Using AUX pins as relays for CHDK

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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      • It was very difficult to find S2 multiport in local electronic store, so I decided to bring RX100II to camera service center to wiring shutter and ground out from camera shutter button.
        And it was work to trigger camera in Pixhawk without any additional circuit. When camera connected to Pixhawk in idle position, sometimes camera take a photo without any trigger command from Mission Planner.
        Then I add Hugues circuit, camera still take a photo without trigger from MP although in longer interval to shut. Is that happen because of voltage in Pixhawk not stable?


    • I don't understand why you need the 5v from the PH. Why is the 3.3v signal not enough to allow the Camera's own power to flow through the transistor? Is it because the Sony is also only 3.3v, and it would need to be higher voltage than at the base of the transistor?
      I have very limited knowledge of circuitry, but I'm eager to learn.
      • MR60

        I do not know for sure but my thought is that in my previous scheme, the focus and shutter pins did not have a reference voltage and were floating. By using the 5v from Pixhawk in this scheme , I am forcing the shutter and focus pins to a fixed 5v and when the trigger order comes on the S line, the transistor conducts , and therefore shorts the focus and trigger pins to ground which is the objective.The 10k resistor is added to limit the current that goes through the 5V line to ground when the transistor conducts.

        It works perfect now with this circuit.

        • Do you think there is any possible risk to the camera? Is was not meant to receive external voltage. I see that resistor probably takes care of the risk, but it is worth considering carefully.
          • MR60

            I do not think there is any risk to the camera as Marco is using the exact opposite circuit than mine by applying 5V to the focus+shutter pins. All is fine.

            The huge 10K resistor makes it so that there is no current going on the pins. So just 5V with no current will never damage the camera.

  • MR60


    As the thread did not show a simple circuit to trigger a Sony Alpha camera yet, here is one I made and tested succesfully today.

    Some explanations to trigger Sony cameras with a multiterminal port  (also named a S2 port): to trigger , you need to short two pins (=focus & shutter pins) to the ground. That is interesting because we do not need any power up circuit from the Pixhawk 3,3V to 5V (like CHDK/Canon cameras would require). We just need a "switch" circuit that will connect the focus, shutter and ground pins of the S2 port. This can be done with one transistor as shown in the schematic/picture below:

    3702059423?profile=originalNote that in addition to making this small circuit (less than 1€), you need to buy a S2 cable (can be found for 10-15€ online).

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