In this project, I show you how to use magnets to make Halloween props on a table move without anything touching them.
To accomplish this, I first embedded magnets inside a severed hand prop. Then I took a second set of magnets and attached them to the end of a linear motor. This motor was mounted to the bottom of a table. When the hand is placed on top of the table, the motor is able to move the hand with the magnets. This process can be controlled manually or with various sensors and a microcontroller.
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Step 1: Watch the Video
Here is a video walkthrough of the project.
Step 2: Materials
Here are the materials and tools that you will need for this project.
Foam/Rubber severed hand prop
Miscellaneous other Halloween props
Strong Magnets (preferably neodymium magnets)
Glue that is safe to use on foam and rubber
Table with a thin top
Motor Controller such as a PicoBoo
Step 3: Embed a Magnet in the Prop
The first thing that you need to do is embed a magnet in your Halloween prop. So start by deciding where in the prop the magnet should be located. This should be some place that naturally sits flush against the table when it set down. On my hand I decided to mount the magnet on the inside of the wrist just past the cut end.
Then take a knife and cut a slit in the side of the prop that is just big enough to fit the magnet. Then put a small drop of glue inside the cut slit. Then insert the magnet. You may want to place a small amount of glue on the outside to help close up the seam. To help hold the magnet and the side of the prop together while the glue dries, I placed two more magnets on the outside of the prop. This held the magnet tightly against the inside wall of the prop. Allow the glue to completely cure before doing any further work with it.
Step 4: Attach Magnets to the Linear Motor
Now you need to attach magnets to the end of your linear motor. The number of magnets that you need to attach will depend on how thick your table top is.
If the extending rod on the motor is magnetic you may be able to simply stick the magnets onto the end and let them hold themselves in place. In my case, the rod was aluminum, but mounting pin was steel. So I just stuck the magnets onto the mounting pin.
In some cases you will need to use a different method of attaching the magnets. Glue will usually do a good job of holding them firmly in place. I recommend trying temporary glues such as hot glue first. That way you will be able to disassemble it after Halloween and reuse the parts. Another option is tape. Tape will usually work but can leave a sticky residue.
Step 5: Mount the Motor to the Underside of the Table
Next you need to mount the motor to the underside of the table. How you do this will depend on the type of table that you are working with, and how permanent you want the setup to be.
The most secure method of attaching the motor is to bolt it in place with metal brackets. However, this requires you to drill holes in your table. If you don't want to damage your table, you have two alternatives.
One option is to replace the table top with a cheap board that you don't care about messing up. In my case, I replaced the table top with an old bulletin board. This also had the advantage of being thinner than the original table top. Unfortunately, this method only works if your table top is detachable.
The other option is to use a non-destructive method of attaching the motor. The best methods for attaching the motor without hurting your table are either hot glue or tape. Both of these have their limitations and they will not stick to all surfaces. You may have to experiment some to find the best option for your setup.
Step 6: Select a Microcontroller
You can manually turn the motor on and off, but in most cases you will want to have the system automated. There are a number of microcontrollers that you can use for this. An Arduino, a Raspberry Pi, or a Beaglebone can all work well. With any of these boards you will also an external motor driver to control a high power motor like one we are using.
For this project, I am using a PicoBoo controller. This is a record-playback system. Rather than programming the exact timing of the output functions, you manually activate each function in the sequence. The controller is then able to play back the sequence whenever it is triggered. The main advantage of this kind of system is that it is fast and easy to set up and requires no programming skills. It also has a built-in relay driver for controlling motors. So no external circuits are required (except for optional sensors for triggering).
Step 7: Hook Up the Motor to a Controller
When hooking to the motor to the controller, be sure to follow the manufacturers directions. Here is how I hooked up my linear motor to a PicoBoo controller.
The wires from the motor are connected to the common terminals on the two relays. Then both "normally closed" (NC) terminals are connected to the positive voltage terminals. The "normally open" (NO) terminals are connected to the negative voltage terminals. When both relays are turned off, both wires from the motor are connected to the same voltage wires. But when one relay is activated the motor will be connected to positive and negative power and the motor will move in one direction. When the other relay is activated, the motor will move in the opposite direction.
Step 8: Hook Up Sensors to Automatically Activate Your Motor
In most cases, you will want to use sensors to automatically activate the motor. I chose to use a motion sensor. To hook it up, I simply connected the four wires from the motion sensor to the trigger terminals on controller. Each terminal on the controller is color coded. So just attach the wires to the matching colored terminal.
Step 9: Program the Controller
Now you need to program the controller. For this prank, I just want the hand to slowly extend and then retract whenever the sensor is triggered.
Press the record button to begin programming the controller. Then press the button that will extend he motor. Hold down the button until the motor is fully extended. Then release the button and wait for a second. Next press the other button to retract the motor. Once the motor is fully retracted, press the record button again to exit the programming mode and lock in the sequence.
Step 10: Set Up Everything on the Table
Now that the basic system is assembled, all you have to do is set up the table with a candy bowl and other props. Place the candy bowl at the back of the table so that people will have to reach past your moving hand to get to it. Set up other Halloween props around the table so that no one will pay too much attention to your hand.
Try to hide your motion sensor out of sight. You also want it to be pointed in a direction that will only register movement when someone reaches for the candy bowl. If you find that the motion sensor is to sensitive, you can place tape over parts of the sensor screen to give it blinders and focus it into a narrower field of vision.
Step 11: Scare Your Friends
When your friends reach for the candy bowl, the hand with slowly reach out towards them. This will scare a lot of people simply because they weren't expecting it to move.