I had a very last minute idea for a Halloween decoration which would feature a ghost like apparition which you could clearly see but could not touch. The ghost would take the form of a small pumpkin in the mouth of a green monster. The pumpkin would spin enticingly, inviting you to grab it from the monster's mouth however when you reach out to touch it, there would be nothing but air in your hand!

This prop could be assembled in a few hours if you have all the parts available. The only specialist item needed is a concave parabolic or spherical mirror which you can get from a science education toy called a "Mirascope". These can be bought cheaply online. I happened to have one already.

Supplies

a geared down DC motor. I'm using a 1:1000 reduction micro metal geared motor
concave parabolic mirror - a spherical mirror might also work. Look online for a Mirascope educational science toy
Two LEDs
a small cardboard box
black paper or cardboard
packaging cardboard
electrical hook-up wire
current limiting resistor 100-500 ohm (optional)
soldering iron and solder
3 AA batteries
battery holder with switch - I used one from indoor battery powered Christmas lights
a small pumpkin - alternatively this could be a small toy or a piece of candy

Step 1: Mount the DC Motor in the Cardboard Box

The DC motor should be about half the length of the cardboard box into which you want to mount it. Cut a strip of packaging cardboard whose width is roughly the same as the length of the DC motor, excluding the shaft. Fold the strip multiple times to form a block. Glue it and clamp it so that it doesn't unfold by itself. Prepare more cardboard strips to build up a block of sufficient thickness so that the motor and pumpkin can spin without hitting the walls of the box. Use glue or double sticky back tape to attach the DC motor to the block.

Step 2: Wire Up the Circuit

Cut and strip wires. I used solid core but stranded will also work. Tin the leads and wires with solder.

Connect the cathodes of the two LEDs together with a length of wire. Connect one terminal of the DC motor to one of the cathodes with a short piece of wire.

Connect the anodes of the LEDs with another length of wire. Notice that the LEDs are connected in parallel, but the DC motor is connected in series with each LED. The DC motor is being used in place of a current limiting resistor.

Test your circuit using the battery pack. Touch one terminal to the anode of one LED, and the other terminal to the free terminal of the DC motor. Don't worry if you get the polarity the wrong way around. The voltage (4.5V) is low enough that it wouldn't damage anything.

Measure the current with a multi-meter to make sure it isn't too high. If it is over 40 mA, it might burn out your LEDs if you leave your decoration on too long. Solder a current limiting resistor (about 100 ohms) if this is the case.

When you're happen everything is working, solder the battery leads to the circuit.

Step 3: Prepare the Enclosure

A cardboard box will serve as the enclosure for the LEDs and DC motor, however only the spinning pumpkin should be visible. Cut black cardstock to cover half of the box. Also cover the walls of the box with black card or with black electrical tape.

Step 4: Mount Motor and LEDs in Enclosure

Secure the LEDs to the cardboard block with gaffers tape or hot glue. Use double sticky-back tape to attach the cardboard block to the back of the box. Make sure there is enough room for the pumpkin to spin freely without hitting the walls or the ceiling. Cover the motor and LEDs with the black cardboard that you cut to size in the previous step.

Step 5: Mount the Concave Mirror

Disassemble the Mirascope. The lower half will be used as the concave reflector.

Cut a piece of packaging cardboard into the shape shown above. Fold it into a stand for the mirror. Trim the edges of the cardboard to fit the mirror as needed.

Mount the mirror on the cardboard stand so that it is almost vertical. Ideally it should be angled to face slightly upward.

Use tape to prevent the stand from unfolding.

Step 6: Final Assembly

Cut out a monster-like shape from card stock of an appropriate colour. The only rule here is that your monster have a wide open mouth through which the pumpkin will be visible. I'm sure your creations will be far superior to mine.

Assemble the illuminated spinning pumpkin apparatus. Position the black card cover so that only the pumpkin is visible. Switch on the battery pack. Turn the box around to face the mirror.

Using double sticky-back tape, attach the cardboard monster to the top of the box so that the open mouth is between the box and the mirror. Give the monster an upward bend so it lies on top of the mirror.

Adjust the distance between the mirror and the pumpkin so that the real image appears inside the monster's mouth or just outside of it. You have to be facing the mirror nearly head-on in order to see the pumpkin's real image.

You might also have to adjust the height of the mirror or the box by placing layers of cardboard underneath. In practice the real image is visible over a narrow field of view. It really depends on the size of your concave mirror.

Place your completed decoration for easy viewing by your intended audience. It will need to be on a low table for kids, or on a high shelf for adults.