LearningForward

Kent Chesnut's technology in education blog.

In my last post, I discussed building input devices for Scratch (see here).  My goal was to create a peripheral that could help get more kids interested in Scratch, programming, and educational endeavors in general (by providing more intrinsic motivation).

My first peripheral - a Scratch Balance Board.  The idea was to allow some neat activities like the Wii balance board.

 Remember that you will NEED the PicoBoard interface discussed in the previous post.  The cost is $50 plus $10 shipping to Oklahoma.

Background

I started this project wanting to build a practical Wii clone.  The tough part of the balance board is pressure sensing.  Load cells are very expensive!  And Force Sensing Resistors are only good for very small loads.  So I changed my goal - to build a very simple balance board using parts that can be easily and cheaply acquired (or hopefully scrounged).

Design

Note that I’m a software engineer and am a real klutz when it comes to mechanical things.  This design is not meant for production - it’s meant so that an adult and kid can put this together and play with writing Scratch programs using the balance board as an input device.

Here’s a picture of the outside.

It’s basically a simple box comprised of the following:

• 2 1″x12″ boards cut 22″ long for the top and bottom.
• A frame around the boards made of 1″x4″ boards.
• The bottom 1×12 is screwed in place.
• The top 1×12 sits on top of an elaborate force management system (more on that later).
• Note that the top 1×12 must slide easily within the frame.
• k
• Sorry about the extra lines
• but needed to keep pics from
• getting screwed up.
• If the pics aren’t lined up on
• the right side, try making your
• window narrower or wider.
• k

Here’s a picture of the inside with the “elaborate force management system”.  This elaborate system is composed of 4 tennis balls which are just placed at the 4 corners of the box.

Switches are placed at the center of each side to detect where the user is standing / leaning.

Note that the switches are mounted to scraps of 2″x4″ boards.   The 2×4’s are glued in place with standard school glue.

• k
• k
• k
• k
• k
• k
• k
• k
• k

As you can see from the closeup, each switch is comprised of 2 small screws.  These screws are parallel to the side of the box frame and 3/4″ away from the frame.  The screws are 1/2″ apart.

The wires are held in place by the screw being tightly screwed into the board.

• k
• k
• k
• k
• k
• k
• k
• k
• k
• k
• k
• k

The switch is activated when the top is pressed down.  This picture shows the other half of a switch mounted to the upper 1×12.

The washer is about 1″ in diameter and held in place with scotch tape.

Note that the objects under the washer are rubber tubing (about 1/4″).  These tubes serve 2 very important purposes:

• Allow washer to flex when contacting the screws.  This flexibility is needed to allow the switches to work even if the screws are not exactly the same height.  This also allows the switches to stay made even when the board is tilted on the perpendicular axis (i.e. you can lean forward and make the up contact even while leaning left and right.
•  Using the tennis balls as the suspension system is easy, but leaves no adjustment.  Adjusting the height of the washer off of the top 1×12 changes the force needed to activate the switch.
• k
• k
• k
• k

The screenshot at right shows a script I have attached to the background.  Notice that the script will work in either of the following 2 conditions:

• The force required to activate the switch is high compared to the weight of the user.  The user must lean toward a switch to activate it.
• The force required to activate the switch is low compared to the weight of the user.  When the user stands on the board without leaning, all the switches (or at least a pair of them - right/left or up/down) are activated.  When the user leans toward a switch, the opposite switch opens.

Note that no message is sent if opposite switches are both met.

The biggest shortcoming in the design is the force adjustment described above.  A system that allows the washer to be flexible but also allows for adjustment toward and away from the top 1×12 would be very helpful.

 I’ll look at 2 or 3 simple programs to use with the board in my next post.  I’d love to hear any comments or suggestions.