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    Operating four Tx-Rx pairs in close proximity

    I know how to send and receive simple slow code (like on/off signals) using an encoder-decoder combination such as the popular Holtek HT12E/HT12D pair and OOK RF modules like inexpensive 434 MHz Tx-Rx pairs.

    I would like to operate four such pairs in close proximity at the same time. Each pair will have its own unique hard-wired address code for identification. The transmitters will be an inch or so apart and the receivers will be some feet from each other.

    The scenario is similar to operating simple RC toy cars close together. The difference is that some of these toy cars don't even use ID address codes and will surely interfere with each other if operated close together.

    No doubt there are more sophisticated ways of doing what I want to do. But will the scheme outlined above work properly? What is the likelihood of cross interference between the different pairs, either from code collision or from swamping of RF signals?

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    Re: Operating four Tx-Rx pairs in close proximity

    I'm not an RF guy, but I have to believe that if you've got four transmitters running at the same frequency, a receiver is going to get garbage.



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    Re: Operating four Tx-Rx pairs in close proximity

    I suspected that, especially with simple OOK signals. Ah well, I suppose I'll have to take another approach. Thanks for the reply.



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    Re: Operating four Tx-Rx pairs in close proximity

    Can you synchronize the transmitters so that only one is transmitting at a time?



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    Re: Operating four Tx-Rx pairs in close proximity

    No. At least two transmitters have to be transmitting at the same time. Each transmitter sends one of two possible signals besides the ID address code.

    I thought of using a single common transmitter, with the four inputs switched in sequence. The problem is that the encoder-decoder bandwidth is limited and the input scanning rate cannot be very high. The output at the receiver will turn on and off at the input scanning rate with a 25% duty cycle. That by itself is not a problem, but a visual indicator, which is vital in this application, will flicker at the scanning frequency and that's undesirable. I could include pulse stretchers at the receiver output but that will add complexity.



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    Re: Operating four Tx-Rx pairs in close proximity

    I'm a bit confused. If your input scanning rate is slow, you can sample an input, transmit the appropriate data, and then wait an eternity before you scan the next input and transmit the next data, right? The time required to transmit the data is tiny compared to your data rate, right? Those decoders have a latched output, so I'm not sure why you're worried about flickering. It seems to me a single encoder/decoder should work.

    Maybe a block diagram will help.



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    Re: Operating four Tx-Rx pairs in close proximity

    I use sequential addressable transmitters with active time of only a few mS, faster than a HT12 can send a single code. Given their close proximities, especially the transmitters it would make more sense, be cheaper and be faster to send to each receiver in turn instead of trying to do it in parallel.

    Despite that, it the TX ends are close together, and as long as the frequencies are different (otherwise the data would collide and nothing would decode it) you principle would work. Placing receivers very close to each other probably wouldn't work as many of the cheap modules use super-regenerative receivers which radiate a small amount of interference and would likely interfere with each other unless you fully screened them and isolated their antenna inputs.

    Brian.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: Operating four Tx-Rx pairs in close proximity

    Quote Originally Posted by barry View Post
    I'm a bit confused. If your input scanning rate is slow, you can sample an input, transmit the appropriate data, and then wait an eternity before you scan the next input and transmit the next data, right? The time required to transmit the data is tiny compared to your data rate, right? Those decoders have a latched output, so I'm not sure why you're worried about flickering. It seems to me a single encoder/decoder should work.

    Maybe a block diagram will help.
    I feel really stupid. I completely forgot about the latching nature of the output. My excuse is that I'd been playing recently with a similar encoder-decoder pair - the PT2262/PT2272. The decoder comes in latching and momentary versions and I used the latter type. Thanks for pointing that out.

    Quote Originally Posted by betwixt View Post
    Placing receivers very close to each other probably wouldn't work as many of the cheap modules use super-regenerative receivers which radiate a small amount of interference and would likely interfere with each other unless you fully screened them and isolated their antenna inputs.
    Good point, thanks. I'll screen the receivers.



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