Or as an alternative, you can use a gear/door sequencer. I was having a problem with the doors interfering with the gear during retraction (since the doors are not held open with positve pressure with the air micro switch method). I use the Mini Hobby unit in conjunction with a second air valve to hold the doors open and with plenty of delay to allow all three gear to retract prior to the doors closing. It's a little more hardware but it simplifies the sequence setup and virtually eliminates the aggravating "partial/jammed retract" issue during flight.

Craig