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Operating a relay with the Pixhawk Aux pin

Posted by Phill Simpson on September 17, 2014 at 2:22pm

I've opened this topic to discuss the question Reuben asked on the 3.2 beta thread regarding the abiltiy to use the Pixhawk to control a 3 volt relay.

My feeling is that you can use the circuit that Marco proposed on the "Using AUX pins as relays for CHDK" thread with a few changes. The spec sheet for the relay that Reuben has lists a coil resistance of somewhere between 25 and 60 ohms depending upon the model of the relay, so the maximum current using a 3.3 volt supply would be 132ma. That is within the range of the 2N3906 in Marco's circuit. So a circuit that looks like the following should work for him. Any other suggestions?

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Replies

- - - - IdefixRC > Henri June 28, 2015 at 5:32pm
        
        Hello Henri,
        
        I'm trying to short 2 pins inside a GoPro remote to trigger the camera shutter and have tried your circuit. Unfortunately it provides power to the reed relay when the relay is off (0v on the Pixhawk pin) and opens the relay once the relay should be on (3.3v on the Pixhawk pin).
        
        Could I drop the 2nd 2N7000 and connect the relay to the first one instead like in the 2nd Diagram below? Or are there issues with that design?
        
        Thanks a lot,
        
        Eric
        
        Reuben > Henri September 18, 2014 at 4:07pm
        
        Hi Henri .. thanks so much for the extra info .. yes I know about using diodes to drop voltage , so I now under stand why you did it..
        
        I see that this solution could also be used and a switch. To ground two wires two each other.. just need to change the incoming feed from 5v to one end of the earth pin and the other pin to the out put of the transistor ..some cameras just need to short two pins to trigger ..
        
        Thanks
        
        Jon Farhat > Reuben May 29, 2015 at 5:30pm
        
        Hi Reuben
        
        Thank you, as well as Henri for this circuit layout. I built it and hooked it up to a HackHD board camera which usually just needs to short two pins for under a second to start and stop. Tactile switch speed. This circuit passes some current through I believe, and though it works, it fires the camera up in Standby mode, usually needing 3 seconds of contact.
        
        I was wondering if you could explain how that circuit would look if if were only a switch.
        
        thanks