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How To Make An Arrow Pointer
Are you tired of
pointing to stars with
just a little red dot?
Do you remember the
days before laser
pointers?
Have you seen the new
super bright “Light
Emitting Diodes”?
If your answer is YES to any of these
questions, then this workshop is for you!
The first version of an arrow
pointer I made, used this
L.E.D. flashlight. It was seen
at the IPS meeting in Wichita,
Kansas.
If you want to make this pointer, you can
download instructions from my web page.
www.pa.msu.edu/people/frenchj
In this newer design, you will make
your own “LED flashlight” which is
specialized for use as a pointer.
The first thing you will need is a
superbright Light Emitting Diode
A good place to get LED’s are the
internet. I bought some from
“superbrightleds.com” Their prices
are good and they ship fast.
The ‘brightness’ or ‘intensity’ of led’s
are measured in millicandela (mcd).
Get one rated over 10,000 mcd.
Superbright red ones are 12,000 mcd
Superbright Yellows are 10,000 mcd
Superbright Whites are 18,000 mcd
White led’s are $1.59, Reds and
Yellows are only 62¢.
Next, get some plumbing parts…
A plastic
PVC tailpiece
will be the
body of your
flashlight.
A six inch long 1½ inch extension
tube (aka, tailpiece) will be used
to hold the lens in place over the
arrow slide and LED. Get the kind
with threads and a big “nut” on
the end to hold the lens
You can use
one without
threads for a
sleeker look,
or a threaded
one if you
want to be
able to “lock”
in the focus.
You can get these at you local hardware store for about $2.00 each.
… and a lens.
This 5 inch Ektanar lens
came from a Kodak slide
projector.
I scavenged this lens
from an old single slide
projector.
This lens is a Cabin 75
mm focal length lens.
This powerpoint shows how make a pointer using the Cabin lens.
It’s a bit different if you use another lens, but still easy.
Get some wire, a switch, a resistor, a 9 volt
battery clip, heat shrink tubing and a 35mm film
can and an extra film can lid.
You’ll need a few tools too. Get a multimeter,
soldering iron, solder, Exacto knife, black tape,
drill and a Dremel tool or saw.
Here’s a quick sketch of what’s in the
pointer and how it goes together.
Note: This was not drawn on Professor Whoopee’s 3D-BB, Chumley!
(Bonus points for anyone that knows what cartoon featured the 3D-BB)
The wiring schematic for the pointer looks like this.
Switch
Resistor
Battery
Diode
It’s important to find the
correct value for the resistor.
Diodes have a + and –
lead. If it’s in backwards it
won’t light up.
The key is to use a resistor that
will limit the current in the diode to
20 milliamps.
Light Emitting Diodes typically
shine brightest at 20 mA of
current. More current than 20 mA
can burn out the diode or shorten
its lifetime. At less current, it
won’t be as bright.
Set the multimeter to measure milliAmps
and measure the current as you try
different resistors. Try and get as close
to 20 without going over.
A small breadboard (socket board) is useful
for testing. But alligator clips will work too.
Start with larger resistors
and work your way down
Or just go with 330 ohms
and call it good!
There are resistor value calculators available on the web that can tell
you the resistor you will need for a particular voltage and current. Look
at http://linear1.org/ckts/led.php as an example.
Make an arrow using your
favorite graphics program…
and turn it into a slide.
Or download some of these
arrows from my web site.
When you have all the parts,
it’s time to put it all together.
Parts List:
LED
Lens
2 Extension tubes
Arrow Slide
Black Foil or Tape
Film can w/extra lid
Dremel Tool
Safety Glasses
Drill
Solder
Soldering iron
Switch
Resistor
9 volt battery
Battery clip
Heat shrink tube
Solder the circuit.
Put the diode and
resistor in series.
Cover all open leads
with heat shrink
tubing. Connect the
switch. Use a heat
sink when soldering
the diode. Too much
heat can damage
the diode.
Drill a hole in the side for the switch.
Stick the wires,
diode and switch into
the extension tube.
Some switches have the nut on the outside and
some have it on the inside. If the nut is on the
inside, you will need to fit it through the hole before
it is soldered together. If the nut for the switch in on
the outside, it is easier to install.
Cut the film can to hold the LED in
the center of the barrel of the tube.
Remove the
bottom of the can
and cut a notch
out for the wires
and switch.
Cut or drill a hole
for the LED.
Stick the LED in
the hole.
Radio Shack sells
little grommets for
holding LEDs. Use
one if you want it
to look pretty!
Stick the can with LED
and wires into the
extension tube.
Put in your
arrow slide:
Cut another hole in
the second lid and
tape your arrow over
the hole
Put this “film holder”
lid over the LED.
Line it up so the LED
shines through the arrow.
Install the lens:
Ream out the top of the extension tube so the lens will fit in the top.
Use the meanest looking
gouger for the Dremel Tool.
This step will get messy as
the bits of plastic fly!
Determine the distance
from the slide to the lens.
Go into the planetarium
dome.
Hold the lens over the
flashlight and focus the
arrow onto the dome.
Measure the distance between
the flashlight and lens.
This will tell you where to cut
the extension tube.
Cut the extension tube
to the proper length.
Be sure to wear safety glasses
when using a Dremel tool.
Put the lens in the extension tube
and carefully screw on the top.
Insert some black foil into the
extension tube to reduce
internal reflections. You can
also use black masking tape.
A very
important
step.
Put the lens/extension tube into the focus
collar and insert it into your “flashlight.”
Slide the lens back and forth
until it’s focused.
Point to the stars!
You’re done.
This pointer design was inspired by a
pointer first made by Tom Ferguson,
an electrical engineering graduate
from Michigan State University and
former planetarium staff member of
the Abrams Planetarium. His first
pointer used a Mag-light flashlight as
the light source. We then switched to
the Brinkman LED flashlight. Now
this one is independent of any store
bought flashlight.
Here we see Tom
using his pointer to
vanquish his foes!
For more information visit John French’s web site at:
http://www.pa.msu.edu/people/frenchj
Here you can download arrow jeg’s, this pointer powerpoint,
the older pointer powerpoint and even link to my fascinating
Moist Towelette Museum web page, which has nothing to
do with this pointer. (That was just a cheap plug for my moist towelette museum!)
Presented by:
John French
Planetarium Production Coordinator
Abrams Planetarium
Michigan State University
[email protected]