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Topic: multiple IR signals interference? (Read 873 times) previous topic - next topic

Hello Everybody...
I was wondering if I could have multiple IR emitters and receivers at different frequencies without them creating interference? For example, what if I have one signal at 38kHz another at 48kHz and then yet another one at 58kHz, and they are transmitting at the same time, is there going to be an issue?

Furthermore, where can I find IR receivers that work at other frequencies than 38kHz...? All the ones I can find are 38kHz...
Thanks for reading :)

MarkT

You can in theory use whatever frequency you want if you deal with the output of a phototransistor or photodiode yourself (though it will use the ADC heavily at that sort of frequency.

The 38kHz frequency is standard for remote controls, hence a lot of chips support it - encoding the signal onto a 38kHz carrier vastly improves sensitivity and reliability in the face of ambient light interference.

Separate frequencies should be able to interoperate if the decoder chips have narrowband filters that properly reject the other frequencies, which means reading the datasheets to find out the frequency response curves.
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Thanks for the replay  :)
I tried searching for IR sensors that are sensitive to other frequencies than 38kHz but couldn't find any... Do you have any suggestions on where I could find them?
(I'm sorry, I'm completely new at electronics, so I have a lot to learn.)

johnwasser


I tried searching for IR sensors that are sensitive to other frequencies than 38kHz but couldn't find any... Do you have any suggestions on where I could find them?


You can get some frequencies (30, 33, 36, 38, 40, and 56 kHz) from Vishay:  http://www.vishay.com/docs/81732/tsop348.pdf

Unfortunately you will probably find the 38 kHz receiver will receive 30 kHz and 56 kHz signals if they are close enough.
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tim7


Separate frequencies should be able to interoperate if the decoder chips have narrowband filters that properly reject the other frequencies, which means reading the datasheets to find out the frequency response curves.


This will only work if only one transmitter is operating at a time.  The frequency in question isn't the frequency of the light, but the frequency of the pulse-train used to encode the signals.  Two simultaneous pulse-trains of different frequencies get mixed up in such a way that it is not possible to separate them afterwards.

As an analogy, imagine sending morse-code messages with a piano.  You can send two simultaneous messages, as long as you use different pitches for different messages.  The recipients can tell which is their message by listening out for tones of the right pitch (perhaps with the help of an audio filter).  But if you try to transmit two simultaneous messages by hitting a single key, even if they are sent at different rates, it's impossible to disentangle the notes into intelligible code.  It is possible to use a single key to send messages to two different recipients if the senders agree to transmit their messages one after the other.  The recipients can work out which message belongs to them by listening to how fast the code is sent.

In this analogy, the piano pitch is the wavelength of the IR light, and the rate at which the morse-code is sent is the IR pulse frequency.

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