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/