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Is there a switch configuration that will do this?

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nuckolls.bob(at)aeroelect
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Posted: Thu Jun 17, 2021 1:36 pm Post subject: Is there a switch configuration that will do this?

At 11:28 AM 6/17/2021, you wrote:

  	  Quote: 	
		  I'd like to have a single switch control both my taxi lights & landing lights. Sounds simple enough, right? Here's the catch. I want to control the taxi lights directly from this switch by making a connection toÂ +12v. The landing lights, however, will be controlled through the Garmin GAD 27 electricalÂ controller "magic box" so that it can provide airspeed-based wig-wag. The GAD 27 light control switch input is active-low.Â 



 How I'd like it to work:

 1. Switch down - both lights off

 2. Middle position -Â +12v supplied to the taxi lights.Â 

 3. Top position - taxi lights no longer connected toÂ +12v, GAD 27 light switch input connected to ground.Â 



 I knowÂ I can do this with a relay, but I'd ratherÂ avoid addingÂ an additionalÂ failure point. It's not the end of the world if I use separateÂ switches for these functions, but I'd prefer they are combined on one switch.Â 	


   There MIGHT be a simple way to do this but it

   depends on the 'pull down' current of the electrical

   controller. Is there a spec for that device that speaks

   to parameters of this input?



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jun 19, 2021 8:20 am Post subject: Is there a switch configuration that will do this?

At 04:58 PM 6/17/2021, you wrote:



  	  Quote: 	
		  >> There MIGHT be a simple way to do this but it

 >> Â  depends on the 'pull down' current of the electrical

 >> Â  controller. Is there a spec for that device that speaks

 >> Â  to parameters of this input?



 Here's what I was able to find in the installationÂ manual: (top of page 26-8, or page 342 in Acrobat Reader)



  https://static.garmin.com/pumac/190-01115-01_an.pdf



 These active-low discrete inputs conform to the following specification:

 Low: Vin < 3.5 VDC, or Rin < 375 Î© (input active)

 High: Vin > 8 VDC, or Rin> 100k Î© (input inactive) 	


   Aha! Exactly what was needed. The 3.5v or 375 Ohms

   suggests that pull-down current at the switch point

   is on the order of 0.01 amps (10 milliAmps).



   The 375 Ohms number is handy, let's consider a

   270 ohm, 2W resistor as an always-connected, pull-down

   value on this input.



   To deactivate that input we need to apply bus

   voltage from the landing/taxi lights management

   switch.



   The switches you cited are the Carling line

   of devices. A manufacturer favored by Cessna

   since the 60s. The full range of functions is

   offered by B&C at https://bandc.com



   The one you want is the S700-2-10 which is

   a progressive transfer, on-on-on function

   illustrated by this schematic from the switches

   chapter of the 'Connection.



   This switch with addition of two

   diodes and the resistor cited above can be configured

   to accomplish the switching functions you requested.

   See attached:



   Lower position applies disable signal to

   landing/wig-wag controller. Mid position

   applies power to taxi lights while maintaining

   the landing/wig-wag disable. Upper position

   removes power from taxi lights -AND- landing/

   wig-wag disable.

   

   



 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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user9253

Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan

Posted: Mon Jun 21, 2021 3:59 pm Post subject: Re: Is there a switch configuration that will do this?

Will this work?

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jun 21, 2021 4:29 pm Post subject: Is there a switch configuration that will do this?

 	  Quote: 	
		  Bob - thanks for the solution design & for the packaging tip. I was also wondering how much heat the resistor would need to dissipate in this application? I'm guessing not much?Â 

 	


    W = E^2/R  =  14^2/270  =  0.725 Watts



    For the 'shrink potted' assembly I would

    probably use two 130 Ohm resistors

    in series to spread out the heat generation

    at 0.360 Watts each.



    It will warm up noticeably but only slightly

    so.



 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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