PixHawk... Chinese clones, 3.3v aux switch & gratitude

Long time member, back again...

I just ordered a *real* Pixhawk (I know, I know, -sigh-  should have done it right the first time...) after being burnt by a Chinese clone whose I/O side seems to have died suddenly. I tried everything I could find on the net to unbrick it, but alas, I accept what is to be... Get what ya pay for right?

No warranty, no assistance, no replies to emails, no cares from the Chinese retailer, (a guy can hope right?) just another bunch of electronics destined for the recyclers.  Makes me sad about the waste.  I wonder if I could offer 3DR enough $$ to see if they could fix it, or would that be an insult of the highest order?  Anyway...

Years back I bought a real APM 1.6 (red and blue boards that connect together) and all its peripherals from 3DR to dive into the hobby and support the 3DR vision-  it still flew when it was decommissioned 2 years ago. That APM never had any problems after one early return to 3DR for glitchy hardware (a great experience as I recall- the customer service, not the grounding of my quadcopter) I have learned alot thru the hobby and put my years of electronics experience to good use.

***Lesson Learned*** Support 3DR/ developers and buy the best hardware from the best in customer service. Ouch, Got it.

Now something to add:

   PIXHAWK AUX RELAY OUT 3.3v EASY SWITCH:

2n3014 transistor (NPN, t-92 case, Hfe of 160+) is perfect for switching 5v (or 12v) with a 3.3v base signal to trigger a tiny 5v (or 12v) relay. This method works great so long as the grounds for the Pixhawk and device are tied together somewhere.

2n3014 may be hard to find, somehow I have a dozen, but any small transistor or FET is fine that saturates (turns on) at less than 3v and will take the load voltage/ current across the switching pins.

The collector pin on the transistor got soldered directly to one coil leg of the relay, +5v to the other leg of the relay, ground goes to the emitter and a 68ohm resistor between the Pixhawk out and the base of the transistor limits output current from the Pixhawk to less than 50ma.

Pixhawk sends 3.3v to the transistor which activates and grounds the coil of the relay. Simple.

Tiny relays can simplify things by allowing multiple unconnected circuits to be controlled by one signal.

I have channel 7 on my Tx set up to turn my RGB orientation LED strips (normally red front, green rear) to all white for landing.

 Thanks to everyone who makes open source projects happen (thats, um, all of us right?) but especially the ones who dare, who try, who risk, who ask the questions others may scoff at;

the ones who take the time to answer those sometimes repetitious questions with patience and wisdom;

and most of all *MANY THANKS* to the DEV crew who put so much into our very addictive hobby to make it what it is- from the amazingly reliable, well tested hardware and software to the excellent forum and documentation. Awesome!!

Gratitude expressed, back to soldering...

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  • So I was just researching more on Pixhawk aux out current limits. I will be changing the input resistor on the 3.3v aux out line to the base of the transistor to a 330ohm  .I just found evidence suggesting that the aux out pins on the Pixhawk are only capable of carrying 25ma @ 3.3v.  3.3v into a 330ohm resistor puts output current at 10ma, well within limits.

    • And today I changed the relay trigger circuit again...

      Clare Semiconductor (et al) makes a great 8 pin IC that houses a pair of optocoupled FET based switches.  It turns on at about 2v and 5ma so our 3.3v works great with a 330 ohm resistor to protect the Pixhawk from over-current on its output.

      Pins 1 and 2 are + and - trigger for one switch whose outputs are pins 5 and 6

      Pins 3 and 4 are + and - trigger for the other switch, outputs are pins 7 and 8.

      This one IC can control 2 relays with separate inputs from the Pixhawk and because it is opto-isolated, it will not add noise back into the ground side of the Pixhawk which can potentially cause other issues in sensitive electronics.

      Just another option, I like this one best so far...

      Ill keep updating if I find other options that work as well, hope it helps. 

  • An easy alternate method is using the PWM output to trigger the "picoswitch," a small relay.  I use this to control lights and a second relay to trigger my camera shutter.  It's worth a look.

    http://www.dimensionengineering.com/products/picoswitch

  • Eric J,  fantastic post. I always enjoy seeing people that are as passionate about their electronics as I attempt to be. Although I have no formal training in electronics I have succeeded in making my UAV fleet fairly robust and customized for my and my customers needs. We trigger all of our lights, landing gear, airbrakes etc. directly from the receiver and not through PixHawk. Your post piqued my interest though with the idea of the AUX RELAY OUT 3.3v EASY SWITCH idea. We fly some larger 1800mm wingspan Skyhunters as well as others and I have always wanted to put a Pixhawk enabled failsafe parachute or triggerable parachute for no landing zone conditions. Your solution makes me think that could be achievable without having to bypass the RC out rail on the PixHawk.

    Thoughts?

    Interested,

    Chris B

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