Building Magic Moving Stairs

This project accompanied several others for our Harry Potter themed Halloween party.

Background:

The Hogwarts School of Witchcraft and Wizardry in the Harry Potter series has many unique architectural features, and some magic ones. One of the more memorable features is the moving section of stairs making up the Grand Staircase. The concept of this project was to play off the existing stairs of our home with a fake moving section. The moving section of stairs would be at the top (far end) of the real stairs giving us:

• a good means to play with depth perception (so the moving stairs don't need to be full size)
• more separation away from traffic areas so guests aren't constantly in the way

The moving stairs would be motorized and setup to oscillate constantly while unattended.

Here's a rough list of what I used to put this together - as the saying goes "your mileage may vary":

• a DC motor with a low RPM (e.g. a 12 volt automotive window or seat motor)
• a 12v battery
• a PWM speed controller to slow down the motor
• on/off switch and hook up wire
• a few feet of aluminum strap or angle for making the linkage
• a 4' x 4' piece of plywood (thin, like .25")
• approx 3, eight foot 1x2" boards
• approx 1, 2x4" board
• one typical door hinge
• two small swivel casters
• liquid nails, a few bolts, nuts, screws, and nylon washers
• approx 1 sheet of 1/2" foam insulation
• paint

The real key of this project is sizing the stairs for the space you have available, and then working out how it will swing through the range of motion. I have access to high end CAD software, so I sketched the limited area I have at the top of my stairs and drew in a staircase module that would fit (from a top view perspective).

Then I designed a mechanism that would oscillate the stairs through 90 degrees while a motor turns continuously. There are more complicated ways to drive the motor - like using a timing circuit, or even an Arduino, to run the motor, then reverse it - but it's hard to beat the simplicity of continuous motion. Plus, continuous motion would help keep people from actually trying to climb on them!

The software I use allows me to sketch all the linkage in simple wireframe and solve the kinematics without bothering with a bunch of solid modeling.


What if you don't have fancy software to figure this out? Well it all could be solved with algebraic equations, but that's probably way too complicated, so just use a simple trial and error method. You can cut strips of cardboard, connect them together with small bolts or wire, and see how they move around. Then try changing the lengths of the strips (or the points at which they are connected) and try again, until you get the motion you need.

Starting with a square of plywood to act as a solid and smooth base, I measured out the two key locations: the center of the stair hinge and the center of the motor shaft. Then I firmly mounted the motor to the base so it's output shaft was in just the right position.

A frame made from 1x2s would become the base of the stairs and was attached to the hinge. Two swivel casters were added to this frame to help carry the weight of the stairs (which isn't much) and let them smoothly roll back and forth. I mounted the hinge to a piece of 2x4, which in turn was attached to the base.

Next a couple couple pieces of aluminum were cut and drilled to make the linkage. The lengths of the pieces isn't critical, but the hole-to-hole distances very much are, so care was needed here. Then these parts were bolted together and secured to the motor shaft and stair frame. Nylon washers were used between the moving parts so they'd slide easily.

After the linkage was assembled and tested, it was time to build the stairs themselves. Some additional 1x2 framing was added to support the steps, but it was kept very minimal.

Some quick calculations were made to figure out the number of steps and sizing. To force the perspective, the steps would get more short and narrow toward the top. In my case I varied the width from 30" at the bottom, down to 18" inches at the top, and from 9" high to 5" - so that's a pretty dramatic difference.

Rigid foam insulation (extruded polystyrene) was used to build out the stairs. First, two long stringers were attached to define the outside taper. Then, working from the bottom up, stair pieces were cut and assembled with liquid nails. After all the stairs were together, masking tape was used over most of the seams to clean up the appearance before painting. The end result is a six foot staircase you can easily lift with one hand!

After everything was painted there was still a little electronics work to do. I was powering the motor with twelve volts, but the motor was a little scary fast. I built a PWM speed controller so I could dial down the rpm with a simple potentiometer. Speed controllers like this control a motor by pulsing the current at the full operating voltage, rather than trying to run it at a lower voltage. I also add a single pole rocker switch inline for on/off.

Lastly I trimmed the base board to a smaller size; just large enough for the casters to ride on it rather than carpet.

The finished prop worked great! As I planned it worked continuously throughout our party giving a unique feature to our Hogwarts home. (Oh, and if you're wondering, we did have a bunch of framed pictures leading up the stairs - some even with moving pictures. But that's another story `ible)