Time Flies: PER3 Interactive

Wow, this interactive was complicated. PER3 (pronounced “per three”) is a gene that has much to do with circadian rhythms and sleep regulation. The intent of the display was to guide the visitor through the process wherein a PER3 gene is constructed using its building blocks, ribosomes. The basic idea is that visitors would take each of seven pucks and place them – in order – into a series of slots to mimic the way genes are stepwise constructed. Here is a picture of the display so that you can visualize the process.

Simple enough in principle, but the devil, as they say, is in the details. Here are the details and solutions:

How would the visitor be alerted that he (I flipped a coin and will be using the masculine third person henceforth) has an incorrect puck in place?

We decided that the visitor would be allowed to make only one mistake in the process at a time. To warn the visitor that he had an incorrect puck in place, we had (imaginatively enough) an OOPS! light. Should a puck be incorrectly placed, this red light would illuminate, providing the visitor a gentle warning and some tips for successful completion of the task. You can see this area of the display below.

How was this accomplished?

Nearly every aspect of this display is controlled by magnets hidden in the pucks themselves or in the actual cabinet.  For the circuit to “know” if there was an incorrect puck in place, it first had to know that there was a puck at in place at all.  Each puck had a small rare-earth magnet at the center.  This magnet let the circuit know (by using a magnetic reed switch in the cabinet itself) that there was a puck in place.  But each puck also had a rare earth magnet near the edge of the puck.  The difference with the perimeter magnets is that they are in a different place for each puck.  In the cabinet, each slot had a magnet at the center, like every other slot, but it also had a perimeter magnet in a location unique to that slot.  So if the correct puck was in place, both the center sensor AND the perimeter sensor would be activated, and the circuit would not light the OOPS! light.  If the wrong puck was in place, the center sensor would be activated, but not the perimeter sensor, and the circuit logic would light the OOPS! light.

What if the visitor placed TWO incorrect pucks?

Not likely.  It is actually physically impossible to have two incorrect pucks in place.  If a visitor made a mistake, and proceeded to the next puck, it would automatically pop out of the slot when it was released by the visitor.

How was this accomplished?

Magnets, my friend, magnets.  Each puck contained a large (3/4 inch) rare-earth magnet at the bottom of the puck.  In the cabinet, each slot had, behind the graphic, a ¾ inch magnet in the corresponding location.  The magnets are arranged so that the north poles are FACING each other when the puck is in place, causing the puck to pop out of the slot.  I called these magnets the “ejection magnets”.

So it seems, then, that the visitor could never put a puck in place.

You’re right, it does seem that way.  But what you do not yet know is that these magnets are movable.  If a visitor places a correct puck in the FIRST slot, the magnet in the SECOND slot will move out of the way so that it can receive a puck.  If the correct puck is place in the SECOND slot, the “ejection magnet” for the THIRD slot will move out of the way, etc. etc., all the way across the panel.

How was this accomplished?

The magnets were placed on the ends of mechanical swing arms that were controlled by the previous slot and the same circuitry that controlled the OOPS! light.  The swing arms were driven by standard electric motors controlled by polarity reversing DPDT relays.  They could exist in either of two positions –  “accept puck” or “defeat puck” – depending on the polarity applied to the motor and the limit switches on the swing arm.  The pictures below show the mechanism and the two positions it could achieve (note the limit switches).

Below is a picture of four of the seven swing arm mechanisms in place in the cabinet, viewed from behind. They are yellow with words on them because they are recycled aluminum sign panels left over from the Big Dig exhibit removed from the museum a few years ago. (Eighth inch aluminum sheeting is an excellent substrate for arranging and mounting various mechanical parts that need to work together.)

Is any other guidance provided to the visitor?

Yes, one final little hint. If a correct puck is placed in a correct slot, a tiny green LED is illuminated that hints at the visitor to proceed to the next slot. If the wrong puck is placed, the LED is not illuminated. These LEDs are clearly visible in the video at the end of this entry.

What happens when all the pucks are in place?

We at the museum have learned that it is good to provide a small reward when the visitor has completed a given interactive. Goal orientation seems to drive the process forward. For this interactive, a short video describing the production of the PER3 gene is started when the last puck is successfully placed in the appropriate slot. The monitor displaying this video is visible to the right of the slots for the pucks.

I see a problem. Won’t the pucks that are left in the slots by one visitor ruin the interactive for the next visitor?

It would, but the pucks do not stay in the slots for very long. When the last puck is correctly placed, a timer is started. The timer runs just slightly longer than the length of the video mentioned in the previous section. When the timer times out, the ejection magnets are rotated back into place, and so the pucks fall out of the slots into the bin below, ready for the next visitor. You can see this bit of magnetic sleight of hand in the this video.