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Feb 06, 2012, 06:57 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Discussion

Displaying TGY 9X RSSI with ET OSD - Success (?)


EDIT: go to post #22 to see a summary of how to make RSSI of TGY 9X display properly on ET OSD pro

EDIT2: Not working anymore after guardian was installed (see post#32) - further investigation needed

Hi all.
This is a thread dedicated to making possible to display the RSSI of TGY 9X on the ET OSD pro.

After doing lots of research I was not able to find a post including a straight forward working solution for displaying the Turnigy 9X RSSI with ET OSD pro.

I did add a RSSI out pin on my rx by connecting the RSSI pin of the TGY 9X rx according to Bakchos' post. The results were not as expected, since the RSSI value displayed was fluctuating way too much (about 100% to 40% with the tx sitting next to the rx)

Then, I did built the RSSI buffer using the LM358 according to BlueAngel2000's post, but that did not improved the situation, since the same fluctuations were evident.

Then, I did built a PC oscilloscope and checked the RSSI output of the LM358. The output puzzled me even more, but I do not have the background to explain it: The LM358 outputs some form of pulse waveform voltage, and not a constant voltage (always tx sitting next to the rx). It seems to me that the RSSI value that the OSD displays is the momentary volt value at the time of sampling, which could be at the top of the pulse, at the bottom of the pulse, or anywhere in between. Maybe some form of R-C circuit could be used to smooth out this pulse, but this just an uneducated guess on my part. Does RSSI outputs digital signal, but the OSD expects analogue signal?

Then, there is the red LED solution that could be a better approach, but I have not tried it yet. I am aware of the RCexplorer tutorial on how to do it on a FASST rx, and the relevant thread by reely340. I suppose it could be done in a similar way with the TGY 9X, but it has only a red LED?

So hasn't anyone ever made the ET OSD pro display correct RSSI information with TGY 9X? That would seem odd, since both of these products are very common.

I kindly invite all the great people of this forum with decent electronics knowledge (unlike myself) to make suggestions in order to help making possible to display orrect RSSI information with TGY 9X.


Best regards,
Stavros
Last edited by sfakias; Apr 02, 2012 at 01:48 PM.
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Feb 07, 2012, 02:54 AM
Registered User
Most 2.4GHz frequency hopping systems have pulsed rssi or lost packet indicator.
So you will need a low pass filter to filter the pulse to a steady voltage, like in the RCexplorer tutorial you linked to.
Feb 07, 2012, 03:34 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Hi Megafluffy.

Do you mean to install a LPF after the LM358, or use the rx red LED and connect it to the OSD through a LPF?

Thanks.
Feb 07, 2012, 08:05 AM
Registered User
You use the rssi signal you got. (I don't know anything about this receiver)

You could do it like this:

ReceiverRSSI -- LPF (Resistor capasitor) -- buffer (LM358) -- eLogger

If you do it like this, you should keep resistor valu high, 100kohm to not load the rssi from the receiver to much. (cap 10nF to 1uF)

Or you could:

ReceiverRSSI -- buffer -- LPF -- eLogger

If you do it like this, you dont have to worry about loading the rssi signal, but you have to think about the input resistance of the temp1 input of the eLogger, that is 5kohm. So then the resistor value should be low (less than 5kohm), and cap value higher. (100uF maybe).

To find the best values you have to try and check the stability and respons of the filtered voltage.
Feb 08, 2012, 11:06 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Megafluffy, thank you for the help.

I followed your advice and opted for the "ReceiverRSSI -- LPF (Resistor capasitor) -- buffer (LM358) -- eLogger" solution, since I already had made the LPF based on RCExplorer's values (68K ohms resistor and a 10uF capacitor).
The results were encouraging: with the tx next to the rx, the RSSI displayed was fluctuating much less, approximately from 100% to 82%, and with tx off RSSI displayed was 0%!

So, I think we are close to pinning this down. However, I do not know much about electronics, so please advise which component ( R or C?) should I change, and how (lower or greater value?) in order to reduce even further the fluctuations?

Once again, thank you for your help.

P.S.: Do you know how I can change the title of the thread?
Feb 09, 2012, 07:23 AM
Registered User
You can increase both to get better stability. I would start with the resistor, to something above 100kohm.
Just note that the higher values you use, that will give better stability, you will also get slower response. Just test both stability and response, when deciding for a value..



PS reply, press edit on the first post, and press advanced, then you can edit title..
Feb 09, 2012, 09:29 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Thank you Megafluffy. I will experiment with greater R and C values, and see how it goes.

I guess a fluctuation within 5% is acceptable, within 3% would be great. However, I am little confused on how to test for response time. As it is now, it takes about 1,5-2sec from the moment I turn off my tx to the moment when the RSSI value goes to 0%. Is this too much or could be, say, 4 sec and still be OK? I do not know the actual instantaneous RSSI value to compare with the displayed value, so how could I judge response time?

Regards,
Stavros
Feb 09, 2012, 12:40 PM
ScratchCrasher
sfakias's Avatar
Thread OP
By adding another 68kΩ resistor in series with the original resistor of the RC filter, the variation was reduced even further: 100% to 92% with tx next to rx, and 0% to 3% with tx off. When switching off the tx, the displayed value drops from 100% to 0% within 5 seconds. I could add more resistance to the circuit, but I feel that it would become very sluggish.

I tried adding a capacitor 10μF capacitor, in parallel to the original one, with or without extra resistance, and it did not apparently improved the fluctuations, although it increased the 100% to 0% display time, so I removed the extra capacitor.

I attach pics of the assembly (prior to heat-shrink), in case anyone is interested.

As it is, the RSSI value shown at the OSD is not perfect, but probably is good enough to give you an approximate, yet reliable indication of signal strength while flying.

Further suggestion for improvement are more than welcomed.
Feb 09, 2012, 10:55 PM
Registered User
stewtheman's Avatar
So what RSSI voltages do you see? I've measured .6V when the transmitter is off, around 1V when it's right next to the receiver, and around 1.25V with my transmitter in a microwave. My OSD, Cyclops Easy OSD, only displays the RSSI voltage, and since there's only around .2V of variation the numbers are pretty worthless to me. I don't get how the LM358 buffer fixes this problem. I'd love to get RSSI working with my OSD but I need a little bit of help. I appreciate all the work you've done so far and hope you can help answer these questions I have.
Feb 10, 2012, 04:58 AM
Registered User
Quote:
Originally Posted by sfakias
By adding another 68kΩ resistor in series with the original resistor of the RC filter, the variation was reduced even further: 100% to 92% with tx next to rx, and 0% to 3% with tx off. When switching off the tx, the displayed value drops from 100% to 0% within 5 seconds. I could add more resistance to the circuit, but I feel that it would become very sluggish.

I tried adding a capacitor 10μF capacitor, in parallel to the original one, with or without extra resistance, and it did not apparently improved the fluctuations, although it increased the 100% to 0% display time, so I removed the extra capacitor.

I attach pics of the assembly (prior to heat-shrink), in case anyone is interested.

As it is, the RSSI value shown at the OSD is not perfect, but probably is good enough to give you an approximate, yet reliable indication of signal strength while flying.

Further suggestion for improvement are more than welcomed.

Next step will be to change the capacitor to a 'low ESR' version. Between 1 and 10 uF. You can find them on an old Pc-motherboard, around the power transistors / CPU.
Feb 10, 2012, 10:17 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Quote:
Originally Posted by stewtheman
So what RSSI voltages do you see? I've measured .6V when the transmitter is off, around 1V when it's right next to the receiver, and around 1.25V with my transmitter in a microwave. My OSD, Cyclops Easy OSD, only displays the RSSI voltage, and since there's only around .2V of variation the numbers are pretty worthless to me. I don't get how the LM358 buffer fixes this problem. I'd love to get RSSI working with my OSD but I need a little bit of help. I appreciate all the work you've done so far and hope you can help answer these questions I have.
Hi. In my system, I measured the RSSI voltage to be 1.11V with tx on and next to rx, and 0.91V with tx off (I do not have a microwave oven handy right now). So, the voltage difference is 0.2V, like your measurements. The ET OSD pro has the functionality to define a voltage input to display RSSI, which means that it "learns" the RSSI voltage range, and displays RSSI instantaneous value as a percentage (0%-100%). I am not familiar with cyclops easy OSD, sorry. Maybe somone with the required electronics knowledge could suggest a DIY circuit to amplify the voltage difference...
Feb 10, 2012, 11:06 AM
ScratchCrasher
sfakias's Avatar
Thread OP
Quote:
Originally Posted by Megafluffy
Next step will be to change the capacitor to a 'low ESR' version. Between 1 and 10 uF. You can find them on an old Pc-motherboard, around the power transistors / CPU.
Megafluffy, I do not know how to distinguish a Low ESR capacitor from a "normal(?)" one. After googling it, I noticed that some low ESR capacitors are colored light blue. Also, I do not have an old PC motherboard handy, but I have some other electronics laying around. So, I found the capacitor that is shown in the attached pics. Is that a capacitor like the one you describe? What would be the benefit of substituting the capacitor that I use at the moment with this "low ESR" capacitor?

Thanks,
Stavros
Feb 10, 2012, 02:10 PM
Registered User
stewtheman's Avatar
Thanks for your reply. It looks like I might need to get a new OSD if I want to use the RSSI readings from my 9x. What kind of range are you expecting to get out of your 9x? I just put a skew planar wheel on my transmitter and a cloverleaf on my receiver to see what happens. My range, with a 1.3GHz video system, was horrible (I lost my RC link at 800 feet out), but I also didn't have a low pass filter installed.
Feb 10, 2012, 05:48 PM
ScratchCrasher
sfakias's Avatar
Thread OP
I also use 1280 MHz A/V tranceiver. With stock antennas I got about 500m (1640 feet) with the rc tx pointed at the plane before I got lockouts, and had to keep the rc tx antenna vertical above my head to bring the plane back. This was at an area without much rf interference, and without a 1280MHz LPF installed.
I installed OMM dual natch filter since then, and trying to make RSSI indicator working properly, while I wait for my ET Guardian to arrive. I want to have a reliable RTH OSD before I explore the limits of the TGY 9X. However, you can not trust the TGY 9X as medium or long range FPV tx, but 1.5-2.0 km would be enough for ny needs. For longer range I would use the Futaba FASST system that I have (only 6ch though...).

Did you put the CL and SPW on the A/V tranciever or on the TGY 9X? Which A/V tranceiver do you use (FOX800 here)?
Feb 10, 2012, 10:53 PM
Registered User
stewtheman's Avatar
I put the SPW and CL on my 9x Tx and Rx. I also just put a CL and SPW on my A/V Tx and Rx. I use a 700mW BOB vTx.


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