One critical part in the honey stick machine is the optical sensor. They will be used in several locations in the honey stick machine to determine if a straw is in position and when honey is in the straw at a particular location. it consists of a very simple circuit containing an IR LED and transistor. The output voltage is detected by the Phidgets UBS interface.
The circuit is setup so that the output voltage increases when the IR is blocked. Testing showed that the circuit was plenty sensitive enough to determine the varying levels of light when a empty or full straw was placed between the LED and transistor. I did observe one very interesting thing when the straw with honey (or water) was centered between the LED and Transistor. In this case the IR intensity increased instead of decreased as one might expect. I suspect in this case the straw is acting as a lens, focusing light on the transistor. Offsetting the IR LED and transistor so that it lines up with the bottom half of the straw eliminates this problem and actually increases the voltage difference between honey and no honey in the straw.