Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Article on Apollo Guidance Computer
Date: Mon, 12 Feb 2001 03:09:25 GMT

In article <flvg3m0c4-1102011924150001@204.156.22.202>,
Mike Flugennock <flvg3m0c4@sinkers.org> wrote:
>Just a quick question: exactly how _is_ an ELD display made, and what's in
>it that lights it up? In photos I've seen of the CM main panel, it almost
>looks as if it's using a really primal LED or Nixie display from the look
>of the indicator mounts and bezels. Flat glass, iirc, and almost totally
>black on the inside, very much the way an LED calculator displays looks
>when powered off.

If memory serves, an ELD is basically just a thin layer of, well, *stuff*,
between a transparent electrode and a back electrode.  Run current
through, typically at a fairly substantial voltage and often preferably
AC, and the *stuff* glows.

It sounds dead simple, and once you've got it working, it is.  But there
are a wide range of possible *stuff* compositions, and designing new ones
is somewhat of a black art, and the real trick is getting one with a half-
reasonable working life.  It's been a perennial also-ran technology for a
long time, because the appealingly simple construction has never quite
made up for the difficult materials problems, less-than-wonderful
performance, and poor lifetime.  (Awkward drive voltages were also a big
headache in the early days, although less so now.)
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

From: glass2@glass2.lexington.ibm.com
Newsgroups: sci.space.history
Subject: Re: Article on Apollo Guidance Computer
Date: 12 Feb 2001 14:49:17 GMT

In <G8MJFp.240@spsystems.net>, henry@spsystems.net (Henry Spencer) writes:
>In article <flvg3m0c4-1102011924150001@204.156.22.202>,
>Mike Flugennock <flvg3m0c4@sinkers.org> wrote:
>>Just a quick question: exactly how _is_ an ELD display made, and what's in
>>it that lights it up? In photos I've seen of the CM main panel, it almost
>>looks as if it's using a really primal LED or Nixie display from the look
>>of the indicator mounts and bezels. Flat glass, iirc, and almost totally
>>black on the inside, very much the way an LED calculator displays looks
>>when powered off.
>
>If memory serves, an ELD is basically just a thin layer of, well, *stuff*,
>between a transparent electrode and a back electrode.  Run current
>through, typically at a fairly substantial voltage and often preferably
>AC, and the *stuff* glows.
>
>It sounds dead simple, and once you've got it working, it is.  But there
>are a wide range of possible *stuff* compositions, and designing new ones
>is somewhat of a black art, and the real trick is getting one with a half-
>reasonable working life.  It's been a perennial also-ran technology for a
>long time, because the appealingly simple construction has never quite
>made up for the difficult materials problems, less-than-wonderful
>performance, and poor lifetime.  (Awkward drive voltages were also a big
>headache in the early days, although less so now.)
>--
>When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
>can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Henry,

I'll add a few comments about EL panels, since I used to work as an
electrical engineer specializing in display devices (although that's
been over 15 years ago).

The *stuff* is some type of phosphor, often a ZnS material with the
appropriate doping.  The phosphor is suspended between two
electrodes, at least one of which is transparent.  There may also be
an insulating layer.  All of the ones I've seen have required AC to
operate.  Depending upon the phosphor being used, a variety of
light colours may be produced, although green, yellow, and white
have been the most common in my experience.

The problem is that since the light output is proportional to the
(displacement) current, and since the device is basically a
capacitor, not much current flows at reasonable voltage levels and
power line frequencies (e.g., 50 or 60 Hz).  While it's possible to
increase the voltage applied across the device to increase the
current, the devices already require on the order of 100 to 150
volts, and voltages higher than this are difficult to create and
control, and risk arcing between the electrodes, which destroys the
device.  The other approach is to increase the frequency of the
applied voltage, and this does increase the current, and thus the
light output.  However, having such a high voltage, high frequency
signal in a delicate area such as a spacecraft computer requires
care in the design of the system.  Additionally, for ground based
systems, the creation of the high voltage, high frequency voltage
requires special circuits, which adds expense compared to
technologies such as LEDs, neon, or vaccum fluorescents.  Thus, EL
displays are rare except in applications requiring their special
characteristics.

Finally, electroluminescent displays have a rather limited life,
although technology has improved in the past few decades.

When I was first involved with EL panels, the light output was rather
small, the life was rather limited, and the cost was rather expensive.
Now, however, the technology has improved, and you can buy EL panels
as nightlights!

For a quick tutorial on EL displays, I've found the following web-page:

http://www.memtronik.com/aelectro.htm

Dave

P.S. Standard Disclaimer: I work for them, but I don't speak for them.

From: OM <om@RE_MOVE_THIS.ccsi.com>
Newsgroups: sci.space.history
Subject: Re: Article on Apollo Guidance Computer
Date: Mon, 12 Feb 2001 18:38:59 -0600

On Mon, 12 Feb 2001 20:04:55 -0800, spaceuk@netcomuk.co.uk wrote:

>DAve wrote:-
>
><snip>
>>
>> Finally, electroluminescent displays have a rather limited life,
>> although technology has improved in the past few decades.
>> <snip>
>
>Its interesting to recall that an Apollo-11 CM DSKY ELD failed during
>Earth entry phase - I think it was the velocity readout!

...and depending on which ELD element went out the results could be
disasterous! If the center element went out, it would be impossible to
determine right off if the number displayed was an 8 or a 0. At the
same time, if they weren't paying careful attention, a 7 and a 9 might
be miscontstrued as well.

...Henry, this sounds like one for you: say the ELP on the DSKY went
out in the first scenario, where the crew couldn't tell a 8 from a 0.
What did mission rules say about abort under these conditions?


						OM

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Article on Apollo Guidance Computer
Date: Tue, 13 Feb 2001 17:27:57 GMT

In article <4gug8t0u2bi4bu34843l5s07s994gt40dc@4ax.com>,
OM  <om@RE_MOVE_THIS.ccsi.com> wrote:
>...Henry, this sounds like one for you: say the ELP on the DSKY went
>out in the first scenario, where the crew couldn't tell a 8 from a 0.
>What did mission rules say about abort under these conditions?

Don't know offhand.  My guess would be continuation if it was a CM DSKY
(since the CM had two) and mission abort for an LM DSKY, but I don't know
for sure.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)