Taste Buds!

I designed and constructed an electronic circuit and related display to illustrate the difference between those who are Phenylthiocarbamide (PTC) tasters and those who are not.  Simply put, those who readily taste PTC (a molecule responsible for bitterness) require the activation of only one receptor to experience bitterness, while those who are not PTC tasters require the activation of several receptors.  The program manager of the Human Body Connection decided on the straight-forward approach of having a display with two “taste buds” –  one representing a PTC taster, the other representing a PTC non-taster.  These are the clear plastic domes seen in the photo.  Each taste bud has four receptors in the form of a ring attached to the dome.  The interactive display is activated by a PTC “molecule” carried by the staff.

While much of the pedagogical content is imparted by the graphics, at least two points needed to be made in a more physical and/or dynamic way.  1) The PTC molecule will fit into a receptor only one way and will fail to activate the receptor otherwise, and 2) the activation of one receptor (for the taster) or three receptors (for the non-taster causes a chemical cascade that eventually reaches the brain and causes the sensation of bitterness.

The first problem was solved by creating a molecule model that is radially asymmetric.  There is a small channel in the molecule that corresponds to a fitting in the receptor on the taste bud. While the molecule will fit in backwards, it will fail to activate the taste bud in this orientation.

PTC “molecule” with groove

I accomplished this by using a magnetic proximity switch for the circuit and a magnet hidden in one end of the PTC molecule. Inserted one way, the magnet is close enough to activate the receptor.  Inserted backward, the magnet is too far from the proximity switch for activation.  This magnetic activation (as opposed to a simple switch) has the added advantage of making the display impossible to activate without the aid of the staff, an important feature insofar as the display is intended to be interpretive.  The second problem is much more complex, requiring different solutions to each of many sub-problems of which the second problem is composed.  First, I needed to illustrate that the receptor has been activated.  This was accomplished by using a Lightwire that is wrapped around the interior of the receptor.  This Lightwire is energized by the proximity switch mentioned above, but only if the PTC molecule is correctly inserted.  The effect is striking.

Lightwires activated

This effect is common to both taste buds.  After this point, however, the behavior of the two taste buds diverges.  For the PTC taster, the activation of a single receptor cause the chemical cascade that results in the sensation of bitterness.  The display illustrates this phenomenon by illuminating a series of lights at two second intervals leading ultimately to the illumination of a model brain, illustrating that the electrochemical signal originating in the receptor has finally reached the brain.  I sequenced the lights using a set of overlapping Elk timers set at two, four, and six seconds for the cascade lights, and a final timer set at eight seconds to activate the model brain.  The PTC non-taster required a more complex solution.  The interpretive aspects of this portion of the display required that the activation of three receptors is necessary to cause the cascading effect.  The tricky part is that it had to be designed so that any three receptors in any order would cause the cascade sequence.  Cleary, some logic circuitry was required.  Since three required inputs of four possible choices results in four permutations, (ABC, ABD, ACD, and BCD) I used four triple-input AND gates of the 9403 IC chip variety.  The output of each gate was attached to a dedicated 5V DPDT relay that activated the cascade circuitry.  The 5VDC relay, in turn, sets off the same display as in the PTC taster cell.

Circuitry for Taste Buds!