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Indicator light circuit with push to test

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nuckolls.bob(at)aeroelect
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Posted: Sun Jun 09, 2013 1:08 pm Post subject: Indicator light circuit with push to test

At 11:39 AM 6/9/2013, you wrote:

 	  Quote: 	
		  



Hey Bob,



This will be a backlit panel using LEDs (Green, Red or Amber).



I see you point about the reliability of LEDs versus incandescent 

bulbs and that makes perfect sense.



It just seems like every panel I've seen (and I'm pretty sure some 

where LEDs) had a push-to-test switch. So I was just trying to 

follow (what I thought) was best practices.


	


   Yep. That's a common feature in aviation . . . TRADITION

   can plow deep furrows . . . and the young bucks are

   reluctant to ask, "why is that there?" So some things

   keep showing up on next year's models even when their

   utility has long since passed . . . like avionics master

   switches . . . battery ammeters . . . battery boxes . . .



   I did have a conversation a few years ago with

   a builder wanting to get some alternator noise

   out of his ADF receiver. I asked why he still had

   the ADF . . . did he shoot ADF approaches? No,

   says he . . . he took the loop antenna off years

   ago. It was already installed and he liked to

   use it as an AM radio. Seems that FM reception

   at altitude was problematic as stations with

   shared frequencies appeared over horizon.



   The noise turned out to be a ground loop in the

   audio wiring.  Hammers are likely to be around

   for awhile too but I'm using mine less-and-less.

   Bob . . .

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Mon Jun 10, 2013 5:28 am Post subject: Re: Indicator light circuit with push to test

The really weird thing with using LEDs is that they will last longer than the socket, longer than the switch and longer than the airframe. So unless you're going full-on Steam Punk, or the press-to-test feature really does something useful, just remove the switch and put a cork in the hole.

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jun 10, 2013 6:34 am Post subject: Indicator light circuit with push to test

At 08:28 AM 6/10/2013, you wrote:

 	  Quote: 	
		  



The really weird thing with using LEDs is that they will last longer 

than the socket, longer than the switch and longer than the 

airframe. So unless you're going full-on Steam Punk, or the 

press-to-test feature really does something useful, just remove the 

switch and put a cork in the hole.


	


    Does the PTT button for a lamp/socket/switch

    combination not produce a false sense of security?



    That's sorta like going out on a high risk

    hike in the woods having only checked to see

    that your sandwiches are packed. We don't

    fly indicator lamps, switches, propellers

    or aileron control cables. These are all

    ingredients that go into recipes for a

    flight system

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jun 10, 2013 1:17 pm Post subject: Indicator light circuit with push to test

At 03:39 PM 6/10/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: John Grosse <grosseair(at)comcast.net>



 Gee, really? The situation I was imagining would be in the air, on approach, but on the ground you would be right:)



 Do you happen to remember Eastern 401 that crashed in the Everglades while the crew tried to figure out if the gear light was burned out. A "push to test" function would have been handy to have. 	


   They DID have a PTT function that didn't light the

   light. So they were borrowing a lamp from another

   fixture. These can be changed without tools but it

   does involve 'fiddling' with the little flanged

   base lamps common to this style of backlit, sunlight

   viewable fixture.



   



   Whoever was supposed to be flying got so involved

   in diagnosis and in-flight maintenance issues

   that a shift in a/p from altitude hold to

   control wheel steering went unnoticed.



   This wasn't a case of not having a 'handy'

   feature, it was a breakdown of cockpit

   protocol.



 

   Bob . . .    [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jun 10, 2013 2:32 pm Post subject: Indicator light circuit with push to test

At 04:24 PM 6/10/2013, you wrote:

 	  Quote: 	
		  



Or maybe the simpler solution is two juxtaposed indicator bulbs 

wired in parallel.


	


   Actually, there are sunlight viewable, dead-front

   annunciator fixtures that mount two bulbs in parallel.

   The idea is that the fixture still performs its intended

   function with only one lamp . . . but with obvious

   gradient of brightness across the face. The likelihood

   of loosing two lamps on any one flight cycle is

   very remote.



   All of the LED sunlight viewable fixtures I've encountered

   have multiple lamps . . . mostly to get the light required

   total light output and sufficient coverage of the back of

   the lens. These too would 'fail gracefully' if single

   lamps were to fail.



   But again, press-to-test verification of lamp functionality

   offers only a meager fraction of the information that

   a pilot would really like to have when making important

   decisions.



   Bob . . .

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