Radio board interferes with my car key system (Solved)

Discussion in 'Automotive Electronics' started by Aclassifier, Aug 7, 2018.

  1. Aclassifier

    Thread Starter New Member

    Aug 7, 2018
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    I am using an RFM69HW board containing a Semtech SX1231H packet radio unit. It will soon transmit data from my aquarium controller.

    I am using the 433 MHz band. I saw that when I sent full effect (100 mW at +20 dBm) and sent 5 timer per second at 433.92 MHz my car key system halted, meaning the car would not open or lock. 10 meters through wooden walls. I opened the key and saw that it sent at the same mid-in-band 433.92 MHz. When I changed to down 1/10 of the band at 433.706 MHz the situation was better: the car opened and closed after a button press or two. If I sent once per second then even 433.92 MHz did not seem to disturb the car key system noticeably.

    The RFM69HW uses data packets (with preamble etc.) at a certain baud rate and CRC16 plus I use a CRC32 in my payload. Sending at 433.92 MHz and listening with another board 15 cm away at 433.706 MHz gave no messages coming through, as expected.

    However, my car key system [1] seems to be much less advanced. Of course it could not be expected to necessarily get through at the same frequency if that frequency was jammed, but it never really came to that I would think.

    How does such a car key system work? Is what I see according to expectations?

    Observe that my packets, even if they hindered the car from being opened, never opened it!

    My car is a 2012 model bought in 2011, but I guess the key system is from the original version of that car series, coming in 2005. It's a German make.

    I am blogging about these matters. See [2] with disclaimer.

    [1] https://fccid.io/MQU-B3333D
    [2] http://www.teigfam.net/oyvind/home/...gh-the-shelf/#frequency_crash_with_my_car_key - disclaimer: no ads, no money, no gifts, just fun and expenses!
     
  2. sghioto

    Active Member

    Dec 31, 2017
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    From what you are describing, Yes! Just simply being jammed by your transmitter.
    From the blog:
    "I have changed to 433.706 MHz and all I need is minumim power level (5 dBm on RFM69HW). And for the aquarium I would probably end up sending once every minute."
    That should do it.
    SG
     
    Last edited: Aug 7, 2018
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  3. Aclassifier

    Thread Starter New Member

    Aug 7, 2018
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    Thanks, sghioto. But, why was it jammed? The frequency was the same, but the transmission is only some ms every 200 ms, and the data format (I do hope and think) is entirely different. The car system should just have sit quiet and don't ley itself be disturbed!(?)

    It says in the car's manual that the key has to be pressed for 500 ms, but I reckon that doesn't mean that the sender wants to send for a full 500 ms continuous wave and the receiver in the car wants a continuos radio wave for 500 ms? The key should send only one small message in some format at the 500 ms timeout? Maybe repeat it is long as the key is depressed. Anyhow, my 200 ms packets should in theory not silence the car?

    It's always good to know why when I try to avoid something.

    Or are these bands's (like the 344 MHz band) units of so many flavours that I could just as well accept it without trying to understand, since it's "impossible" to know about all the radio modulation etc. techniques?
     
  4. sghioto

    Active Member

    Dec 31, 2017
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    Most of those low power systems send data by the OOK system, ON-OFF Keying. When you press the button on the car remote it's sending coded data by switching the carrier on and off. The receiver is just programmed to require 500ms of data before opening or locking the door.
    So if you have a strong enough carrier on the same frequency as the car system then that on-off carrier will wreck havoc.
    SG
     
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  5. DickCappels

    Moderator

    Aug 21, 2008
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    From memory, Part 15 of the CFR, which governs unlicensed radio in the United States and a couple of other related things, that band is only to be used for intermittent signaling. Continuous transmission is not permitted, and now you know why.
     
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  6. Aclassifier

    Thread Starter New Member

    Aug 7, 2018
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  7. Aclassifier

    Thread Starter New Member

    Aug 7, 2018
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    In Norway it's also 10% at max 10 mW [1].

    My power was worse for the test, but not the time on. I am still rather surprised!

    [1] In Norwegian: https://lovdata.no/dokument/LTI/forskrift/2001-11-05-1251
     
  8. sghioto

    Active Member

    Dec 31, 2017
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    Most likely just a unique on/off pattern that may be sending 50 times a second. Like I said before the receiver is programmed that it must see that continuous stream of data for 500ms before operating the doors for added security.
    SG
     
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  9. Aclassifier

    Thread Starter New Member

    Aug 7, 2018
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    Thanks, guys! I'm considering this as solved now! (If there is a button to press to make this seen in the header, then please tell me. I'm new here today! Great start!)
     
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