I'm confused and a bit frustrated, would appreciate any help as I'm not sure how to proceed.

Setup: I have a pixhawk autopilot running Arduplane 2.77, using Mission Planner 1.2.95

I've been following this guide to get my CHDK enabled camera working with my pixhawk. I'm interested in getting the camera triggering based on distance traveled.

Obviously the guide was written with APM in mind, so it needs to be adapted a bit for pixhawk; specifically the setting for RELAY_PIN. I've kept the rest of the settings the same as in the tutorial.

Pixhawk has 6 AUX ports according to the quick start guide, I am assuming that the numbers above the pins correspond to the AUX port numbering. When you mouse over the RELAY_PIN parameter, it says that pin 54 corresponds to Pixhawk FMU AUX1.

Tried setting RELAY_PIN to 54, no joy. Did a bit more looking, discovered in the release notes for 2.77: 

"Improved relay code
The relay and servo set code has had a major overhaul, with up to 4 relays now supported for MAVLink control and much better support for the DO_SET_SERVO, DO_SET_RELAY, DO_REPEAT_SERVO and DO_REPEAT_RELAY MAVLink commands. Along with these changes is a new parameter BRD_PWM_COUNT which allows you to specify how many auxillary PWM outputs to enable, with the remaining outputs being available as digital relays. This allows you to re-assign some of the aux servo outputs on Pixhawk for use as relays, by setting the RELAY_PIN, RELAY_PIN2, RELAY_PIN3 and RELAY_PIN4 parameters. The pin numbers for these pins start at 50 for the first aux servo pin, and go to 55 on Pixhawk."

Additionally,I found this github issue about this same problem that states that the Pixhawk can't trigger CHDK because its relay pins operate at 3.3V instead of 5V.

So i suppose my specific questions are:

1) Is AUX1 pin 50 or 54?

2) Can anyone confirm that the relays, when working properly, won't be able to trigger CHDK?

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I have all the components ready to be soldered, but I am now a bit confused about the recommended resistors…

What would be the best values for R1 and R2? Thanks.

Anything over 100 ohm is ok. There is no best value here. There is practically no difference if you are using 220 or 220k ohm resistors.

Circuit design is always a question of trade offs.  If the output impedance of the pixhawk was higher (i.e. current limited) then the value of R1 (the one limiting base-emitter current) would matter.  And if the input impedance of the camera's USB +5V pin was lower (i.e. more current needed) then the value of R2 (the one limiting collector-emitter current) would matter. As it is, the range over which R1 & R2 will work is large.   So the trade off is then the extra current used as you lower R1 & R2 (which at some point will overload the pixhawk) versus noise immunity degrading as you raise R1 & R2.   Personally, I'd go with 1K resistors given that trade off.


Have you received the cable?

how has it worked?


Have you receibved your cable?

Is it working for you?



Hi Joe

I am in fieldwork time, but I consulted about my mailing in my city, and one package from Australia arrived the last friday. The sender is Andre Preller. I am pretty sure that is your shipmment. I am excite to review it. I am returnt to the city the next week. So I estimate that i be able to test it in two weeks, I hope. Sorry for this long period but This is a bussy time for me....Thank you so much for your confidence...

Hi Marco, maybe an old topic.

I'd like to add a micro fuse to your setup as you suggested.

What kind of microfuse did you added? Rated?

Thanks in advance

Hi hugues i have some problems with your circuit. What is the usb pinout connection ?. Shutter and focus goes to V+?


How did it go?

Hi Plino,

Did you get it?

Did it work for you?

Hi all, 

I just want to say that I tried the Marco's circuit and worked OK on my Pixhawk to trigger a Canon S110.

My settings:

- 5V BEC connected to AUX pin 4 (RC12)

- Servo cable to circuit Input: connected to AUX pin 5 (RC13)




CAM_DURATION: 10 (1 sec)

CAM_TRIGG_DIST: 0. Click "trigger camera now" in Flight Data and worked OK!




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