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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 8:26 am Post subject: Indicator light circuit with push to test

At 07:15 AM 6/9/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "donjohnston" <don(at)numa.aero>



 I'm working on an annunciator panel circuit that will have a push-to-test function and dimmer. No problem there. 



 The challenge that I'm having is that some of the indicators will be powered by the line which drives the device (pitot heat, fuel pump, etc.) and others will be switched to ground. 



 I'd be fine if it weren't for the push-to-test function.  

 	


   Are these lamps incandescent or LED?  Keep in mind that

   the original application of lamps as annunciators for

   important functions could not escape the fact that

   a lamp's filament had a demonstrated service life which

   was a tiny fraction of that for an airframe. It was prudent to

   'test' each lamp before takeoff (or in flight should

   a questionable condition be noted) to make sure that

   the filament was intact and the lamp was CAPABLE of

   doing its assigned task. Hence, legacy design philosophies

   have birthed a plethora of press-to-test techniques.



   [img]cid:.0[/img] 



 

   The fixture shown above has a press-to-test feature

   built in . . . which does a fine job of testing the

   lamp. But what drives the lamp? A canopy open warning

   switch, an low oil pressure switch, low voltage warning?

   What confidence is gained for system performance knowing

   that the lamp is good when the device driving that lamp

   is subject to its own, possibly hidden failures?



   PTT for lamps made sense when lamps had an MTBF that

   was a fraction of that for the constellation of

   other devices in the airplane. With LED annunciators,

   the philosophy putting dip-sticks into system

   functionality in pre-flight are changed.



   Low oil pressure should be illuminated before the

   engine is started. The canopy unlocked light can

   be 'tested' by observing the condition of the light

   after the master switch is on but before the canopy

   is locked. Low volts light will be illuminated any

   time the master is on but engine not running.



   In other words, the greatest confidence in assessing

   system integrity calls for and end-to-end testing

   of the annunciated condition. This philosophy is

   valid irrespective of the type of lamp used. Better

   to poke a stick at the warning SENSOR and see

   that it lights the lamp than to push a button that

   lights all lamps thus encouraging you to believe that

   all is right with the universe.



   So before you spend $time$ and panel space crafting

   a PTT system, see if you can figure out a way to build 

   end-to-end, check list functionality testing into all

   the annunciated conditions before taking the active

   for departure.



 

 

   Bob . . .

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donjohnston

Joined: 13 Dec 2009
Posts: 232

Posted: Sun Jun 09, 2013 8:39 am Post subject: Re: Indicator light circuit with push to test

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. 



Thanks,

Don

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