Snotrag the Ghost

Animated Halloween Decoration

This year (October 2006,) my daughter and I decided to make an animated "flying ghost" for the front yard. We already had a big pile of bicycle parts available, so we decided to use those as a basis. This project is fairly simple, and can be completed in a day.

Tools you'll need:

Dremel tool with cutoff wheels
Hacksaw
Flat file
Drill with drill bits
Hammer
Screwdriver
Step ladder

Materials you'll need:

Donor bike with two good wheels (perfectly round wheels aren't required, but good bearings are a must)
Drywall screws (1-5/8" long are preferred)
Flat steel mending plates
Nylon tie wraps
Small 6" bungee cord
30-lb. monofilament fishing line or heavy-duty thread
Small nylon pulley
Small electric motor
Brass tubing
Wire (low-voltage landscaping-light type)
Variable power supply
Two 8-foot 2x4 posts
Four 36" steel fenceposts
Two tent pegs
Mason's line (we used #18 available at Home Depot)
Rubber bands
Glow stick
Tissue paper (the stuff for wrapping gifts works better than the stuff for wiping noses)
Yellow cellophane
Clear tape

The basic concept involves two posts with vertical pulleys. The pulleys are made from salvaged 12" kid-scooter wheels. You could just as easily use the chain-drive from the donor bike to provide motion, but that'll require a little more metalworking. We also hacked the chain-stay bars from a donor bike to make the attachment points on our posts. The desirable wheels are the 5-spoke nylon types. We cut down the spokes for clearance for the drive mechanism. Using a traditional wire-spoked wheel will work, but you'll need to be careful to avoid entanglement issues between the spoke nubs and the carrier line.

Hack the rear wheel mounting lugs out of the donor bike. Bash them flat-ish with the hammer, drill two holes, and mount each onto the end of a 2x4. While not required for this method, you probably want to make a mirror-image pair of poles - you'll have one side clear of obstructions should you decide to add larger dangly-bits.

Hack the spokes on one wheel for clearance. You're objective is to cut down the obstructions such that the wheel hub is smooth and can be used as a friction-drive component. There's more detail in the photos, which should make it more clear why this step is necessary.

We used a windshield wiper motor to drive this contraption. The motor was purchased from All Electronics for about $17. I used a hacksaw and the Dremel to remove the worm-drive housing. I found a close-fit piece of brass tubing that I could press onto the output shaft. The nylon drive pulley was hacked out of a screen door hanger/roller bracket I'd purchased at Home Depot. I drilled out the center slightly smaller than the brass tube, and just pressed the two together. A little 5-minute epoxy would have been appropriate if I expected this contraption to operate for more than one evening.

The drive belt is made from a 6" bungee cord. Snip the crimped ends off the cord, discard the metal hooks, and splice the cord ends together with the 30# monofilament. If you don't have fishing line available, use heavy thread from a sewing kit. The splice doesn't have to be perfect, but you do want it neat enough that it won't jump out of the nylon drive pulley. Choose a drive pulley with high sidewalls.

Place the drive belt on the wheel with the modified spokes, and then mount the wheel to one of the post brackets. Mount the other bike wheel to the other 2x4 post. The "idler" post assembly is complete. Check the photos for more detail on the drive side. The two flat mending plates screw to the 2x4 such that they support the motor in the "right" place. You'll need to adjust as necessary for your specific motor. Try to make sure the motor axis is aligned parallel with the wheel axis. Too much misalignment will make the drive belt jump out of the drive pulley. Secure the motor with nylon tie wraps.

Okay, now it's time to install the posts in your yard. A second pair of hands is helpful, but not required. Pound the two fenceposts into the ground such that they form the base of a triangle. The 2x4 forms the third leg of a tripod. Secure the two fenceposts to the 2x4 with drywall screws. Hammer a tent peg at the base of the 2x4 to prevent it from falling over backwards when the line isn't connected betweeen the two posts. Install the Mason's line as a loop over the two pulleys. Make sure there's some tension on the line - the Mason's line will stretch slightly during use. Tie a nice knot that won't slip loose. Sight down the line to make sure the bike wheel pulleys are in-line with the string. Perfect alignment isn't critical, but too much skew will make the string jump out of the wheel.

Break out the wire and the power supply. A variable power supply is desirable because you can tweak the voltage to get the ghost's speed "just right." A leftover PC supply provides +3.3V, +5V, and +12V, which may provide enough choices if you're lucky. Simple and inexpensive power supply kits are available from Electrokits, but you'll need to do a little soldering to put the kit together. Another alternative is a semi-variable power supply like this one. Expect to spend some time fine-tuning the alignment of the motor to the bike wheel. That's the fussiest part of this whole assembly. Rubber bands can be used as a drive belt as long as the load isn't too high.

Finally, make the ghost. We used a 3" yellow glow stick draped in strips of tissue and cellophane. Be creative, but remember that you need to keep it lightweight. Don't make it too fragile either, as the ghost gets smacked pretty hard every time he goes around the pulley. Loop a rubber band over the main string and back through itself, securing it to the string. Pass the free end of the rubber band through the hole at the end of the glow stick, then loop it over the hook to secure. If your glowstick doesn't have a hole in the end ... improvise. The ghost should now be hanging from the Mason's line with a little bit of flexibility - too rigid a mounting will make the Mason's line pop out of the pulley.

Fire it up. Adjust the ghost speed to about a walking pace. We've got about 40 yards/meters between the two main pulleys, and the ghost makes the round trip in about 30 seconds. If the ghost is moving too fast, you'll tend to make the main string line jump out of the pulley when the ghost is changing directions. High speed operation also beats the heck out of the cellophane.

Movies of the ghost in operation:

From the pulley side [10.7MB]
From the motor side [6.4MB]

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