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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No, you can not. See previous posts on these converters. You need to use Marco's or Reuben's circuit.

Tks  Phill Simpson! I will build Reuben's circuit to my CHDK. My step-up 3.3-5v goes to the drawer :( haha.

Sorry, one more question... Can I use FrSkyRx 5V output for power the circuit? I don't have 5v bec on my OPTO's ESC's...


Is your FrSkyRx powered from the PixHawk? The PixHawk instructions are pretty clear about not putting any additional load on the power outputs. I don't know how much current will be drawn by the circuit you use but it might cause a brownout if you are also have other devices plugged into the Pixhawk. Standalone 5 volt BECs are cheap compared to the cost of a crash.

Plinio  .. It is always best to use a two 5v supplies .. One used only for the Pixhawk flight controller .. The other for Aux Devices.. all you have to do is plug a turnigy 5v bec into the 5v rail on the bottom of the Pixhawk .. then you can drive other devices without worrying about stealing 5v from the PixHawk operating system.

Regards Reuben..

P.s It is always a bad idea to use 5v becs from an ESC .. do not do this .. use a dedicated 5v supply.

Add Aux 5volts like this.  Only to pins 1 to 6

I think I just had my light bulb moment.... Regardless of the voltage you put on the output Vcc rail, the signal voltage will only ever be 3.3 Volts. Is this correct? If so, you actually need two things;

1) You need 5 volts on the Vcc rail to feed to the USB connection 

2) You need to use the 3.3 V signal to switch the 5V.

Can someone tell me that I have finally worked this out?


You have finally worked this out!

Eureka !!

Yes, it was like a blinding light going on inside my head! I couldn't understand the problem with the whole 5V thing. I assumed that hitting the rail with 5V meant that the PWM signal also went between 0 and 5V......noooooo!  I think that has been confusing everybody. Where you get the 5V from is irrelevant. The trick is switching with 3.3. This now becomes a trivial task.

While I have no doubt that Rueben's circuit will work nicely. I think this should do the job;;


Hang on....does anyone know the "off" voltage for the AUX pins?

Hi ..Nice find, but i see the price $35AUD each ..plus shipping ? My little unit does the same in fact i believe it is more or less the same circuit.. only diff is mine only costs Less then $2 and a little time to solder the wires on ..Lol

The Pixhawk 3.3 volt output has a limited ability to source current. The switch you listed appears to require at least 15ma. I couldn't find the specifications for the Pixhawk 3.3 volt output but it might be too much. Reuben's and Marco's circuits are tested and known to be reliable.

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