AM transmitter interference

Discussion in 'Wireless & RF Design' started by chowdhas, Apr 19, 2012.

  1. chowdhas

    Thread Starter New Member

    Apr 6, 2012
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    Dear Forum,
    I am trying to overcome a problem with interference between multiple 433Mhz remote controls. I built a simple "off after delay" circuit which I installed into my motorized gate controller. It activates a wired up keyfob remote to transmit a 90 second burst to an AM receiver board that then activates a relay to keep a floodlight on for 90 secs after the gate has been activated. The problem is that while the transmitter is transmitting no other remote keyfobs can get their signal through including those to lock our car doors. I thought of changing the gate controller transmitter to the FM version but this also works on 433MHz so I am not sure if it would still interfere. Data sheet of equipment installed (HiRK 433A) attached. The light on signal only needs to be transmitted 15m - can anyone advise how I can transmit using UK approved bandwidths and not interfere with standard remotes?
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    there is 315MHz version too.
    but just how many FOBs are used at the same time to open and close same gate? and while at it, how long one FOB transmits? it should be single burst of data (blip) not a continuous signal.
     
  3. chowdhas

    Thread Starter New Member

    Apr 6, 2012
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    The 315MHz version does not seem to be on sale in the UK (perhaps the frequency isn't approved here).
    The setup is:
    The gate is activated by several access points (1 FOB, 1 DoorPhone controler and one hard wired keypad). For the FOB there is a proprietary receiver /relay in the gate controller housing that acitvates the gate motor. These all work on blips of transmission as you point out.
    I have connected this same receiver relay to my own circuit as well. This circuit activates relays of my own that stay on for a adjustable time after the gate FOB has blipped. These I have hard wired to control power to gate post lights. I also wanted the wall mounted flood light to stay on for exactly the same time but did not want to run wire. Hence a the rigged FOB controlled by the same relay so it transmits for exactly the same time - the light stays on so long as it sees the transmission.
    Problem being that all other remotes are blocked during this time. Is there any way of achieving this control element wireless?
     
  4. chowdhas

    Thread Starter New Member

    Apr 6, 2012
    10
    0
    I have found the following modules which will operate at 915MHz FM. is that far enough away to not block signals from 433MHz AM keyfob transmitters?
    Can anyone direct me to a schematic of the additional circuitry I need to build around these for them to work in a simple remote switch application.
    many thanks in advance.
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    check the page 2 for transmitter connections, other than power and antenna everything goes to microcontroller (you need to use SPI to program it).
    receiver is on page 3 and it is the same story, you connect power and antenna - everything else goes to microcontroller.
    on both sides (sender and receiver) you have to write code to move the data through SPI interface
     
  6. chowdhas

    Thread Starter New Member

    Apr 6, 2012
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    Thanks Panic Mode
    I am not really familiar with the terms you are describing but have researched (googled) SPI. I am not familiar with PIC microcontroller programming and I can only program in Visual Basic. I only really want to transmit a simple on when signal received command rather than a data stream. Am I barking up the wrong tree here???
     
  7. panic mode

    Senior Member

    Oct 10, 2011
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    hi chowdhas

    first of all SPI is not that complicated, there are ready routines for just about any MCU and language. it allows reading and setting parameters or data of another chip. most of the other signals in the datasheet are just flags to monitor or control the radio module directly rather than through polling dozens of registers.
    each message is 16-clock long (16-bit). in those 16bit you can encode command, target register, value etc. for example you can choose one of three bands, fine tune frequency etc. there are flow diagrams detailing handshaking process.

    but if you have never dealt with MCUs, this will be a challenge simply because there are many other hoops to jump through before you even get to SPI (learning how to program MCU, configure it, etc.).

    for those who never dealt with microcontrollers, Arduino is the platform of choice because it offers smoothest introduction (many of the hoops are taken care of, it's like bicycle with training wheels).

    but when you are dealing with MCUs, there are not many ways around. you can either program in assembler (steep learning curve but efficient memory use, different brand or family uses different assembler), or C (pretty much same on every platform, efficiency depends on compiler, some compilers are expensive and their free versions have various limitations such as code size etc).

    there are couple of odd balls that can be programmed in basic (BasicStamp for example) and although other languages do exist, by far the most common ones are ASM and C.

    if you are going to consider this radio, you better read and understand every line of the datasheet. good luck
     
    chowdhas likes this.
  8. WBahn

    Moderator

    Mar 31, 2012
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    Before you settle on a solution that has you transmitting anything, at any frequency, for a continuous 90 seconds, please consider that if such a transmission caused problems for you, then it might cause problem for others as well. So you might not notice a long transmission timeon 915MHz, but your neighbor might be greatly inconvenienced.

    The best solution, arguably, would be for your jury-rigged key fob to transmit one burst signal to turn the light on and another to turn the light off. Is there some reason why the two sets of lights HAVE to come on and go off at exactly the same time? Would it be disastrous if one set went off, say, five seconds before or after the other set?

    How about something like this:

    Have the transmitter send a heartbeat signal by transmitting a burst every, say, five seconds. The light controller at the receiving end turns on the lights for, say, sixteen seconds everytime it receives a command. If the lights are already on, it simply keeps them on and restarts the sixteen second count down timer. The transmitter could send a longer initial burst or several bursts at one second intervals to make sure that the receiver has a real good chance of hearing at least one of them. After that, since the transmitter will transmit three bursts during any sixteen second window, any two consecutive transmissions could be missed without causing a problem. At the end, the transmitter simply sends its last burst sixteen seconds before the lights on its end will shut off. If that burst is missed, then the lights at the transmitter will shut off five seconds early, but that should be a fairly rare event. The timers at both ends should easily be capable of keeping the difference between the sixteen seconds one each side from being noticeable.
     
  9. w2aew

    Member

    Jan 3, 2012
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    64
    Or, consider using a time delay relay at the floodlight. Use your same transmitter to turn on the relay, then stop transmitting, and let the time delay relay hold the floodlight on.
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,765
    4,801
    That was my immediate thought, but the problem is that it is a one-way comm system so if the receiver misses the initial transmission (because someone nearby is transmitting at the same time, for instance), then the light won't turn on at all.
     
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