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Old Aug 16, 2014, 04:01 PM
Who let the dogs out?
phil_g's Avatar
Pontefract, Yorkshire, UK
Joined Jul 2007
1,069 Posts
Kraft Series 71 retro-refit

Sometimes its good to try something new! My Godson had been given an Arduino kit for his birthday, and he was starting to ask questions I couldnt answer – as a dedicated PIC assembler programmer I'd never touched an Arduino - so I thought I'd better learn something about them, and another retro transmitter conversion seemed a good place to dive right in at the deep end.
So this is an Arduino-Noob's first and only coding project!

After 17? years of MPALC/MPASM I found the Arduino really easy to code, just a simple c++ -like script. Some functions (like 'map') made me giggle out loud, its almost cheating!

When I was a kid the Krafts were very desirable sets but way beyond our budget, so its great to not just own one at last but to fly it frequently too, and with the benefit of a few convenient bells & whistles on top. I'm calling this project 'finished' but you know how these things go...

This has been a fun project in that the rewards come easily, the Arduino is simplicity itself and most of the hard work has been done for you by the designers, leaving the user with just a basic scripting job.
Let me admit up front that my Arduino code is very simplistic, there are many far more complex and very clever encoder projects out there in interweb land, including the Microstar, 9x, ArduinoRC, Patolin and more, but this one suffices for a retro conversion where there is no lcd screen or fancy programming interface, and its probably easier to make than any of the posh ones. I enjoyed doing it.

You can even make one without soldering, using one of these.

With the back off (screws long-lost...) you can see the Arduino Nano, a 2S lipo and the Frsky DHT module.
I removed the LED from the Frsky bind board and fitted a new extended one behind the Kraft meter, and I removed the bind button and extended the connections to what was the Kraft trainer button.

On power-up for the very first time, the sticks need to be set up. To calibrate, you hold a button down, switch on, and still holding the button, move each of the sticks into all four corners (including the mechanical trims, or not, as you prefer). When done, you let go of the button and it saves the maximum and minimum values from each pot to flash. Every time you power-up it reads the calibrated values from flash and uses these for the stick maps. You can repeat the calibration as often as you like but it only needs to be done once unless you change the stick configurations.

The 'calibrate' button isnt dedicated - its also my little surprise which you’ll see in the video!

In this case, channel 5 is a conventional 2-position switch for retracts, flaps etc and has built-in servo-slow - but any combination of propo/switched/momentary channels is possible.

The Arduino is a piece of cake to work with, its very quick to try ideas although I found the sequencing can be a bit awkward with the enforced setup() and loop() constructs but coding is laughably easy – for example the ppm frame output including sync can be done in only 5 statements by having the sync as another (long) channel.

This particular encoder is intended for simple retro transmitter refits like the Kraft, so no LCD or anything fancy, its very basic but it does have some useful (and some novel!) features:
  • Frsky 1way or 2way switchable
  • Self-calibrating sticks - no trimpots or voltage measuring to do
  • Up to 6 propo/switched/momentary channels plus one servo-slowed switched channel (I'm uncomfortable with more than 7ch in a 20ms frame)
  • Servo reversing by holding stick over on power-up, saved to flash
  • Rates, Expo (thanks to Paul Luby for the expo maths)
  • Single-handed range-check mode with throttle held off
  • Two mix ratios, 50:50 for V-Tails and 75:25 for flying wings like the 'Wildthing' EPP slope-soarer
  • Throttle lock-off for when retrieving electric models leaving the tx on the flight-line
  • Auto throttle lock on powerup (on switch on, throttle has to be closed to enable throttle channel, thereafter it acts normally until throttle-lock switch is thrown)
  • Inactivity warning after 10 minutes, stops you leaving it switched on and ruining the lipo!
  • No bootloader delay for instant ppm.
  • ... and ... Full single-channel escapement emulation mix, compound and sequential !

The set has been flying my test-bed Multiplex Cub at every opportunity for several weeks now and it feels lovely, just like a Kraft did in its heyday! The Single-Channel mix is great for newcomers to 'the button' as you always have the propo 'safety net' to save the day. You could even extend the button with a fly-lead, 'buddy-box' style! It's helped me to re-learn the art of sequential rudder-only flying, which we used to do effortlessly back in the 60s!

This video is a quick summary of the completed project, I've tried to keep it short as I've been told my videos are too long and boring!

Kraft Series 71 refit completed - final rundown (7 min 23 sec)

So thats my first and only Arduino project so far, I realise that in the context of RCGroups contributions its all very simplistic, but it has taught an old dog a new trick, which after all was the object of the exercise - and at least now I can answer some of my Godsons questions!

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Last edited by phil_g; Aug 18, 2014 at 07:53 PM.
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Old Aug 17, 2014, 03:47 AM
Registered User
Romania, Dolj, Craiova
Joined Sep 2007
14,425 Posts
No OpenTx?! You broke my heart
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Old Aug 19, 2014, 11:44 AM
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MAVA's Avatar
El Dorado Hills, California, U.S.A.
Joined Mar 2000
548 Posts

Thank you for sharing your project. Now i know what to do with the dozen or so Kraft Transmitters.

Good job,

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