Obtain thin metal tubing that is free of kinks, dents, and bends. It must have an inside diameter that fits snugly around the outside of your phototransistors. Use a grinding cutoff wheel or metal saw to cut the tubing into lengths of 8 inches (20 cm). Be careful not to crush the tube while cutting it. Gently remove the burrs using a flat sanding block and fine sandpaper. Clean any oxidation from the inside of the tubes by forcing a ball of steel wool through each tube several times with a dowel. Then force a ball of paper towel through each tube to remove any dust. Check each piece for straightness by looking through it at a sheet of plain white paper. Straighten by hand as needed.
Apply metal primer to the inside of each tube. After the primer has dried completely, apply a coat of flat black paint to the inside of each tube. Allow the paint to dry. Look through each tube at a sheet of plain white paper to verify that the entire inside surface has been covered, and that the tube remains straight.
Obtain stiff, thick-walled tubing with an inside diameter that is larger than the outside diameter of the tubes prepared above. It should be straight enough for one of the smaller tubes to fit inside without touching. Cut to the same length as the narrow tubes. File and sand the ends to remove burrs.
Wrap a narrow band of tape around each end of each of the smaller tubes. Apply multiple wraps until the tape is just thick enough to fit snugly inside the larger tubes. Slide each of the smaller tubes inside a larger tube and secure it there with epoxy or hot glue. (See the diagram.)
Copyright © 2001 Kristin Hammond
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