Introduction: Wave Viewer - a Mechanical Oscilloscope
First Prize in the
The Teacher Contest
What It Is: This "wave viewer" creates a zoetrope-style effect that lets students see a snapshot of the waves on a vibrating string.
Learning Objectives: A kid-powered, mechanical oscilloscope helps students explore wave phenomena. Younger students can get a feel for the concepts of waveforms, frequency, and amplitude. Older students can gain a greater understanding of the functions of an electronic oscilloscope (such as sample rate and amplification).
Grades: Kindergarten through high school
Sources: This project was begun as part of a university's science outreach program to an elementary school. Inspired by the concept of "The Exploratorium Science Snackbook," which offers instructions on how to create classroom-sized versions of science museum exhibits, and by the amazing artwork of Norman Tuck, we created this project.
Cost: $20 - $30
Step 1: Parts and Supplies
For the main assembly:
- A plastic bucket, 5-gallon size or similar
- A lid to match that bucket
- Black spray paint, suitable for use on plastics
- Black elastic cord, about 24" long, and the thicker the better. (This is available at most craft stores.)
- A "lazy Susan" turntable bearing. (I used a 6" version.)
- White paint
- A wide board to use as a base, about 12" x 18"
- Two small eye hooks
- A small cleat, such as the kind used for window blinds' sash cords
- Three lengths of 1" x 2" wood, each about 16" long (or about the same height as your bucket)
- Two triangles of scrap lumber, or two store-bought angle braces (see step 6)
- Wood screws, 1" length or similar
- Various nuts and bolts. (I heartily recommend grabbing a $2 assortment of machine screws, rather than tracking down each part individually)
For the handle:
Either Version One: - One bearing from a skateboard wheel (see step 8)
- One half-inch PVC pipe coupling
Or, Version Two: - A length of half-inch dowel (about 4" to 5")
- A length of half-inch PVC pipe (of the same length as the dowel)
Tools and other equipment:
- An electric drill
Extra supplies for lessons:
- Black electrical tape
- A black Slinky (the small kind sold as party favors by dollar stores)
Step 2: Painting the Bucket
The oscilloscope illusion results from watching a vibrating, black string against a background of alternating light and dark lines. We'll need to create a black cylinder with white, vertical "pinstripes."
Paint the Bucket: Flip the bucket upside-down and paint the outside completely black with spray paint. (Make sure that you use a paint that's recommended for use on plastics.)
Add the Stripes: Once the black coat is dry, paint twelve evenly-spaced, white "pinstripes" around the sides of the bucket. Masking tape makes for super-straight lines; aim to make them about 1/8" thick.
To help place the lines, I first made twelve white dots in a clock-face arrangement on the "bottom" (now "top") of the bucket, then used these marks as a guide when adding the tape and paint.
While the white stripes dry, move on the to next steps.
Step 3: Align the Bearing and Lid
The bucket needs to be able to spin freely on the base board. We'll use the turntable bearing to attach the bucket's lid to the baseboard, and then install the bucket on the lid.
1.) Center the turntable bearing on the "inside" surface of the bucket's lid. Mark where the bearing's holes go. (A white correction-fluid pen is great for making marks on a black surface!)
2.) Drill four holes in the bucket's lid in the places you just marked.
3.) Find four machine screws that will match the holes in your turntable. (For mine, 1/8" x 1" screws were a good fit.) Test-fit the screws through the turntable and bucket lid to make sure that everything fits. Then, remove the lid from the turntable, but leave the four screws in the turntable's holes. (See the photos.)
Step 4: Attach the Bearing to the Base Board
1.) With the four machine screws still protruding from the turntable bearing, place the bearing near the center of your base board. (It's essential to leave those four screws in the bearing during this step, since there'll be no way to re-insert them once you've screwed the bearing down to the base board.)
2.) Mark where the four remaining holes in the bearing will be attached to the base board.
3.) Attach the bearing to the baseboard with wood screws.
(Did you remember to leave those four machine screws protruding from the bearing?)
Step 5: Attach the Lid to the Bearing
1.) Hold the lid over the bearing so that the "inside" surface of the lid faces upward. (This is so that you can attach the bucket to the lid.)
2.) Align the four holes in the lid with the four machine screws, and pull the screws up through the holes in the bucket. (This part takes a bit of fiddling -- it helps to have a second pair of hands!)
3.) Use the appropriate-sized nuts to attach the lid to the turntable bearing. Use pliers to tighten the nuts as far as possible.
Step 6: Install the Frame
For this next step, we'll install a U-shaped frame of wood to the base board. The height and width of the U-shape should more or less match the height and width of your bucket. For me, this meant three 16" lengths of 1" x 2" wood.
1.) If necessary, cut the three 16" lengths from a longer board. (Hardware stores usually sell 6' lengths of 1" x 2" board.)
2.) Use wood screws to attach the three pieces together into a U-shape.
3.) Attach this U-shape to the front edge of your base board. (Whichever edge you attach this frame to will become the "front" edge, so you might want to choose the edge that looks nicest. Or not.)
4.) To add kid-resistant strength, you'll need to reinforce the corners of the frame. You can either do this (as I did) with two triangles of scrap wood; or, if you don't have any scrap wood (or would like a more-polished appearance), you could use some store-bought corner braces.
Step 7: Attach Eye Hooks, Cleat, and String
To support the vibrating string, we'll attach two eye hooks and a cleat to the frame we just installed.
1.) Attach a small eye hook to the inside edge of each tip of the U-shaped frame. (The string will eventually be attached between these two eye hooks, so it's important that they be placed at about half the height of the bucket. I found that 6" from the top on each side was a good height.)
2.) Attach the cleat to one side of the U-shaped frame, near the bottom. (I positioned mine on the right, since I'm right-handed. The cleat will be used for winding up any extra elastic cord once the string is attached.)
3.) Tie the length of elastic cord to one of the eye hooks, thread it through the other eye hook, and then wind any excess around the cleat.
Step 8: Make a Handle for the Bucket
The students will need a handle to spin the bucket, and it's important that the handle both turns freely, and is attached securely. In this step, we'll go over how to build a sturdy handle from a PVC fitting and a skateboard-wheel bearing. However, in case you can't easily find these parts, the next step will cover an alternate, simpler (albeit flimsier) handle.
1.) Use a screwdriver to pry one of the bearings out of a skateboard wheel. (Every skateboard wheel contains two bearings, and packs of cheap skateboard wheels are sold in the sporting-goods aisles of most Mart-style department stores. If you have access to a hardware store that will sell you individual bearings, you might just go ahead and purchase one bearing with an internal diameter of about 5/16" or 8mm, and an outside diameter of about 7/8" or 22mm.)
2.) Slide a 1/4" by 1" machine screw through the center hole of the bearing, and tighten it down with a nut.
3.) To make a tight "press-fit" between the half-inch PVC coupling and the bearing, you'll need to heat the coupling in an oven. I recommend about 10 minutes at 200 degrees F in order to soften the plastic.
4.) Once the plastic is soft, you can press the PVC coupling onto the bearing until the bearing is flush with the end of the coupling. (Be sure to use heat-safe gloves!)
5.) Drill a 1/4" hole in the bottom of the bucket near the outside edge. Slide the handle's screw through the hole, and attach it with a nut.
Step 9: Alternate Handle
In case you couldn't get a skateboard bearing, here's a simpler handle you can whip up from a piece of dowel and some plastic pipe. However, this is much more prone to getting snapped off when twirled by young hands.
1.) Cut an approximately 5" length of half-inch dowel.
2.) Cut a matching length of half-inch PVC pipe
3.) Sand down the dowel until it will just barely fit into the pipe.
4.) Rub all sides of the dowel thoroughly with a pencil to cut down on friction, then slide it into the pipe.
5.) Similarly to the previous step, use a wood screw to attach your handle to the outside edge of the bottom of the bucket.
Step 10: Finish It Up!
Next, just press the bucket into the lid on the base board, and your construction phase is complete!
For a few tips on how to use this oscilloscope as part of your lesson, see the next step.
Step 11: Lesson Tips
The Basics: To use your wave viewer, have one student spin the bucket with the handle while another plucks the string.
Cranking a bucket at high speed is a lot of fun for little hands, so you may want to encourage the "cranking" student to start slow, and gradually build up speed.
Most students' first impulse is to draw the string back like a crossbow, and snap it against the bucket. While this does make a very satisfying "thwack!" sound, it doesn't let us see much about waves. Be sure to ask the "plucking" student to pluck the string vertically, so that it oscillates up and down.
Once the kids have gotten over the initial "oohs" and "aaahs" of seeing their waveforms frozen in time, challenge them to see what changes will result from a few variations: Can they make a small wiggle appear on the string? How about a large one? What changes if they spin the bucket slower and faster? What's the slowest speed that will still make the illusion work? What happens if they adjust the tension in the elastic string?
After that, it's assessment time! Have the students identify which wave-related vocabulary terms apply to the oscilloscope. Can they demonstrate a wave with a different amplitude? A different frequency? A shorter or longer wavelength?
Once they've explored the basics, you can have the students introduce a few new variations by tinkering with the oscilloscope: if they replace the elastic string with a dark-colored slinky, can they demonstrate the difference between transverse (or "wiggle") waves and longitudinal (or "squish") waves?
Challenges for Older Students: Older students can use black electrical tape to obscure some of the white stripes on the bucket. This is why we made twelve stripes; it allows students to reduce the number of visible stripes while preserving even spacing.
Have the students use the tape to leave, say, six evenly-spaced stripes, and crank the bucket at a certain speed. Then, have them create three evenly-spaced stripes, and crank the bucket at the same speed as before. How has their view changed?
What happens if they use the tape to leave stripes that aren't evenly-spaced? How does their waveform look when viewed against, say, three stripes in a row, with the other nine blacked out?
If the students (at high-school or college-level) are beginning to learn how to operate an electronic oscilloscope, have them identify which concepts/controls have analogs in the mechanical oscilloscope. What is this scopes "trigger"? Sample rate? Gain?
I hope you enjoy playing with your own oscilloscopes! If you have any new lessons and experiences to share, please post them here!
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