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Wily Widget

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Musical Chairs

Wily was reading one of his science magazines when he got a call from a company who puts together various interactive systems for museums around the country.  They were given the challenge by their client to design a musical chair game for small children at a museum in a big city.

They experimented with various pressure switches and strain gages to detect when a child was sitting in the chair but they couldn’t seem to find anything that worked consistently.  Could Wily help?  Wily listened to the companies requirements and thought that a capacitance method would work.  After gathering some more information he agreed to take on the project.

The company already had the computerized system all designed and tested.  They only needed Wily’s help with the child’s fanny detection. The sensor needed to detect when small children where seated in small wood chairs.

The first thing Wily did was attach some aluminum foil strips to the bottom of a hard wood chair as shown below.  This formed a two capacitor plates.  The capacitance between the two metal plates on the bottom of the chair should increase when the child was seated. 

To prove his theory, he connected his battery operated capacitance meter to the strips.  He then “borrowed” some neighborhood kids to sit in the chair, so Wily could make some measurements of the capacitance change associated with the small surface area of a child’s fanny, while sitting in the chair. The capacitance change was small but it proved that such a sensor should work for this application. 

After doing some research on the Internet, Wily found a nice capacitance switch made by a Colorado company.  Wily bought a couple and was very pleased when they worked perfectly.  The switch had a multi-turn adjustment on it, which could be set, so the switch changed state with just ten picofarads of capacitance change.

Wily drew up some sketches and sent some information on the switches to his client.  To increase the capacitance change, Wily suggested the smaller children’s chairs be made of thinner wood.  A few weeks later he was told that the system was up and running.  A photo of the complete system is shown below.

October 2010     Issue 12

Page 1 Circuit
Good Idea
gone Badly
New Products What the World
needs Now
Wily Widget

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