For this Instructable I'll be teaching you how to make your very own ted dot reflex sight. This quick and simple project is fun to make and the possibilities are endless when you think of the things you can do with it. This project should only take somewhere between fifteen minutes and half an hour, making it great for a rainy day craft or just a way to overcome boredom. If you are going to make this project, you might want to do a little research on basic circuits or have someone nearby who can help you out.

Step 1: Materials

For this project you will need:

x1 - Cardboard Tube (any kind)

x1 - Plastic scraps

x1 - Red Led

x1 - 1,000 Ohm Resistor (1 kOhm)

x1 - Pen/Pencil

x1 - Scissors

x1 - Tape Measure or Ruler

x1 - Hot Glue Gun (and Glue)

x1 - 9v Battery Clip

x1 - 9v Battery

Multiple sheets of paper

Step 2: Cut Your Tube

The first, and by-far simplest step is to cut your cardboard tube to a length of five inches. Measure with your ruler or tape measure and mark your tube at five inches with the pen or pencil - I found it helpful to make several marks all the way around the tube. Now, carefully cut the tube along your mark(s). Be especially careful not to squish the sides of the tube.

Step 3: Math!

This is one of the hardest steps for this project. For this step you will need your paper, pen or pencil, tape measure or ruler, and you may want a calculator. The Pythagorean Theorem states that A^2+B^2=C^2. What this means is for any right triangle, one leg squared, plus the second leg squared equals the hypotenuse (the longest side) squared. To solve this equation for our purpose, let A be the diameter of your tube (the width), and B should be 1/2 the length of your tube (2.5 for a five inch tube). Now if we fill in the Theorem with these numbers we can find the length for our piece of plastic. 1.5^2+2.5^2= length or plastic^2. If we solve this equation the answer is root 8.5, which is equal to 2.9154... which we can round to 3 for our purposes. We now have the length for our piece of plastic, three inches.

Step 4: Cut Your Plastic

For my plastic I used the packaging of some breadboards I bought from Radioshack. Cut your plastic so you have a length of three inches (as determined in step 3) that is as wide as your tube, for me this was 1.5 inches. Now mark the center of each side of the plastic you cut out with a permanent marker. After you have done this cut the plastic in the best oval you can with the edges being where you have just marked the plastic. This plastic will be like a mirror to reflect the dot made by the led into your eye.

Step 5: Finnish the Job

Place a bead of hot glue on one of the short edges of your plastic and slide this into your tube. Now reach through the other end with your finger and hold the plastic to the bottom of the tube while at the same time holding the other end to the top of the tube, right next to the opening. After the glue dries, hot glue the other end of the plastic to the top of the tube, right next to the opening. Now starting from a little ways past the opening cut a slit about three quarters of an inch long above the plastic (represented by LED in the drawing). Place your LED module (see next step) in this slit and move the led around until you can look through the end of the tube and see the dot in the center of the tube. This might take a while and you may have to fiddle with the slit to get it to work, but you should eventually get it. Once you have the LED where you want it, hot glue it in place.

Step 6: LED Module

To make the LED module, solder or twist the red, positive wire from the clip to the resistor and the resistor to the longer leg of the LED (anode), and then the black wire from the clip to the short end of the LED (cathode). For a circuit diagram download the INSTRUCTABLE.svg file and open with internet explorer.

Step 7: Make It Better

Now you can decorate and personalize your sight any way you want. I wrapped mine in electrical tape to give it a sleek look and cover the hot glue.

<p>This doesn't work, it's a cheesy decorative piece that will only look cool to people who don't know anything about guns or optics.</p><p>I just 3D printed one of these LED + mirror type gun sights, and found out without a collimating lens in front of the LED, it's useless, way too much parallax, essentially useless as a gun sight</p><p>The other way is to make a spherical lens, or use a collimator in front of the LED and use a flat lens.</p>

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