View Full Version : RSSI on futaba receivers.

22nd August 2012, 12:30 AM
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.

22nd August 2012, 02:01 AM
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/2011/11/adding-rssi-to-futabas-r149dp-pcm-receiver/

And this is how I used a buffer from DPVAC: http://www.rcflighttest.com/home/2011/11/dpcavs-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.

22nd August 2012, 05:50 AM
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?

Vitamin J
22nd August 2012, 10:44 AM
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.

22nd August 2012, 09:33 PM
ViJ, are you saying that you didn't even mod the receiver? I assume this might work, but is it accurate?

Vitamin J
22nd August 2012, 10:43 PM
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:

22nd August 2012, 11:12 PM
Yeah thats the mod I plan to do.

28th August 2012, 08:05 AM
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.

28th August 2012, 05:58 PM
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.

29th August 2012, 01:15 AM
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 (http://www.rcgroups.com/forums/showpost.php?p=10991631&postcount=6).

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 (http://www.rc-cam.com/forum/index.php?/topic/234-rssi-on-rc-receiver/#entry6143)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 ;-)

Vitamin J
29th August 2012, 10:24 AM
I was using a Corona rx and the voltage range for RSSI was only from about .8v to 1.5v and it still worked perfectly with the Trace. The Trace has a built-in buffer for the RSSI signal.

29th August 2012, 09:33 PM
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 (http://www.rcgroups.com/forums/showpost.php?p=10991631&postcount=6).
Please mark sure you get a correct RSSI signal,[Power ON] signal should be greater than [Power OFF] signal.

29th August 2012, 11:40 PM
Hiya Skylark.

Thanks for jumping in Skylark.

In that case I'll need to build a buffer that alters the RSSI voltage. Currently mine has 0.26v when at close range and 1.2v when the tx is off.
But that does not explain why it does not work when I try it in reverse. It still does not see any voltage with the configuation tool.

The next step for me is to try it another way. I'll test by applying a 5v voltage to the RSSI pin from a onboard uBEC, just to see if the OSD is able to read the voltage correctly. If that works, then I'll build a difference buffer for the OSD using an op-amp.

This should give me 4.74v at full power and about 3.8v when the tx is turned off. I assume this will be acceptable to the OSD?

@kieth: can you confirm what voltages you saw from your 149 receiver?

30th August 2012, 02:43 AM
OSD can accept 0-5V voltage.
But for RC receiver, [Power ON] signal should be greater than [Power OFF] signal,
so I guess you maybe get a wrong signal(maybe wrong pin) from receiver

30th August 2012, 08:30 AM
OSD can accept 0-5V voltage.
But for RC receiver, [Power ON] signal should be greater than [Power OFF] signal,
so I guess you maybe get a wrong signal(maybe wrong pin) from receiver

I don't think so, others have reported similar readings with this rx. I'll do some more testing and report back here.

30th August 2012, 08:53 AM

Download this manual from ImmersionRC as here is a mod for Futaba FM receivers...


Hope it helps


30th August 2012, 10:12 PM
That's a R168 in IRC's manual, it's a little different than the 149s. The problem is there's at least two versions of the 149, and getting RSSI from some of them are sketchy. I know I've seen a post detailing the "other" 149DP that MrFollie's likely has, it was on RCGroups a long time ago. I wasn't able to dig it up though. :( I do remember it being a little tougher to do, there was a tiny pin on one of the chips that you soldered to.