I tried to test some different modes and frankly nothing else comes to my mind anymore other than to put a heavy 40A relay to a main power line and attach a servo to it, so I could map it to one of taranis switches so in a case of malfunction or lost orientation I would be able to kill drone in the air or on the ground and stop props _immediately_.

Any other option I tried - a combination of 'land' mode with throttle cut off, an improved failsafe on remote switch off - none of that cuts power feed to props immediately, and quite often it leads to horrible results, like if in loiter mode drone suddenly gets to a GPS free zone or glitch, and starts something erratic, or flips over and tries to destroy everything around spinning props at full power ignoring remote commands to cut throttle input to zero.

I had only 3 incidents like that and I know now what it is, but it does not make my platform any safer.

What do you do for a physical kill switch? Do you have one built? If yes, how was it done?

Please respond.

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  • @Thomas

    I used 10 because of thermal issues. 3 of them would dissipate ~21w@60A load but 10 would only dissipate ~6.3w@60A and safe without heatsink(?)

    BTW. Looks like Emcotec has a turnkey solution


  • @Patrick,

    Yeah! That's what I was talking about. They use three 24A/190A burst MOSFETS in parallel with a couple of flip-flops set by PWM. Although it is only for 3S, some beefier MOSFETs would fix that.

    Additionally, probably some control on the gates to slow the power to the capacitors in the ESCs. A bit more logic and it would function as a main power on/off switch.

    Perhaps with a full-current XT60 "safe" jumper; removed when not flying to prevent inadvertent power-on.

    Good sleuthing Patrick!


    Why so many MOSFETs? Ten? Those IRFZ44s are rated 49A continuous(160A burst). What was the continuous load you were aiming for?

    Happy flying!

  • @Thomas,
    Looking at these rules

    There is an official Kill Switch design that looks a lot like your design !! :-)

  • Another reason not to use the motor-interlock/emergency stop feature is the of the pilot's usage of an APM2.6/2.8 board.

    These features are not compatible with these versions of APM, if anyone has managed to overcome this, I would accept that as a valid solution in my use case.  

    I understand the benefits of creating an auxiliary system to cut the power, but for the meantime, if it can be done in software that would make my flights considerably safer. 

    Keep me posted. 

  • I did something like this a time ago, inspired by a product already on the market(can't remember it's name).

    Put 10 IRFZ44N in parallel, added some pullups and pulldowns and two switch connectors. One connector turned the MOSFETs off when shorted and the other turned it off then not shorted. This would give the ability to plug in a switch, with the obligatory "remove before flight" tag, and safely handle the aircraft on ground. The other switch would be tied to a servo, that would remove the jumper remotely and scuttle the aircraft.

    Unfortunately I never got the prototype working, and the project was abandoned.

    Here's a picture


  • @Ultrafudge,

    The objective is to shut down the copter when the software has gone haywire...i.e., no reliance on the flight controller. A totally independent failsafe way to turn off. Usually the copter is out of control BECAUSE the flight control software has failed.

  • Others just use 'Emergency Stop Motors' or 'Motor Interlock' .

    Auxiliary Function Switches (3.6 and earlier) — Copter documentation
  • hey @Patrick, this is good! Getting some design issues talked about for our new "kill" switch...I suppose it would be a kill switch that takes advantage of the added circuitry to create a master power switch too. I have not looked at the 3DR Solo, but I think it has a master power switch...need to check the schematics. I don't think getting 100% reliability is an issue considering the quality of components available today.

    IN my design, the switch will NOT require a constant RC signal; just like the other RC controlled attributes.  The kill would be user activated, so whatever circuitry is required to enable that type of action is what would be created. Not that big of a deal...manual power on to set a latch and then say a full PWM to trip the kill; 2ms. HobbyKing sells a PWM controlled switch, but a better solution would be a custom circuit. Perhaps with a counter to insure the 2ms is active for 0.5sec (or more) as a debounce or glitch prevention of inadvertent tripping. Of course then we need a hardware cover to protect the kill switch to prevent inadvertent tripping.

    @Henry I saw that ST 100A MOSFET you suggested. A better one would be 60V 200A IRFS7530. You specify 100A but a typical UAV would require 100A as a burst for a few seconds; I"m sure you know full power is not used for continuous flight; MOSFET size also depends on your motors. The continuous current would be a fraction of the burst. Additionally, you will want the voltage overrated so the circuit can take up to a 6S cell and handle ESD, ringing, etc. Using several 7530s MOSFETs in parallel would provide some failure protection if one blew.

  • @ Thomas, if you think your design is good, just call it a Remotely Controlled Master Switch .... not a Kill Switch ;-)

    To illustrate the difference and the risk associated, lets say that you need a constant radio signal to activate the Master Switch, what happens if this radio signal is lost ?  Technically the  Master Switch would turn OFF , causing an unintentional crash...  This is what we call a risk of failure....

    Statistically, the Master Switch must be reliable at 100% at all time of the system operation and the Kill switch must be reliable 100% at failure, depending on the design of the system , it can be  less than 0.01 % of the time, so your Master Switch design must be 10,000 times more reliable than a Kill Switch.

  • Forget about added parts and weight for the moment. What we are pursuing is a method that will work which will allow the user to turn power on and off remotely.  Then we can worry about the costs...and optimize!

    Adding more MOSFETs is NOT going to increase the risk of failure except by one red hair...look at an ESC which has ten or more!

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