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Thread: Evohome cylinder strap on sensor incompatible ?

  1. #41
    Automated Home Legend paulockenden's Avatar
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    The spacing thing is a huge issue. It's incredible that the RF comms is this susceptible to interference.

    I once considered mounting each BDR91 in a screened metal box....

    ... until 5 seconds later when I realised the flaw in that plan!

  2. #42
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    AintBigAintClever on YouTube has confirmed my suspicions because he shows what's inside the controller and there are no conventional RF components visible, telling me like most of these products that the receiver front end is a wide open to the RF spectrum and what these receivers do is process any signal at their front end, so no selectivity such that two transmitters on the same or near-by frequencies will be received as if they are valid, just like the 433Mhz units used world wide, this means that a nearby unit will block the front-end and prevent communications, usually solved by increasing spacing from the near field; comprehensive error detection, checking and correction and two-way protocols. Typically designers us off-the-shelf transceiver designs then integrate these into their design and hence these issues. For the most part they work well, but what with 868 being the 2nd harmonic of 433 then problems abound, let alone when customers have 868Mhz equipment too.

    Often these little 868MHz receivers use a fast-Fourier transform to convert signals from the time to frequency domain, then select the desired signal with a digital filter, then demodulate it to a data stream, but the FFT routines are not that selective either, hence compounding the problems. When I compare the front end circuit used by Honeywell to my UHF transceiver they are orders of magnitude different in complexity - the Evohome unit has nothing to prevent in-band or co-channel interference, but of course budget driven.

    My Davis Weather station uses 868Mhz, so I'm running a trial later today with it switched off to see if that makes a difference, but I have to get up and disable the remote sender as the base unit is receive only. I'll report the outcome of 6-hours of testing later today.
    Last edited by g6ejd; 20th October 2016 at 01:03 PM.

  3. #43
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    I've left the controller in the same position where it was reporting an average of 3 communication errors a day since installation and have fitted EMC filter clamps to the supplies of the BDR91's and turned off my weather station and since 12:30 today there are no reported errors, so tomorrow I'll switch back on the (868Mhz) weather station which sends an update every 2-secs and I'm expecting some of the errors to return, but hopefully not with a nice clean supply to them.

  4. #44
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    There appear to be good and bad days wrt the comms issues. So run each test for longer and report back. I'd be interested to hear your results.

  5. #45
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    Has anyone looked at the design of the "antenna" on the board ? Is it vertical or horizontal polarisation ?

    If it's horizontal, the unit will be sensitive to its orientation relative to the controller in the yaw axis - perpendicular to the controller should give the best signal. If it's vertical it shouldn't matter which way it is oriented in the yaw axis.

  6. #46
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    I haven't seen inside one yet, but it's likely to be a 'typical' PCB type, see: http://www.ebay.co.uk/itm/like/15214...0794543&crdt=0

    Polarisation will be random by the time the RF gets reflected off any metallic objects - hot water cylinders, door handles, hinges, nails, plaster board corner re-enforcements, etc.

    As I speak to more people about the Evohome communications system I learn they are experiencing what appear to be random communication faults.

    So-far today no more communication faults, that's a total of ~12-hours. But as I sit hear I've just heard the pump run for about 10-secs and I know the schedule goes low at 23:30, so that's strange. I know other people are reporting that. Could be a room is at or near the night-time 15'C - checks - no all the rooms are currently at least 3'C about the 15'C night time temp.

    I'm leaving the experiment running until Friday at 12:00, then I'll restart what could be an interfering weather station.

  7. #47
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    Well I turned my weather station back on and the faults (Boiler Relay Fault) have returned, I've had 4 in a short period this afternoon, but what's interesting is they are spaced almost exactly an hour apart, well within a few minutes of each other and all at HH:39 HH:42 or thereabouts. There is no change in the schedule, its set for on at 0700 and off at 2330, so nothing obvious there, but I'm thinking this is a bit more than interference.

    So new tests tomorrow...

    Updated:
    Hmm.. some time later, I have switched off my weather station and the errors are still flowing, almost at random. Now I've moved the controller to within 1M of the BDR91's that are wired 25cms apart, not 30cm now could they be that sensitive to space, I'll let you know.

    Updated:
    I have been monitoring the 868Mhz transmissions and there sounds like other units on the band, I have not yet been able to caputure the received audio on my digital storage scope but my ears tell me the transmission type is similar, so am I receiving packets from another system (house) that are being interpreted as erroneous communications, noting my system is working just fine despite these error messages...

    The controller should only report errors from known/bound devices, I wonder if that's the case?
    Last edited by g6ejd; 22nd October 2016 at 12:39 AM.

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  9. #49
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    A good find, thanks, that's answered a lot of questions. I'm going to re-site one of the BDR91's today to see if that's the cause.

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