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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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PostPosted: Thu Jun 17, 2021 1:36 pm    Post subject: Is there a switch configuration that will do this? Reply with quote

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 to +12v. The landing lights, however, will be controlled through the Garmin GAD 27 electrical 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 to +12v, GAD 27 light switch input connected to ground.Â

I know I can do this with a relay, but I'd rather avoid adding an additional failure point. It's not the end of the world if I use separate 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
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PostPosted: Sat Jun 19, 2021 8:20 am    Post subject: Is there a switch configuration that will do this? Reply with quote

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 installation 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 Ω (input active)
High: Vin > 8 VDC, or Rin> 100k Ω (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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Joined: 28 Mar 2008
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Location: Riley TWP Michigan

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

Will this work?

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nuckolls.bob(at)aeroelect
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PostPosted: Mon Jun 21, 2021 4:29 pm    Post subject: Is there a switch configuration that will do this? Reply with quote

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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