How To Make An Arrow Pointer

Abstract:
This paper shows how to make a pointer that projects an arrow on the planetarium dome, using an LED, some plumbing parts, an old lens and a few simple tools. Pictures of this pointer can be seen at: http://www.pa.msu.edu/people/frenchj/pointerglpa2005/ along with images of arrows for downloading.

The first version of an arrow pointer I made, used a Brinkman L.E.D. flashlight. It was seen as a poster paper at the GLPA meeting in Wisconsin 2002. One problem with it was the availability of the flashlight. It was only sold at Wal-mart and there's no tellin' how long Brinkman will keep making them . Also the switch was located on the bottom of the flashlight. To overcome the drawbacks of the first design, I came up a new LED arrow pointer. These instructions are for a pointer that does not use a store bought flashlight. It uses a superbright LED. The switch can be placed on the side where it feels more natural and can be used with one hand. Although I do recommend holding any pointer with both hands just to make it steadier.

The first thing you will need is a "Superbright" Light Emitting Diode. There are several places to buy them. Most electronics parts supply stores sell them. Another option is to buy a keychain flashlight and remove its LED. I bought mine over the internet. A place called superbrightleds.com has a large selection, low prices and fast shipping. Red and yellow LED's were 62¢ and the white LED cost $1.59 each.

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.

The lens is the next item. I have had success with lenses from old single slide projectors. I found a Cabin 75mm focal length lens that works nice. Many of you might have some of these old projectors in your surplus closet. I've also used a 5-inch Ektanar lens from a Kodak carrousel slide projector. The 5 inch lens projects a smaller image than the 75mm lens.

The body of the pointer and the lens holder is made from plastic plumbing parts. Get two, 6 inch long, 1½ inch extension tubes. These are also known as tailpieces. It's the part of your bathroom sink, just under the drain, before the P trap. They cost about $2.00 each.

From your local electronics store, get a push button switch, a resistor, some heat shrink tubing, and solder. For the switch. You can get a momentary-on switch or an on/off switch. A momentary-on switch will be "on" when you push the button and "off" when you let go. Don't get a momentary-off switch unless you want it to be on all the time and off when you push the button! Also watch where the nut is for the push button switch. 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 will be easier to install.

The resistor is key. You need to get one that limits the current through the LED. Light Emitting Diodes typically shine brightest at 20 milliamps 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. A resistor costs just a few cents. However, they are usually bought in multipacks. Radio Shack sells a pack with a variety of resistor values for just a few dollars.

To determine how many ohms your resistor needs to be, use a multimeter. Set the multimeter to measure milliamps and measure the current as you try different resistors. Try and get as close to 20 mA without going over. If you don't have a meter, there are calculators available on the web that let you input your voltage and current and output the correct resistance. Or you can just go with 330 ohms for most LED's when using a 9 volt battery. LED's do vary a bit though, so this isn't going to be exact for every diode.

Next, make an arrow. I made my arrow using PaintShop Pro which is a graphics program like Photoshop. Then I sent it out to Gamma Tech to have it made into a slide. You can make any type of arrow or download one that I made from my web page. It's important to make sure that your arrow on the slide is about the size of the LED. So it does have to be quite small on the film chip. The arrow I used is only about 3.5 mm from tip to tail.

The diode is going to be mounted inside the lid of a 35mm film can. Before the days of digital cameras, everyone had these laying around in the darkroom. If you can't find one. Go to your local camera store and ask for some. They should give them to you for free. Most places just throw them away. I picked up a bag full at my local CVS drug store in the film developing section. Get the black with grey caps. Not the clear ones.

Once you have all the parts, start putting it together. Make the circuit first. Solder the resistor in series with the LED. Solder in the switch and the battery snap. Be sure to get the LED in the right way. LED's have a positive and negative lead. If it's in wrong, it won't work. Also be careful when soldering the LED. It can be damaged by the heat of the soldering iron. Use a heat sink to protect it.

When the circuit is done, put the LED through a little hole in the top of the film can lid. Cut out a notch for the switch in the side of the can. Drill a hole for the switch in the tailpiece. Stick the circuit in the tailpiece. Clip on a 9 volt battery and see if it lights up.

Take another film can lid, cut a hole in it and tape your arrow slide to the second lid. Place it directly over the light emitting diode.

Now, work on the lens holder. This will be a bit different for each lens. For the Cabin lens, I had to ream out the inside of the second tailpiece with a Dremel tool to make it fit. For the Kodak lens, I trimmed the outside of the tailpiece and slipped it over the top. I used a small piece of plastic from a film can as a shim to make a tighter fit.

Cut the tailpiece to the proper length. Take your almost finished pointer into the planetarium dome and hold the lens over the arrow and get an idea how far above the arrow, the lens has to be. Cut the tailpiece to that you have some room to slide the lens up and down in the body of the first tailpiece.

Put some black masking tape or black foil inside the lens holder. This will cut down on internal reflections. Without it, you might get a faint halo around your arrow.

Then, stick your lens holder into your pointer and slide it up and down to focus it. I put a small piece of masking tape on the barrel of the lens holder to make for a more snug fit. You are done. Go point to some stars. Pictures of this pointer can be seen at: "http://www.pa.msu.edu/people/frenchj/pointerglpa2005". Also you can download jpg images of arrows that will be the correct scale when converted to slides.