I know this simple shock gag has likely been covered elsewhere in the forum, as it's a Halloween classic, but I wish to relate my own experience with it.
Parts:
1 electric RC (radio controlled) car with remote. Choice of car is important. You need the cheaper 1-channel model that is always “On”; the remote control only makes it go in reverse. I like the size car which takes four AA batteries; it has enough power and the motor isn’t loud enough to give it away. The remote control should be small and a simply push button design so you can operate it discretely, or even use it as a foot switch.
6ft. strong string – it may get tugged on…
1 small universal power supply.
1 spool of twin lead light electrical wire.
1 eight inch rubber spider (your choice of design).
Build:
Remove and discard the body of the car. Solder wires to the car’s battery terminals and connect them to the power supply so you don’t have to mess with car batteries. Observe polarity and set power supply to correct voltage. Pry a drive wheel out so there is about a ½” gap on the axle between the wheel and the frame of the car. If the wheel is too loose, apply a drop of super glue. Tie one end of the string to the car’s axle (I tie it to the axle first and then tie a knot through the wheel’s spokes so it doesn’t slip). Tie the other end of the string hanging from the axle to your spider; I usually pierce a spider at its center of balance this so all eight legs hang down in a decorous fashion.
Install:
I affix the car to the ceiling with several strips of duct tape and lots of thumbtacks because the adhesive in the tape won’t hold it up alone. Try to get the car’s axle square to the ground. Duct tape conducts electricity; don’t let it come in contact with the circuit board or the car’s RC antenna as this will adversely effect its operation. The spider will likely be swatted at and get swung around during operation so make sure it is sturdily affixed to the ceiling and there are no entanglements (such as webbing, etc) in its swing area.
Operation:
Plug in the power supply and the car will run forward, winding the spider’s string around the axle and holding it up until needed (it may be necessary to trim the string to length). Push the remote and the car will go in reverse dropping the spider on a trick-or-treater’s shoulder – way too much fun! By quickly pushing and releasing the remote control button you can get the spider to hang motionlessly in space; release the button and the spider returns up to the car until called for again. It is useful to employ a distraction so people don’t hear the car motor dropping the spider and become forewarned; I use the usual spooky sounds Halloween CD to cover up the motor noise.
Note that I have used the same old remote control car and universal power supply for about eight years now (from Radio Shack originally) and it has never failed. The car’s motor stays “On” all night long and doesn’t seem to mind and the power supply gets warm, but everything keeps working! The dropping spider is a very simple but highly effective Halloween gag!
Yes, the car motor is always "On". This provides the torque necessary to hold the spider up in place on the ceiling. If power is lost; the spider drops about a foot as it unwinds itself a bit...
I've always thought that this was a very abusive thing to do with the little electric car motor, but it hasn't suffered any ill effects from this obvious mistreatment.
It is my understanding that only the cheapest RC toys have the single channel forward/reverse design. Once you turn the car on "On", it keeps going until you switch it "Off" -- the radio control only reverses its direction of travel.
A few reasons why the single channel RC car works best, at least for my application anyway: When you let go of the controller and the car motor is "Off" the spider may tend to unravel and drop down a bit from its own weight -- you need the motor to remain "On"" to constantly hold it up (this also depends on the car's gearbox...); when dropping the spider, if you just turn the motor "Off", the spider may tend to keep on dropping a bit due to Newton's first law of motion...; having something more complicated than is absolutely necessary violates the K.I.S.S. principle -- Keep It Simple, Stupid!
But after saying all that; I'm sure that with a little bit of tinkering you can adapt any multichannel RC car into a perfectly adequate dropping spider gag. Note that I pretty much destroy a perfectly good RC "Car" when I repurpose it into a halloween prop, so I like to buy a cheap one.