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dballin
Joined: 11 Nov 2007
Posts: 11
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 Posted: Wed Jan 19, 2011 7:21 am Post subject: Ray Allen position indicator dimmer |
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I have a couple of ray allen trim servos with the LED position
indicators. I'd like to hook the dimmer circuit (white wire) to my
panel light switch so when I turn the lights on the indicators dim.
This would work fine in a 12 v system. Mine alas is 24. Does anyone
know the correct value of a resistor I can put inline to drop the
voltage to around 12. I called Ray Allen and they couldn't give me an
answer other than the circuit draws "milliamps". I could also use a
relay, but would rather go the resistor route.
Thanks
Dan Ballin
Lancair Legacy N386DM
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mdnanwelch7(at)hotmail.co
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| Posted: Wed Jan 19, 2011 8:08 am Post subject: Ray Allen position indicator dimmer |
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Dan,
I'll take a stab at this, but I need the real electronic gurus to
verify I was on the right track.
Re: the 10 LED bar graphs, I have many of them, but they are either
all red or all yellow. The ones that contain all three colors are just a combination
of the available LED colors, namely red, yellow and green. See below for an 'all one color";
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330271806124&ssPageName=STRK:MEWAX:IT
Although this particular bar graph is offered in one color, eBay has several
of the 3-color bar graphs that are exactly what Ray Allen uses. If you scroll down
far enough in this eBay listing, you'll see that ALL THREE (green, yellow, & red)
colors have a MAXIMUM forward voltage of 2.5 volts!!
This should be sufficient enough information to use our handy LED formula
to calculte the necessary resistor.
R= (Vs-Vled)/I
Vs=24 volts
Vled=2.5 volts
I=20mA
Let's solve for R now. 24 volts-2.5 volts= 21.5 volts
Therefore; 21.5 volts/20mA = R
But we need to make sure we convert our mA into Amps first. So 20mA / 1000 = .02 A
Well, continue..... 21.5 volts / .02A = 1075 ohms
You're going to want at least 1075 ohms to keep from popping LEDs. I found it makes
VERY little difference in brightness by making small resistance changes, so if it was me, I'd be
shooting for close to 1200-1500 ohms (probably 1500 ohms).
Hope this helps, Dan
Mike Welch
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jan 19, 2011 8:56 am Post subject: Ray Allen position indicator dimmer |
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At 10:18 AM 1/19/2011, you wrote:
| Quote: | --> AeroElectric-List message posted by: Dan Ballin <dballin(at)gmail.com>
I have a couple of ray allen trim servos with the LED position
indicators. I'd like to hook the dimmer circuit (white wire) to my
panel light switch so when I turn the lights on the indicators dim.
This would work fine in a 12 v system. Mine alas is 24. Does anyone
know the correct value of a resistor I can put inline to drop the
voltage to around 12. I called Ray Allen and they couldn't give me an
answer other than the circuit draws "milliamps". I could also use a
relay, but would rather go the resistor route. |
Not easy to deduce without more data. I suspect the
bar graph array is driven by an LM3914 (or similar)
device.
www.energies.alba-annuaire.fr/data/lm/LM3914 .pdf
I presume further that the chip is configured
to illuminate only one led at a time as opposed to
a variable length bar of multiple leds. The data
sheet says that an LM3914 controls LED current
INTERNALLY as a function of the current flowing
in the reference voltage divider (R1, R2). Adding
resistors in series with the leds will wrestle
with the brightness control feature built into
the chip.
So driving your stock indicator directly from
the dimming system will similarly wrestle with
the chip's built in features.
Use an LM7812 or equal to provide 12v
power to run the indicator from your 24v
system.
http://tinyurl.com/2fwxrgm
If you want to do real dimming control, you'll
need to get inside the indicator and "jeep"
the design to add a dimming feature that exploits
the chip's built in feature.
Bob . . . [quote][b]
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mdnanwelch7(at)hotmail.co
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| Posted: Wed Jan 19, 2011 9:17 am Post subject: Ray Allen position indicator dimmer |
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Dan, everyone else, too,
Regarding those little RYG (red, yellow, green) 10 LED bargraphs, like
I said, I have several! I bought a bunch of them years ago, and so far
have never really found a good use for any of them. However, recently I
thought I'd see how they work, and if they look good enough (brightness,
notice-ability, clarity, etc.) I'd find a good home for several of them
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jan 19, 2011 9:54 am Post subject: Ray Allen position indicator dimmer |
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At 12:12 PM 1/19/2011, you wrote:
Dan, everyone else, too,
Regarding those little RYG (red, yellow, green) 10 LED bargraphs, like
I said, I have several! I bought a bunch of them years ago, and so far
have never really found a good use for any of them. However, recently I
thought I'd see how they work, and if they look good enough (brightness,
notice-ability, clarity, etc.) I'd find a good home for several of them.
Excellent experiment. But know that there are leds and then there ARE
leds. They come in a constellation of capabilities ranging from the ALARM
ON light in your bedside clock (where 6 mCd is enough) up to those used
on full color billboards were 15,000 mCd is marginal to the builder's
design goals.
The market for bar-graph displays is consumer products most of which
are used in the living room and have no sun-light viewability issues.
Further, the customer seldom has an opinion or desire with respect to
color, brightness or other viewability wishes.
So be aware that most of what's out there is probably not suited for
use on the panel of an airplane. Further, MOST bar graphs of any
manufacture will be hard pressed to make it into the cockpit of a
TC aircraft.
First, I thought I'd see how they 'operate'! Can I hook all the anodes in
parallel, and dispense with a resistor? Do the yellow ones look sharp
enough?
Can I use a stack of 3-5 high, and make them look better?
Sure. The LM3914 data sheet I cited earlier says you can stack chips
and displays. One of the guys I worked with at Electro-Mech about
30 years ago did a 100 lamp display from a 10x array of chips and
displays. 1 percent resolution. Cool!
First, I found out the yellow ones are near useless. At least, that's
my opinion. They look more like off-color white, somewhat diffused, and
not very 'attractive'. IMO!
Most "yellow" is not very aircraft oriented except for painting
a J-3. What you're really wanting is an amber led. Hard to find
in bar graphs assemblies. That's why you mgiht consider building
your own. It's just 10 leds in a suitable housing . . . and the
LEDs are plentiful, bright and cheap.
So, I thought I'd put one (yellow) on a breadboard, hook up ALL 10
anodes to the
positive 12V transformer, combine all 10 cathodes to the negative, and give it
a 'go'. Pop!! about 8 of them blew out, and a couple of
others looked dazed
and confused!! If I'm not mistaken, I may have had a little puff of
magic smoke.
In the trash it went!!
Okay, so far, I found out that just because they are all in a
tight little rectangular package,
they're STILL LEDs!!! They will still pop if you don't give each and
ever one of them
their proper individual resistor. Duh!! But, hey, I was experimenting!!
The next thing I did was look on eBay for someone selling some of
these, so that i could get
a 'data sheet'. That's where I found that link I shared.
Yup! Fortunately it was a low cost experiment . . .
LEDs are current driven devices and CANNOT be practically
connected to any hard voltage source. EACH led needs to
be driven by its own current limited source (unless wired
in series). In the case of the LM3914 driver, the current
controls are built in. But if you're going to light these
fellows up on the bench, pay attention to their requirements.
They are generally NOT fragile devices. We had a discussion
back in 2004/2005 where I cited an experiment where I'd
wired a 30mA rated LED to a wall-wart biased at 5x rated
current and plugged it into a little used outlet in my
garage. A least a couple years later I terminated the
experiment with the LED still illuminating the whole
garage at night.
In a nutshell I found out;
a) 10 LED bargraphs are still regular LEDs, just neatly packed in a
line....and really small
b) they burn out---just like the regular kind if you don't allow for
proper resistance
c) never buy 20 of them when you don't have a use for one of them
d) the yellow ones aren't worth buying....at all!!
Mike Welch
Learning LEDs 101
Good for you. Keep us apprised of your discoveries. There
are no doubt others who would like to incorporate your
own experiences into their bags of tricks. In any case
be aware that most led arrays you might put your hands
on will want to be up under the glare-shield to be
visible while flying in sunlight. Consider also putting
them behind a 'smoked' plexiglas . . . while you toss way
1/2 or more of their intensity, the CONTRAST ratio
is so much higher that they GAIN sunlight viewability.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jan 19, 2011 10:02 am Post subject: Ray Allen position indicator dimmer |
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| Quote: |
First, I found out the yellow ones are near useless. At least, that's
my opinion. They look more like off-color white, somewhat diffused, and
not very 'attractive'. IMO! |
Check out these critters.
http://tinyurl.com/4v7clqr
Really good intensity numbers for a bar graph assy.
Plus DUAL color red/green which means you can light
both lamps to get yellow?
Bob . . . [quote][b]
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mdnanwelch7(at)hotmail.co
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| Posted: Wed Jan 19, 2011 10:46 am Post subject: Ray Allen position indicator dimmer |
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Does anyone
| Quote: | know the correct value of a resistor I can put inline to drop the
voltage to around 12. I called Ray Allen and they couldn't give me an
answer other than the circuit draws "milliamps". I could also use a
relay, but would rather go the resistor route.
Thanks
Dan Ballin
Lancair Legacy N386DM
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Dan,
In re-reading your question....and my response, I think Bob's
answer was more what you're looking for. (as usual!!)
Since you wouldn't normally need ANY resistor attached to
the white wire (for dimming the LEDs), your question is more
directed toward dropping 24V to 12V.
In that case, obviously, the LM7812 is the correct way to go.
Mike Welch
learning from the best, all the time.
[quote][b]
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