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Transmitting using this transmitter seems to jam many wireless devices (possibly using the same frequency) like car keyless entry remotes, garage door openers etc. Moreover, two such transmitters cannot be used simultaneously as they jam each other and the receiver gets no signal from either. One workaround is to use an oscillator to transmit discountinously (a short burst every few seconds). But I was wondering if there is a way to solve the jamming issue and transmit signals from both the transmitters simultaneously. The receiver is common for both the transmitters (assigned different addresses using HT12E) and hence signal from just one transmitter is used at any given point of time. ASK RF transmitter and receiver Data sheet(not of the exact product but is similar) https://electrosome.com/wp-content/uploads/2014/10/MO_SAWR.pdf

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  • \$\begingroup\$ They're not really intended for continuous transmission, but for short bursts (like the entry systems you mention). Your options are basically some sort or CSMA/CD, CSMA/CA, or time-based (possibly timeslots like GSM if you're going to have a lot of them). \$\endgroup\$ – pjc50 Jul 6 '16 at 15:15
  • \$\begingroup\$ I just have two of them. The other components involved are a priority encoder, ht12e encoder etc. I was thinking about keeping it really simple. But if there is isn't any other way, I will consider your suggestion. I am off to do some research on CSMA/CD and CSMA/CA as I haven't dealt with them before. But it would be really helpful if you could get me started 😄 \$\endgroup\$ – Adarsh Pryce Jul 6 '16 at 15:17
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Your receiver is fixed to a single frequency and can only receive the signal from one transmitter at a time, so your only option is some kind of time division multiplexing (ie. each transmitter sends a burst in a different 'time slot' when the other one is idle).

Problem is how to make sure that each transmitter stays in its own time slot and not stomp over the other one? You need some way to synchronize them. One way might be for one transmitter to include a receiver which listens for transmissions from the other one, then it can avoid the other transmitter's time slot. Or you could occasionally send a synchronizing signal to both transmitters, which then stay in sync long enough to avoid colliding in the mean time (how long would depend on how accurate the clocks are in each transmitter).

Alternatively you could just let them collide and rely on the decoder to sort it out. For this to work reliably each transmitter needs to send bursts at a different (asynchronous) rate or with random delays. Occasionally they will collide and then the corrupted signal will be rejected by the decoder, but mostly they will send at different times so their signals will get through. Since RF transmissions can be corrupted by noise and interfering signals anyway, this is not much different from using a single transmitter (just a few more lost data packets).

The HT12E takes about 40ms to send a burst, so the absolute maximum rate for receiving two transmissions is about 12 per second. However if the transmitters are not synchronized then you may need a much lower rate to reduce collisions. If that is not acceptable then you might have to look at using more sophisticated transceivers.

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  • \$\begingroup\$ Adding another receiver with the second transmitter to form a transceiver and the "listening and if idle transmission" would work great.The maximum delay that I can afford between each transmission from a transmitter is 8 seconds. Setting one transmitter to transmit at every 7th second and the other at every 8th second would also do as only the transmission at the second corresponding to the LCM of their individual time periods would be lost (Both transmitters are powered by the same source and hence their relative time periods are unlikely to change much). Thank you \$\endgroup\$ – Adarsh Pryce Jul 7 '16 at 5:51
  • \$\begingroup\$ One problem with the "listening" trick is that once it "listens" to the other transmission and starts transmitting its own signal,it can no longer listen to the other signal as it would be jammed by its own transmitter.Hence I would have to modify it to "LIsten, if idle transmit for a fixed time and then go back to listening". Given all this, transmitting at different time periods would be less sophisticated and the relatively seldom data loss is tolerable considering that the transmitters in discussion aren't that high end. \$\endgroup\$ – Adarsh Pryce Jul 7 '16 at 5:57
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You need to find a medium access control mechanism that will suit your needs.

The easiest is to designate one device as a master and the two others as slaves. The master will ask only one of them for data at a time. The slave will only transmit if asked by the master, so you can easily avoid collisions.

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