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Old Apr 13, 2010, 09:17 PM
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Building a short range PPM TX/RX system (Wireless Head Tracker)

I would like to build a wireless connection between my headtracker OEM module which outputs serial PPM and my trainer port. The idea is to integrate the headtracker module into my goggles and have a small receiver "dongle" (the smaller the better) which plugs into my trainer port on my radio.

Since the distance will be VERY short (from me head to my radio which is held in my hands) it can be very low power i think and hopefully can remain very small (tx & rx both). I was thinking something like 433mhz for this short range link but i have no idea where i could buy some radios that would do this. Ideally, the TX would be able to run off 1s lipo and the RX able to handle up to 12v (3s lipo).

Does anyone have some ideas or can point me in the right direction for some small modules and host boards which would be up to a task like this? Would it be possible to simply send this sort of signal over a wireless audio link?
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Old Apr 13, 2010, 09:28 PM
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It looks like i might need to convert the PPM to a frame based protocol which can be sent over serial and then just choose any 2 radios that support wireless serial send and receive and then at the receiving side decode and convert back to PPM ?

Does that sound right? and if so, is there is an easier method where the signal can just be sent as is with out any conversion?
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Old Apr 13, 2010, 10:20 PM
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Quote:
Originally Posted by AnnihilaT View Post
Since the distance will be VERY short (from me head to my radio which is held in my hands) it can be very low power i think and hopefully can remain very small (tx & rx both). I was thinking something like 433mhz for this short range link but i have no idea where i could buy some radios that would do this.
The small cheap 433mhz tx/rx (aka OOK) modules are definitely a good choice for sending PPM short distance. They are available for a buck or two from china and can sometimes be used to transmit PMM a foot or two connected directly. For longer (few feet) a simple modulation scheme can be implemented using a PIC or AVR.
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Old Apr 13, 2010, 10:32 PM
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Hi rich,

Thanks for the reply. Do you have any links to these modules you mention?

What do you think about the possibility of using something like these:
http://www.sparkfun.com/commerce/pro...oducts_id=8945
http://www.sparkfun.com/commerce/pro...oducts_id=8947
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Old Apr 14, 2010, 07:18 AM
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Or these?

2 x http://www.sparkfun.com/commerce/pro...oducts_id=9582

EDIT: I Dont recommend these if you want to keep it quick and simple.
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Old Apr 14, 2010, 08:53 AM
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Yes, those first links are exactly what I refer to. Sparkfun is a little misleading when they imply you can just feed rs232 and get full range. In fact you may go a foot or two with some tricks(maybe enough for you) but generally must modulate/demodulate to go farther. I suggest you try and if you need any more info I'll be glad to help. I am very fond of these little OOK modules and use them in my company, hobby, and security for my house.

That last link I have no first hand experience. I'm not a huge fan of complicated protocols and I2C. I'm a KISS kinda guy.
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Old Apr 14, 2010, 09:11 AM
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Quote:
I'm not a huge fan of complicated protocols and I2C. I'm a KISS kinda guy.
Me too, Rich!

So as a follow to your post where you said:
Quote:
The small cheap 433mhz tx/rx (aka OOK) modules are definitely a good choice for sending PPM short distance. They are available for a buck or two from china and can sometimes be used to transmit PMM a foot or two connected directly.
What do you mean by connected directly? Do you mean i could simply run serial PPM to the TX pin of the transmitter and expect to get serial PPM out on the RX pin of the receiver? Or do i need to put a microcontroller at each end to convert to something digital and frame based?

Im quite a newb when it comes to this stuff. still learning.... so as much 101 for dummies detail you can give me would really be much appreciated.
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Old Apr 14, 2010, 09:23 AM
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You "might" be able to get away w/o a micro. The problem is these are AC coupled and transmit noise when there is no signal unlike expensive RF modules. If the signal is well balanced (NRZ) and continuous then they work very well. PPM is almost like this so connecting direct will probably work for you short range. I got PPM quite far with some of the $.50 chinese versions.
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Old Apr 14, 2010, 11:04 AM
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cool great tips! I will order some pieces and start doing some testing and inform of the results.
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Old Apr 14, 2010, 11:31 AM
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rich one more question. I see on this transmitter:
http://www.sparkfun.com/commerce/pro...oducts_id=8945

it can do 2400 or 4800 baud. there are code examples for setting baud rate and starting to transmit. That made me start worrying that i do in fact need a controller of some type to set baud rate and tell it to start transmitting.

see here:
http://www.sparkfun.com/datasheets/R...alkthrough.pdf

Can you advise me on this?
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Old Apr 14, 2010, 01:25 PM
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Those are just ASK transmitters and receivers, ie basically just AM with a digital input. Within limits, whatever digital signal you feed into the transmitter will come out of the receiver.
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Old Apr 14, 2010, 01:43 PM
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Not true. As mentioned these are AC coupled so any low frequency components will show up as bursts of noise. That is why only NRZ signals like the key fobs these are designed for will work. The Sparkfun hype is pure baloney as anyone who has actually tried these will discover.

Unlike XBee and other more expensive systems they are not:

Quote:
Originally Posted by sparkfun pdf
acting as if you had a wired serial connection between them, minus the wire!
There are no actual "baud rates" associated with these except for frequency response limits. If you feed rs232 bytes in with every other byte a complement the range is better but still not close to what a proper NRZ like manchester will provide.

They work with signals like PPM only because it is repetitive and fairly well balanced but only over very short range (1'). Performance with normal rs232 is dismal.
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Old Apr 25, 2010, 01:23 PM
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Hi Rich et al,

So here is my story to date...please keep in mind im very new to DIY electronics and have no training in this area.

My parts came in (ask xmitters and receivers) and i got to work building a test setup. My initial results were promising. I fed a PPM stream into the xmitter and hooked the receiver up to my tiny DIY scope to see what was coming out and the signal was very nearly the same coming out of the receiver as going into the xmitter. Of course some noise to be sign from time to time but thats to be expected i guess with such a setup as this.

So next step was to see how the Futaba 9C would like this signal. I fed the output of the receiver into the Futaba 9C trainer port and.... nothing. Threw the signal back on the scope with the recevier rig still hooked up and powered to/by the trainer port and first thing i noticed was that the signal now looked VERY attenuated. Whereas the PPM train was nice and tall on the scope before, the moment i hooked it up to the trainer port the "peaks" of the PPM signal became much smaller/lower.

Armed with my limited knowledge i started trying to figure out why this could be and in the end the only thing i found was this:

If i measure between ground & signal of a normal healthy ppm signal which the 9C DOES see and accept as usable, i get a voltage reading of about 4.85 volts. If i measure between signal and ground on the output of the ASK receiver module, its only something like 0.27volts.

Is this my problem? If so can someone explain in dummy terms whats going on here and how i can go about fixing it/boosting voltage/amplitude on the receiver output signal? Im making some assumptions here that the signal is just too small for the 9C to see as valid so its just ignoring it. Im sure that greatly oversimplifying things and being curious to learn, i would love to know if im on the right track and actually have a better explanation of whats going on here.

If im totally on the wrong track, would love to know that as well and have someone point me in the right direction.

Thanks alot for any help
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Old Apr 25, 2010, 01:49 PM
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If your measuring volts with a meter those results are normal which means you have a polarity problem and need to invert. 4.85v indicated an active low PPM (idle high). .25v is active high (idle low w/5v pulses).

Also be aware that even a tiny bit of noise will crash a PPM system. What range are you getting? I'd be surprised if it went very far w/o modulating.

Also I'm curious about your DIY scope. Details?
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Old Apr 25, 2010, 03:23 PM
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Hi Rich,

Here is the DIY scope i was talking about: http://www.sparkfun.com/commerce/pro...oducts_id=9484

The range im testing right now is only a meter or so which is all i need.

Im thinking the issue with the voltage has to do with these module only able to deliver 10 micro amps and i think the futaba radio needs more than that to drive it so im working out how to amplify the signal before it goes into the 9C
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