are wanting to use.

Another idea is to use a modulation scheme which has a "built-in" clock, such as biphase or Manchester coding.
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