Here's my solution for automatically operating the DX6i's timer off the throttle stick. Why design yet another one, when there are already two available? One is commercial, the other isn't open source. Both are more complicated than they need to be. My design is so simple that you don't even need a pcb. Just wire the PIC chip directly into the throttle stick cable, and add one extra wire connecting to the Throttle Cut button.
You could use the Trainer switch instead, but I chose the Throttle Cut button because it's otherwise redundant on an electric model, and I don't have to worry about the timer getting accidentally activated by other Trainer switch functions (binding, range check, etc.). The Throttle Cut button can still be used to manually operate the timer if desired, though it might get out of sequence if the throttle stick is not in the appropriate position.
On my radio the timer starts when the throttle stick gets up to 20%, and stops just below that. The exact switching point may vary from one radio to another depending on pot calibration. 20% is low enough that most power systems are idling and drawing minimal power, so it's virtually off as far as timing battery usage is concerned.
If you need to disable the automatic timer to use the Throttle Cut button for its original purpose (cutting your ic engine's throttle) simply hold it down it when turning on the radio. The PIC will then go to sleep until the radio is turned off and on again.
My circuit is obviously not applicable to a collective helicopter running in idle-up or 'stunt' mode. In this case it would be better to connect the throttle input (pin 3 of the PIC) to the Flight Mode or Throttle Hold switch.
Hex files are included for PIC12F675, 12F615 and 12F683. So far I have only tested it with a 12F675.
Source code and hex files