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 paper is for you!
The first version of an arrow pointer I made, used this L.E.D. flashlight. It was seen as a poster paper at the GLPA meeting in Wisconsin 2002. 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. SuperbrightRedones are 12,000 mcd SuperbrightYellowsare 10,000 mcd SuperbrightWhitesare 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. 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. 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 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.
The wiring schematic for the pointer looks like this. Switch Resistor Battery Diode Diodes have a + and – lead. If it’s in backwards it won’t light up. It’s important to find the correct value for the resistor.
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!
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. Before After This step will get messy as the bits of plastic fly! Use the meanest looking gouger for the Dremel Tool.
Determine the distance from the slide to the lens. Go into the planetarium dome. Hold the lens over the LED/arrow and focus the arrow onto the dome. Measure the distance between the tailpiece 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 design is independent of any store bought flashlight. Here we see Tom using his pointer to vanquish 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 • Abrams Planetarium • Production Coordinator • firstname.lastname@example.org