Hi,

Like a lot of you, I own a Graupner MX12 transmitter. Great device but it is only a 6 channel transmitter. I surely could use 2 extra channels .
Curious as I am, I hooked up my scope to the ppm data line comming form the MX12 logic board connected to the RF board.
Guess what...there are 8 channels... But channel 7 and 8 are fixed. Is there a way to modify it? Well with no schematics or source code available I hardly believe that there is something to achive.
I noticed the ppm signal is TTL level, exellent. What if, we replace the last 2 channel data with our own channel data. A small micro-controller could do the trick.

So, what do we need:
To get an accurate measurement of the 2 extra channels (10 Bit ADC) we need at least a 16 bit timer.
The smallest AVR having these features is the Tiny24/44. It also comes with an internal calibrated 8mHz oscillator.
Further components needed
-5 volt regulator, using the supply voltage for the RF board.
-2 Potentiometers
-a few pasive components (6p. program header and capacitors)

Optional: 8 mHz Crystal if the internal oscillator isn't (calibrated) accurate enough.

The ppm signal is connected to the Input capture pin and triggers an interrupt for eigther the rising or falling edge.
After detecting a minimum startpulse it directly copies the ppm input signal to the PPM out. After doing this for the usual 6 channels, the
timer is reconfigured for using COMP.A. The interrupt handler for comp.A loads the 10bit ADC value of channel 7 and 8 potentiometers as 0..1ms timer value.
When the extra channel information is processed, the timer is then reconfigured again for Input Capture to synchronise the start pulse again.

Because it uses edge trigered interrupts there is no traceable delay in the ppm output signal.

Very straitforward project. Only one ADC ready check at startup, the rest is processed by interrupt handlers only.
At this moment the compiled code uses 741 bytes Flash and 55 bytes SRam. I'm using the Tiny44 for now,but the Tiny24 could also do the job. It could save you another 10 Eurocent

I haven't designed a boardlayout yet, but I will post my little project soon, also depending on your input.
So If you have any additional ideas, "nice to haves" or even "must haves", please post your comments!
I will post some pictures tomorrow.


Regards,
Jacques

Sounds really interesting!
I'm waiting for something visual

Is it outputting those additional channels out of the blue, or is it reading two pins with nothing attached to them? If the latter, there might be a quicker way to add a switch or so. I think I read something about this a while back, but can't find it.

Really interesting.

Please show more of this project!

Thanks
Kind Regards

neat,

i could also use some extra channels on my mx-12

regards
Rob - subscribed

@Arthur,
If I knew how to connect two potentiometers directly to the MX12 Transmitter logic I would chose that option.
But I do not think Japan Radio/Graupner left us this option open.

@All,
Here's the schematic:
externer Link:

Heating up the controller with my girlfriends hair dryer did not show any change in timing with the internal oscillator nor using the external oscillator.
You can download the Eagle schematic file as example for your own mod: http://www.file-upload.net/download-846607/MX12-2-ppm-Channel-Expander.sch.html

Here some pictures of my MX12 mod
externer Link:

I used an external housing (RF shielded)

externer Link:

and put the handlebar on the new housing

externer Link:

Only a small hole in the MX12 housing is needed for getting the wiring out.

externer Link:

Scope view of the original ppm signal (on top of the scope) and the modified ppm signal.

externer Link:

A close-up of the channels 7 and 8. Channel 7 (bottom is set to maximum (total 2ms) > ADC value = 1000. Channel 8 is minimum 1ms > ADC value = 0.

externer Link:

Now Channel 8 (bottom is set to maximum (total 2ms) > ADC value = 1000. Channel 7 is minimum 1ms > ADC value = 0.

I've made 2 videos of my MX12 modifation:
http://www.youtube.com/watch?v=V_oXNHI-sjk
and
http://www.youtube.com/watch?v=5AAQWFyhPu0

The project file is made with IAR workbench. Due to lack of time I did not manage to convert it into a WinAVR project.
I will post the source code later.

If someone wants to finalize this little project in a more proffesional way please sende me a pm.


Jacques

As promised, herewith the source code for v1.0. It is build with IAR Workbench V5.11B

http://www.file-upload.net/download-847597/MX12-6-to-8-channel-ppm-expander-v1.0.zip.html


Jacques

Hi Jacques,

did you check what happens if you use pcm mode? In this case the signal should only be bypassed..

Steffen

@McClean,

There is no bypass for PCM at the moment and I did not test the software on PCM mode eighter. However, I believe it would be possible and it should detect PCM mode in the future, but I'm not using PCM at the moment and do not have the time to investigate this.

Perhaps someone with enough spare time could help us implementing the PCM detection.

Volunteers?

Jacques

[OT: good Beer-taste: Brand ]

Hello Jaques,

Good work! I had tried the same work based on the BL-Ctrl or Flight-Ctrl H/W, but i hat not enough time for that. Your way is very simple and it is easy to realize it on a little prototyping board. It is possible to integrate the expander inside of the MX12. There is enough room between the existing electronics and the battery. The potis can be placed somewhere at the case, so there is no need for an addistional box. But i think, you need an pulse shaping circuit (lowpass) at the output of the expander to ensure that you have the same pulse form as the original signal. I'm not shure, but please compare the puls form of the original signal and the modified signal. If you have the same pulse form (rise and fall time) you can ensure that the transmitter produces no additional harmonics in the RF signal. This is important to avoid interferences to neighbour channels.
It is also possible to use the existing (not used) switches for some additional functions.
I think you can add one further additional channel for more functionallity

dynahenry

@Jacques

Did you tried to add a 9th channel. In the PPM timing is still some time for an additional channel and you have an 3rd ADC available at the Atiny. The PPM Frame is then not within the PPM16 specification but this should not influence the sum signal decoding at the FC.

This additional expansion would be even more interesting also for other RCs like my 7 channel MC12.

Regards Greg

@dynahenry and killagreg

Thank you both for the feedback.
A 9th. channel, hmm, why didn't I came up with that obvious but great idea.

I've finished the software for the 9th. channel and test it comming weekend.
One extra feature: If pin (13) PORTA0 is connected to ground whilst starting up, the software is in 9th. channel mode.

Regarding the lowpass filter; If someone has a sugguestion about typical low pass filter. I do not have to find out myselve 82k/100n ??

Does the MC12 RF board use ppm TTL signal levels? If so, would it be wise to have a version that only add channel 8 + 9 or replace the 7th. channel of the MC12?


Jacques

I would pick up the PPM signal from my MC12 at the learner port and send it further to the RF board using the teacher port as a kind of a loopback function with tha atiny in between.

Jacques meinte

Regarding the lowpass filter; If someone has a sugguestion about typical low pass filter. I do not have to find out myselve 82k/100n ??

It's not that difficult:
externer Link:

killagreg meinte

I would pick up the PPM signal from my MC12 at the learner port and send it further to the RF board using the teacher port as a kind of a loopback function with tha atiny in between.

When the PPM Signal comes out of the learner port, the RF is deactivated.

Chavotronic meinte

When the PPM Signal comes out of the learner port, the RF is deactivated.

I dont think so, because in the manual of the MC12 is written that the transmitter crystal must be removed at the learner rc.
(http://www.graupner.de/fileadmin/downloadcenter/anleitungen/4725.41.pdf see page 12 left column line 8.)
But I will try that measuring the PPM at the receiver with an oszilloscope at the same time I capture the PPM from the learners port.

Regards Greg

I am sorry, i was talking about the mx-12, not the mc-12

@Vaid

Zitat

It's not that difficult:
externer Link:

You are absolutely right, but what frequency should we use?

Cheers,
Jacques

I am not sure if the lowpass will do the thing. Here we need more something like a raised-cosine filter.
Does someone have the schematic of the mx-12? I suppose it has its own pulse shaper so that its not important to do it on our own.

I've tried to get my hands on a MX12 schematic....no luck....
Let's forget the pulse shaping circuit (lowpass) for a moment.
Here's the demo of the 9th. channel in action:
http://www.youtube.com/watch?v=WBAfNVgm9zg

The proto board does not make use of SMT components yet, but here's a impression:
externer Link:


Note: The External Oscillator is placed but not used!
I have dropped the idea of 8 or 9 channel mode, just trying to keep it simple.

To do:
- Determen if an pulse shaping circuit is nessecery
- PCM detection
- decrease board size by using SMT components
- ...
- ...
- Publish complete project


Jacques

Perfect!

looks real good so far.. i am very interested in upgrading my mx-12 now.
good work!

regards
Rob - looking forward to a SMD board

Hey Jacques,
I´m impressed.
This is amazing and i hope there will be something like a HOW TO published soon. I do no a little about circuits...but this is way to deep into it. I like my MX-12, but the channel restriction really sucks

So...keep on going the nice work and I hope we get some detailed instrucktions later

greetings
Phil

Hello Jacques, i could route the SMT board if you like to.

Hello,
I have a MX16S transmitter.
Is this model expandable for some additional channels too?

Dietrich