I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

"homebuilt aircraft"... any radio interference concerns from the
switching currents?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith


Make one for a bike that runs on 2.4 Volts (two rechargeables in series)

Instead of a dimmer function, make it a flasher of various patterns.

Charlie Smith wrote:
I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith



Since it is for a flying machine, how about a little redundancy?




--

Richard Lamb
http://www.home.earthlink.net/~cavelamb/


"The clock of life is wound but once, and no man has the power
to tell just when the hands will stop, at late or early hour...
Now is the only time you own. Live, love, toil with a will.
Place no faith in time. For the clock may soon be still."

"cavelamb" wrote in message
...
Since it is for a flying machine, how about a little redundancy?

I'd be tempted to toss in a switch that connects +14V-Big resistor-All the
LEDs (with their individual current-limiting resistors) -- so if the 7805 or
555 fail, you can still have full brightness backlights.

But perhaps just having a flashlight around is just as good, if there's a
mount such that it can be pointed at the display. :-)

Charlie Smith wrote:
I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

You'll generate a lot of heat in the L78S05 regulator - worst
case, about 4.2 watts. You could use a L78S09 instead, and reduce
the worst case heating in the regulator to about 2.4 watts. You'd
need to change the 68 ohm resistors to 270 ohms.

Ed

"Charlie Smith" wrote in message
...
I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary.

Well, it looks like it'll work fine. If you care, you can gain some
efficiency by just using a IC meant for the task (regulating LED current)
directly -- these are really just a switching power supply controller IC
that's been tweaked to regulate current rather than voltage; you can do such
tweaking yourself if it's a cost sensitive design (using, e.g., an LT1070 or
one of its many variants). If you look at, e.g., Linear Tech's web site,
you'll find plenty of example circuits and application notes on this sort of
thing -- LED controllers are a Big Deal these days since of course pretty much
every cell phone, laptop, and TV needs one.

---Joel

Will it work as drawn? Yes. Is it the most efficient design for the
job? No. Is it pretty simple and easy to build? Yes. However,
there are several single-point failure points, but if this aircraft
isn't designed for hard-IFR or if you choose to carry a backup
flashlight you should be OK.

Jim




On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

Safety- it definitely needs fusing and, as Jim suggests, EMI from the
switching could conceivably screw up your radio communications or
other instruments. Maybe not enough DC current to worry about, but you
also should be aware that heavy DC currents can affect compasses, so
wiring layout may need a bit of care.

You could also consider a premade solution like this one:

http://www.digikey.com/scripts/DkSea...me=788-1023-ND

The 350mA constant-current dimmable version is less than $20 at
Digikey. There still be some EMI, so it has to be checked that it
doesn't cause interference (eg. with weak radio signals). You could
probably reduce the 68 ohm resistors to 33R without any problems.

You also need some kind of reliable backup for when this thing fails
and it's dark. Consider an on/off/bypass switch that would replace the
electronics with a resistor.

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

It's not very efficient. You could put the LEDs in series clusters and
run the substrings from a somewhat higher voltage.

A mosfet would be nice, instead of the antique 3055.

Come to think about it, PWM isn't any more efficient here than linear
regulation. A variable-voltage regulator, LM317 sort of thing with a
pot, could replace the 555 and all that stuff. Much simpler.

John

On Tue, 12 Jan 2010 18:46:13 -0800, the renowned John Larkin
wrote:

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith

It's not very efficient. You could put the LEDs in series clusters and
run the substrings from a somewhat higher voltage.

A mosfet would be nice, instead of the antique 3055.

Come to think about it, PWM isn't any more efficient here than linear
regulation. A variable-voltage regulator, LM317 sort of thing with a
pot, could replace the 555 and all that stuff. Much simpler.

John

Good point, also I don't think running 20mA through 365-1467-ND 3mm
white LEDs is a very good idea. That's the abs. max. current at 25C.
If you want it not to fail at high panel temperatures/brightness (ever
come in out of the sun into a dark hangar?) and don't want to be one
of those folks who complains about crappy LEDs (because they abuse
them and they die fast) you should derate to something like 10mA. Two
in series at 10mA will give you about the same amount of light, but
draw 1/2 the current (at twice the voltage), but you need twice as
many LEDs.

So, say the voltage regulator goes from 5V to 12V, the resistors in
series with each series LED pair will be 560 ohms. A TO-220 LM317
should not need hardly any heatsink (and there will be no RFI
generated). Say 180R from LM317 out to sense input, and a 1K pot in
series with 510R to ground from the sense input, to give about a 5V to
12V adjustment range. Maybe a mechanical switch to give off/bypass
(full bright)/dimmed.

Disadvantage of this over the PWM is that the apparent brightness will
not vary nearly as linearly (most of the action will be down near
where it just comes on), so check it out on the bench to make sure you
can live with it if you decide to go this way.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Spehro Pefhany wrote:
On Tue, 12 Jan 2010 18:46:13 -0800, the renowned John Larkin
wrote:

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering if
those qualified in this group would mind reviewing the schematic and
making constructive commentary. I'd like this thing to be reliable and
not cause a host of other electronic gremlins. Just keep in mind, I'm a
chemist, not an electronics professional. But, I do learn quickly.


Thanks.
Charlie Smith
It's not very efficient. You could put the LEDs in series clusters and
run the substrings from a somewhat higher voltage.

A mosfet would be nice, instead of the antique 3055.

Come to think about it, PWM isn't any more efficient here than linear
regulation. A variable-voltage regulator, LM317 sort of thing with a
pot, could replace the 555 and all that stuff. Much simpler.

John

Good point, also I don't think running 20mA through 365-1467-ND 3mm
white LEDs is a very good idea. That's the abs. max. current at 25C.
If you want it not to fail at high panel temperatures/brightness (ever
come in out of the sun into a dark hangar?) and don't want to be one
of those folks who complains about crappy LEDs (because they abuse
them and they die fast) you should derate to something like 10mA. Two
in series at 10mA will give you about the same amount of light, but
draw 1/2 the current (at twice the voltage), but you need twice as
many LEDs.

So, say the voltage regulator goes from 5V to 12V, the resistors in
series with each series LED pair will be 560 ohms. A TO-220 LM317
should not need hardly any heatsink (and there will be no RFI
generated). Say 180R from LM317 out to sense input, and a 1K pot in
series with 510R to ground from the sense input, to give about a 5V to
12V adjustment range. Maybe a mechanical switch to give off/bypass
(full bright)/dimmed.

Disadvantage of this over the PWM is that the apparent brightness will
not vary nearly as linearly (most of the action will be down near
where it just comes on), so check it out on the bench to make sure you
can live with it if you decide to go this way.


Best regards,
Spehro Pefhany


An audio taper pot should help with the brightness question?


--

Richard Lamb
http://www.home.earthlink.net/~cavelamb/

cavelamb wrote:

An audio taper pot should help with the brightness question?


They really aren't intended for DC current. They will become noisy
in a hurry, with multiple dead spots in the carbon track. Delco found
out the hard way when they built some early transistor car radios with
DC on the volume control, then had to do a recall to repair them, and
modify them to prevent future failures. A wirewound Rheostat would be a
better choice, but wouldn't have a linear change in intensity.


--
Greed is the root of all eBay.

Sure they are, you just can't run a LOT of DC current through them,
especially on the ends of the tracks.

Jim



On Wed, 13 Jan 2010 09:07:25 -0500, "Michael A. Terrell"
wrote:


cavelamb wrote:

An audio taper pot should help with the brightness question?


They really aren't intended for DC current.

cavelamb wrote:
Spehro Pefhany wrote:
On Tue, 12 Jan 2010 18:46:13 -0800, the renowned John Larkin
wrote:

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering
if those qualified in this group would mind reviewing the schematic
and making constructive commentary. I'd like this thing to be
reliable and not cause a host of other electronic gremlins. Just
keep in mind, I'm a chemist, not an electronics professional. But,
I do learn quickly.


Thanks.
Charlie Smith
It's not very efficient. You could put the LEDs in series clusters and
run the substrings from a somewhat higher voltage.

A mosfet would be nice, instead of the antique 3055.

Come to think about it, PWM isn't any more efficient here than linear
regulation. A variable-voltage regulator, LM317 sort of thing with a
pot, could replace the 555 and all that stuff. Much simpler.

John

Good point, also I don't think running 20mA through 365-1467-ND 3mm
white LEDs is a very good idea. That's the abs. max. current at 25C.
If you want it not to fail at high panel temperatures/brightness (ever
come in out of the sun into a dark hangar?) and don't want to be one
of those folks who complains about crappy LEDs (because they abuse
them and they die fast) you should derate to something like 10mA. Two
in series at 10mA will give you about the same amount of light, but
draw 1/2 the current (at twice the voltage), but you need twice as
many LEDs.
So, say the voltage regulator goes from 5V to 12V, the resistors in
series with each series LED pair will be 560 ohms. A TO-220 LM317
should not need hardly any heatsink (and there will be no RFI
generated). Say 180R from LM317 out to sense input, and a 1K pot in
series with 510R to ground from the sense input, to give about a 5V to
12V adjustment range. Maybe a mechanical switch to give off/bypass
(full bright)/dimmed.
Disadvantage of this over the PWM is that the apparent brightness will
not vary nearly as linearly (most of the action will be down near
where it just comes on), so check it out on the bench to make sure you
can live with it if you decide to go this way.

Best regards, Spehro Pefhany


An audio taper pot should help with the brightness question?



Thanks to all who contributed comments. I appreciate all the suggestions
for improvement.

I am somewhat puzzled by the characterization of it being inefficient. I
guess it depends on perspective. I think this approach has to be a
whole lot more efficient than incandescent bulbs and a lot more
reliable. This was my main goal here.

I appreciate the insight on noise. This is something I had not even
considered but, in hindsight, is really obvious. The linear regulator
seems a great idea. What do you think about the LT3080? It looks like I
would need a 100K trim pot to dial in my voltage. The required 50K
should be right in the middle of the range for the pot.

I was also surprised about the comments regarding If. The lit advertised
20 mA as typical current with 50 mA max and that's why I designed around
20. But, there it was in Fig 4 showing a steady drop off hotter than
25°C and the interior could easily see 35-40°C peak temps. That's a good
catch. Thanks. What I may do here is de-rate down to 16 or 18 mA since I
won't be spending much time in a 40°C cockpit. Promise. At 5V and 16
mA, it will take 100 ohm resistors. Even RadioShack should have those on
the shelf.

The board will be fused with a 2A fuse at the buss. Backup is a
flashlight, same as with the rest of us flying behind incandescent
bulbs. If I can mount them easy enough, I can consider putting them in
in pairs for higher illumination.

Thanks again.
Charlie

On Wed, 13 Jan 2010 17:15:57 -0500, Charlie Smith
wrote:

cavelamb wrote:
Spehro Pefhany wrote:
On Tue, 12 Jan 2010 18:46:13 -0800, the renowned John Larkin
wrote:

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith
wrote:

I have cobbled together a circuit from available internet sources to
illuminate a homebuilt aircraft instrument panel. I was wondering
if those qualified in this group would mind reviewing the schematic
and making constructive commentary. I'd like this thing to be
reliable and not cause a host of other electronic gremlins. Just
keep in mind, I'm a chemist, not an electronics professional. But,
I do learn quickly.


Thanks.
Charlie Smith
It's not very efficient. You could put the LEDs in series clusters and
run the substrings from a somewhat higher voltage.

A mosfet would be nice, instead of the antique 3055.

Come to think about it, PWM isn't any more efficient here than linear
regulation. A variable-voltage regulator, LM317 sort of thing with a
pot, could replace the 555 and all that stuff. Much simpler.

John

Good point, also I don't think running 20mA through 365-1467-ND 3mm
white LEDs is a very good idea. That's the abs. max. current at 25C.
If you want it not to fail at high panel temperatures/brightness (ever
come in out of the sun into a dark hangar?) and don't want to be one
of those folks who complains about crappy LEDs (because they abuse
them and they die fast) you should derate to something like 10mA. Two
in series at 10mA will give you about the same amount of light, but
draw 1/2 the current (at twice the voltage), but you need twice as
many LEDs.
So, say the voltage regulator goes from 5V to 12V, the resistors in
series with each series LED pair will be 560 ohms. A TO-220 LM317
should not need hardly any heatsink (and there will be no RFI
generated). Say 180R from LM317 out to sense input, and a 1K pot in
series with 510R to ground from the sense input, to give about a 5V to
12V adjustment range. Maybe a mechanical switch to give off/bypass
(full bright)/dimmed.
Disadvantage of this over the PWM is that the apparent brightness will
not vary nearly as linearly (most of the action will be down near
where it just comes on), so check it out on the bench to make sure you
can live with it if you decide to go this way.

Best regards, Spehro Pefhany


An audio taper pot should help with the brightness question?



Thanks to all who contributed comments. I appreciate all the suggestions
for improvement.

I am somewhat puzzled by the characterization of it being inefficient. I
guess it depends on perspective.


No, it's quantifiable. If you're running a 2 volt LED from a 14 volt
supply, with only resistive-type (lossy) current limiting, the
electrical efficiency is about 15%. The electrical-to-optical
efficiency will be a lot less, worse than incandescents probably. Most
of the input power is being burned up in the resistors.

If you put pairs of LEDs in series, efficiency doubles. Three or four
per string is even better... if efficiency matters at all here.

John

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith wrote:



Been reading the thread. A combination of switching to a 7809 regulator and using
two LEDs in series may save some power/reduce heat. (Ed and Speff IIRC)

You also need to protect the input of the regulator just like automobile applications,
able to withstand up to 100 v spikes in either direction for 5 ms duration. Also for
30 to 40 V for 200 ms.

If you go with PWM you may wish to consider a lower resistor on the base of the 3055,
typical Beta is about 50 at 500 mA IC.

I also suggest some isolation between the regulator and the LED block, say a lossy
inductor with about 1 ohm resistance. Inductor size should be premised on discontinuous
conduction at low brightness and continuous conduction at max brightness. A couple more
diodes may be appropriate.