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Hacked FailSafe for the Spektrum DX6
This project is kind of obsolete since Spektrum released a bot-specific version of the DX6 receiver with bot-appropriate fail-safe functionality. It can be purchased here.
The Spektrum DX6 appears to be a wonderful radio for use with combat robots.
For $200 you get a 2.4GHZ system that should be reliable and isn't subject to frequency conflicts. No more crystals.
One big problem - the failsafe is only settable for the "throttle" (weapon) channel. All other channels hold the last known position in event of failure. This can result in a runaway robot and failing the safety inspection.
I found this out after I'd already ordered my radio, so I decided to hack up a solution.
The Barello ANT100 board uses an ATTINY26 microcontroller with an exposed reset pin (see http://www.atmel.com/dyn/resources/prod_documents/doc1477.pdf). A little bit of precision soldering is needed. I put together a little board using a PICAXE 8M to monitor the throttle channel for the failsafe signal. The throttle channel "passes through" the board - and can still be used for a weapon.
Total cost is under $10. A little more if you need to purchase a PICAXE development kit - but probably still under $20. See http://www.phanderson.com/picaxe/picaxe.html. or http://www.rev-ed.co.uk/. Dr. Anderson probably offers a better deal on shipping for U.S. customers.
I used a PICAXE because it was cheap, easy and had some "in stock." This design could probably be easily adapted for other microcontrollers. Should also work for other speed controllers which have a "Reset" pin enabled.
The trick here is to set the failsafe at the lowest throttle setting, and then trim the throttle up just a little. This way, the board will only trigger the failsafe when the signal is actually lost, not just if you pull the throttle stick all the way down. You may need to adjust the code a little to get things working perfectly.
Weight for the completed board is about 4 grams.
Disclaimer: This board seems to work for me - no promises that it will work for you. It may fry your radio and burn your house down. Some of the resistors may not really be needed. I also suspect that someone with more of a clue than me might put some capacitors in there someplace.
'SpamButcher Combat Robotics - http://www.spambutcher.com
'Use this code freely!
'Spektrum DX6 Failsafe module for use with Barello ANT100 (http://www.barello.net)
'PICAXE 8M / 4MHZ (http://www.phanderson.com/picaxe/picaxe.html)
'pin 0 connects to ANT100's ATTINY26 reset pin
' (see http://www.atmel.com/dyn/resources/prod_documents/doc1477.pdf)
'pin 2 is status LED
'pin 3 is throttle output
'Solid - good signal
'Fast blink - fail safe signal
'Slow blink - no signal (result of pulsin timing out)
'if throttle is low failsafe
if w1 < 120 then goto failsafe
'set ANT100's reset pin high (enabled)
'turn status led on
'set ANT100's reset pin low (disabled)
'blip the status LED
pulsout 2, 7500