Electricity!

Project: I designed and constructed a display so that visitors could make simple electrical circuits and observe the way they behave under different configurations.

Challenges: The display needed to show the effects of variable voltage, series and parallel circuits, and polarity sensitivity. Further, the circuits needed to be easily assembled and disassembled.  Finally, the modules needed to be color coded, more of a problem than it might first appear.

Solutions: Supplying a variable voltage to the display proved more difficult than I had initially imagined.  As a rule, one should not provide a knob for the visitors to turn – it simply won’t last.

So we decided to have a big honkin’ knob with three strong detents that would provide three discrete voltages (3V, 6V, 9V). The mechanism and the circuitry can be seen clearly in the picture below.

To make the circuits easy to assemble and disassemble, I used modules with magnets hidden behind the conductive aluminum plates on each end.  This is a typical arrangement, seen in many science museums.  The magnets are arranged so the left side of one module will always attract the right side of the one to which it is paired.  This arrangement tends to lead the visitor to make somewhat circular circuits that align well with the positive and negative terminals provided in the display.  Further, the modules can be attached side-to-side as well as end-to-end.  This ability allows the visitor to make parallel circuits along with the series circuits.  One problem with this display is that – inevitably – a visitor will do whatever it takes to short the two terminals of the display, blowing the fuse.  One visitor was seen removing his keys from their ring and placing them end to end in a protracted effort to short the circuit.  Faced with this kind of determination, I simply had to develop a solution that would not reward any such effort.  I could have used a current limiting circuit, but for complicated reasons this was a bad choice.  Rather, I developed a little circuit that would simply turn the entire unit off for 30 seconds if the terminals were shorted.  I put a 1.5 ohm power resistor in line with the display’s circuit and connected a 1.5 V relay parallel to it.  If there is more than 1 amp drawn across the power resistor, the relay picks up and engages a standard Elk timer that shuts down the display for 30 seconds.  No bells, no buzzers, no lights.  The display simply goes dark.  Any sort of flashy reaction on the part of the circuit would be seen by the visitor as a reward and would therefore reinforce the behavior. One problem with this system is that the motor module draws more than 1 amp when it is first energized, and this brief transient was enough to engage the timer and disable the display.  Rather than raise the current setting, I decide to put a large capacitance in parallel with the power resistor.  This has the effect of damping the inrush current of the motor’s start up and allowed me to keep the setting at 1 amp.  The capacitor – as well as the power resistor – are clearly visible in the picture below.

Showing polarity sensitivity was easy enough.  Three of the modules show polarity sensitivity: the motor module, the buzzer module, and the LED module.  The motor shows sensitivity in that it simply rotates in opposite directions when the module is flipped in the circuit.  To emphasize this phenomenon, I attached a small wheel to the motor that is painted with an arrow, making it obvious that the motor has switched directions.  The buzzer demonstrates polarity sensitivity in that is simply doesn’t work if installed backward.  The LED module has a green LED and a yellow one.  Hooked up one way, the green LED lights but the yellow is dark.  Switch the module in the circuit, and the opposite occurs.

Finally, I encountered the problem of how to make these little modules color coded.  Colored plastic is available, of course, but, is usually purchased in large sheets which can cost hundreds of dollars.  Since I needed only a few square inches per module, this was not an option.  I stumbled on the perfect solution walking by the kitchen area of the museum: color coded cutting boards.  Commercial kitchens now use color coded cutting boards to prevent cross contamination of vegetables with meat, etc.  They  come in a variety of colors, are high quality, high density plastic, and cost just a few dollars each.  I was able to make three modules from each board I purchased.  Here are the Motor Module and the LED Module:

Motor Module

LED Module