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Thread: RSSI on futaba receivers.

  1. #1
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    RSSI on futaba receivers.

    I'm flying a lot of FM these days. The band is much clearer than it was a few years ago and (I hope) it's a cheap way to get 5kms.

    There is a lot of info about getting RSSI output from the futaba R149DP receiver. So I'd like to mod my rx and hopefully get RSSI output to my Trace III (or is it Trace IV?) OSD.

    I've read that the RSSI voltage is between 0.7v and 2.2v depending on how close the TX is to the RX. I've not gone so far as to test this for myself.

    So I have two questions for Skylark (or anyone else that's done this). First off, can you calibrate teh OSD such that these voltages are seen as 100% and 0% (does not need to be terribly accurate I guess).

    Second question is, does the Trace OSD draw much current to measure the RSSI? I have heard bad things happen to the Rx when the RSSI pin is loaded down.

    If it is drawing some load, then I'll make up some kind of isolation circuit to get the RSSI value into the Trace OSD.

    p.s. When I get this going, I'll do a how-to for others to play with.

  2. #2
    bonafidepirate Keith's Avatar
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    I can't say much about calibration RSSI in the Trace, I haven't played with that one. You will likely need to buffer the RSSI output from your receiver though.

    Here's how I got the RSSI out of my 149DP: http://www.rcflighttest.com/home/201...-pcm-receiver/

    And this is how I used a buffer from DPVAC: http://www.rcflighttest.com/home/201...s-rssi-buffer/

    I also made another buffer later using a buffer IC which was pretty easy to do, and worked fine. If I need to, I can look up the part# on the IC that I used, I still have a few left I think. There was a diagram posted somewhere on making one.
    AKA Bonafidepirate

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    Thanks Kieth,...

    That's the procedure I saw as well. And the connector that you made for the buffering is also what I envisiged. (Using the 5v for power and the signal for RSSI).
    Did it work out ok for you?

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    Yeah the Trace will take the RSSI no problem and you don't need a buffer. I just used a servo connector to take 5v and RSSI from an empty channel on my 72mhz rx, easy.

    To calibrate you need to use the Skylark computer program and the USB hookup, the process is really simple just click a button turn your radio on and off and then click another button.

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    ViJ, are you saying that you didn't even mod the receiver? I assume this might work, but is it accurate?

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    I used a different rx and had to tap RSSI on it. Not sure what you'll need to do with your rx though, here check this out:
    http://www.rcflighttest.com/home/201...-pcm-receiver/

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    Yeah thats the mod I plan to do.

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    Has anyone else done this? I've modded my rx and I can see rssi data with a multimeter (0.26v when tx is in the same room, 1.25 when the tx is turned off).

    Tried the calibrate but the TraceIV does not read the RSSI (in the PC program it's showing 0.1).
    I'll try with my TraceIII.

  9. #9
    bonafidepirate Keith's Avatar
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    Something doesn't sound right, it should be lower with no signal, and higher with a good signal. I seem to remember there being at least two versions of the R149DP that were slightly different inside, and RSSI was in a different spot too. There was a post on RCGroups about it a while back, if I can dig it up I'll link it here.
    AKA Bonafidepirate

  10. #10
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    Hiya Kieth.... My Rx does not look the same as the one on the RCflighttest page.
    Mine is the one in the first picture of this post.

    So I de-soldered the diode, but I didn't cut any tracks (no idea which one I would cut).
    Also, I am not using a buffer.

    I flew LOS with the receiver today (<400') and all seemed OK. Next step is to figure out the RSSI bit.

    With the Tx on I could not get the rssi voltage to change from 0.26. The furtherest I moved it was about 20' away into another room (antenna down).
    Only when I turn it off does it jump.

    I guess I should have done a test at the field with someone taking the Tx and driving away while I watched the voltage.

    Once I figure it out, I'll solder up a buffer. I can add some gain that way to get the signal between 0v and 5v.

    This post seems to indicate that the RSSI voltage is inverted with respect to siglan strength.

    From a skylark trace point of view, I tried doing it backwards (tx off then on) to see if I could get a reading but no go. I suspect that it's because I don't have a buffer.

    If we assume that I need to take the 0.26v - 1.2v range as RSSI, how best should I design the voltage buffer circuit?
    Perhaps skylark's trace would like to see 0v - 5v? or perhaps 5v - 0v is also ok? Perhaps 0.26v to 1.2v is ok as well but it will only work with a buffer.

    I think the only thing I can do is to just do up a simple circuit and test it to figure out what works.... I'm a newb to electronics, but I'm learning fast ;-)

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