DIY: Range Finder
Rangefinders are used to find how far away an object is. Today they are used in a variety of recreational activities including hunting and golf. Originally, however, they were used for military purposes to establish a target's distance. The coincidence rangefinder is a type of optical rangefinder that uses mirrors to display the distant object. When two images from separate mirrors are in focus or coincide with each other, the distance can be calculated.
Things You'll Need
- 2 square mirrors, 3 1/2 inch
- 3 x 1-inch plywood, 1 foot long
- 3 1/2-inch 3-ply plywood, 7 inches long
- Hobby knife
- Compass and pencil
- Jigsaw
- Drill
- 1/16-inch bit
- Countersink bit
- 1-inch self tapping screw
- No. 6 metal washer
- Screwdriver
- Wood glue
- Epoxy adhesive
Instructions
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1
Lay one of the mirrors face down on a flat surface and draw a line across its length using a pencil. Scrape the mirror backing away using a sharp hobby knife until you can see the glass.
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2
Draw two 3 1/2-inch diameter circles on the 7-inch piece of plywood using a compass and pencil. The circles should be drawn side by side so that they do not overlap. Cut the circles out using a jigsaw.
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3
Take one of the circles and draw a line across it so that it passes through the center point. Place the second mirror at right angles to the plywood circle along the pencil line. Mark the mirror's width and draw two lines across the circle marking the width, parallel to the center line.
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4
Push a drawing pin into the circle with the pencil lines at the point made by the compass. Draw around the drawing pin with a pencil and then remove it. Use a jigsaw to cut all the way along the two outer lines marking the mirror's width. Discard the thin piece of plywood left over. Cut out the semicircle on each piece of plywood that was made by drawing around the drawing pin.
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5
Drill a hole in the second plywood circle using a 1/16-inch drill bit two inches from one end. Countersink the hole using a countersink drill bit. Glue the semicircle plywood pieces to the complete circle using wood glue so that you have one plywood circle with a groove along its center on one face.
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6
Lay the one-foot length of plywood on a work bench on its 1/2-inch side. Measure 1 1/2 inches from one end and mark a line across the plywood. Measure 1/4 inch along this line and mark the spot with a pencil. Drill a one-inch hole in the plywood at the pencil spot using a 1/16-inch drill bit.
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7
Place a washer over the one-inch hole and place the plywood disk over it so the two holes line up. Insert a one-inch self-tapping screw and tighten the screw until the screw head is flush with the disk surface. The disk should be able to rotate around the screw.
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8
Insert the mirror with the uncleared backing into the groove on the disk and glue in place using epoxy adhesive. Push pins into the plywood on either side of the mirror to keep it straight while the glue dries. Draw a line in the center of the plywood length just adjacent to the disk to use as a measuring marker.
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9
Draw a line at a 45-degree angle across the plywood length at the end opposite the disk. Cut a groove along this line wide enough for the second mirror to sit in. Glue the mirror in place with the mirrored half facing the mirror on the disk.
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10
Set the range finder on a flat surface and place an object exactly 10 feet away. Look through the clear half of the fixed mirror with the range finder parallel to the object. Rotate the disk until you can see the object in its mirror. Adjust the mirror by rotating it backward and forward until the reflection from the half mirror lines up with the image in the full mirror when viewed through the clear glass. If you see a double image, keep adjusting until the images overlap completely.
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11
Make a mark on the disk where it lines up with the measuring mark on the arm and write "10 feet" next to it. Continue to calibrate the disk in this way using objects at different known distances.
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