More LED's

Little while back, on the subject of LED's, I mentioned that if they weren't
bright enough simply use more of them. That produced a couples of queries from
homebuilders wanting to know how many and how-to.

The how-to is easy, especially if you're a ham radio operator, pick your teeth
with a soldering iron and have been passing notes to your buds in Morse code
since the fourth grade.

Circuit board is an insulator such as phenolic or fiberglas with a layer of
copper glued to one or both sides. Single-sided stuff is inexpensive and
commonly available. To mount an LED by the bulb you simply drill a 5mm hole
(.196" givertake), poke the LED thru the hole and give it a dot of crazy glue.
That leaves the LED with its head sticking out one side and its legs out the
other. One leg is for power, the other for ground. Bend the ground leg over
and solder it to the copper then collect the other legs singly or in sets,
according to how you want to wire it, solder them to a dropping resister and
connecting the other of the resister end to power. Let there be light... and
there usually is.

Want them to flash? Then you gotta pony up another dollar fifty. That'll buy
you a NE555 timer chip, a capacitor, three quarter-watt resisters and a generic
PNP transister. Crazy-glue the timer chip to the circuit board 'dead bug,'
meaning it's legs are sticking up, solder the thing together in the proper
manner and your LED will blink sixty times a minute with whatever duty cycle
you've selected. If we're talking a nominal 12vdc and up to 45 LED's, a 1:10
duty cycle will give you a nice flasher that should last about 100,000 hours...
mebbe a little less.

That's one way.

Trouble is, with the LED just poking through the circuit board it's staring
straight ahead; most of its light is going to be in a cone that's only about 20
degrees wide. Even a hundred LED's won't put out very much light if viewed
from the side. To be seen from the side the LED has to be pointing to the
side... which means it can't be pointing anywhere else at the same time. So
you use a buncha them, pointing ever whichaway.

Do you need FORTY-FIVE of those suckers? At about half a buck each the red
ones aren't too expensive, their production having been subsidized by their use
as automotive tail lights. And ultra-bright white LED's are down to about two
bucks because they are coming into common use for flashlights and such. But
those green jobbies cost the earth... nearly three bucks each. So you're
looking at two hundred and fifty bucks worth of LED's... which is about a big
handful.

On the up-side, it's a one-time cost. Build them right into the airframe, GLUE
on the molded Lexan cover, forget about it.

Okay, but why FORTY-FIVE of those suckers?

Got an orange? Okay, a grapefruit then. Cut it in half. Now you got a
hemi-sphere. That's your tail light's coverage.

Take the other half of the grapefruit and slice IT in half. Now you got two
quada-spheres, or whatever. They are your wing tip nav-light's coverage.

Remember the part about the 20 degree angle? Take one of the wing tip chunks
of grapefruit and divide it into twenty-degree slices. (Don't cut it, use a
marking pin on the rind; you can eat it for breakfast tomorrow. Yeah, I
know... but she already thinks you're crazy anyway.) 180 degrees, you need
about 9 LED's. 90 degrees, about five. (yeah, it overlaps a bit) Nine times
five is 45.

So how do you do that? How do you arrange 45 LED's so their output covers a
quarter of a sphere?

Take a look at an LED. The tip is about the size of a grain of popcorn. Now
imagine your wing-tip nav light is little EAR of popcorn with nine kernels from
top to bottom that tapers smoothly to the end FIVE kernels away. Now all we
have to do is replace the kernels with LED's :-)

Looks something like the eye of a bug.

To make something like that I used the comptuer to make a drawing showing rows
of holes for the legs of the LED's. Glued it to the non-copper side of the
circuit board. Started drilling. (#80 carbide burr; about 12,000 rpm. SOP
for anyone who fiddles with electronics.)

(The pattern is about 3-3/16" long, 1-3/16" wide at its fattest point, which
happens to be 1-3/16" back of the nose, and is a smooth tear-drop shape. The
holes are logically placed so that their LED can be bent to the proper angle.)

Over on the copper side of the board the hole for the grounded leg is left
straight. The leg will be poked through it then bent over and soldered to the
copper cladding. But the hole for the power leg gets deburred with a regular
1/8" diameter drill bit having an angle of 116 degrees. That gives me a
shallow NON-CONDUCTIVE hole through which the hot leg of the LED protrudes.
From that point on the wiring is exactly the same as for the first example,
with a dropping resister (about 4 cents each) attached to each leg, or to pairs
or even sets of three... depending on how you want to wire it.

The LED's get poked into their holes to a depth determined by the ANGLE to
which they have to be bent, which I worked out on the comptuer before I started
drilling. Since the pattern is symmetrical this isn't as tough as it sounds.
Basically, the LED's in the middle stick up about 3/4" and stand up straight.
The ones on the outside only stand up about a quarter of an inch and get bent
over at 90 degrees. Between the middle row and the outer row you've got three
LED's, each standing about an eighth of an inch higher, each bent at a
different angle.

Not a very good description but you should get the idea.

The computer also provides the profile of the Lexan cover, which starts life as
two poster-board templates to which you glue some crunchy urethane foam then
sand it into a smooth surface. Coat that with something that will harden up
nice -- I used sheet-rock mud -- then use it to make a female mold and from
that pull a solid plug of either Portland cement or Plaster of Paris. The
cement shrinks a bit more than the plaster but both are saturated and have to
be cured for about ten days before you can use them to make your cover, which
you do by gluing flannel to the plug, greasing it up and popping it into the
oven with a hunka Lexan balanced on top. When the Lexan gets rubbery, protect
your hand and mold the plastic to the plug. Do that until you have a nice
symmetrical shape. Now do it again for the other wing. If you know about
thermoplastics and canopies and who shot John you know you can use the original
mold to flare the skirt so as to form a flange. Trim that, apply sealant and
attach it with glue, rivets or whatever... you won't be removing it.

Did you want fries with that? Or mebbe a strobe? Then order a batch of item #
06040 from Harbor Freight. You can throw away the case; all you want is the
guts. It has its own plastic shield. Trim a piece of aluminum so the clear
plastic is a nice fit then glue the guts to the aluminum... or whatever... and
mount them ALL OVER your airplane... wing tips, top & bottom of the fuselage,
top of the vertical stabilizer, ass-end of the rudder... wherever you want a
bright little spark of light.

The little strobe costs about eight bucks and runs on two AA cells. Forget
that and pony up another buck per unit for an LM340T-5 voltage regulator and a
couple of caps, an electrolytic 470uF/16v for the input and a disk ceramic .1uF
for the output. Screw the LM340T-5 to the piece of aluminum to which you've
mounted the guts and solder it all up.

18ga. wire is over-kill for either the stobe or the flashing LED's

The strobes aren't very bright but then, neither are they very large nor
expensive... use a buncha them. It won't keep the Big Boys from running your
ass down but it might scare the crows out of your way.

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Too expensive, right? I think so too. But the cost of LED's continues to drop
and ultrabrights will probably be down to a nickle each by the time the bird is
ready to fly, if ever.

-R.S.Hoover