A switch an be made using a popsicle and a comparator where the pop serves as a resistor and as it melts, its resistance drops, triggering a change of state in the comparator. This is made possible by the fact that ice does not conduct electricity while water does. You can test this with a multi-meter by sticking two probes into an ice cube and watching the resistance drop.
The design is simple: two wires with exposed ends are frozen into a popsicle about 1cm apart. The popsicle can then be treated like an ordinary variable resistor. To use the popsicle as a switch, you will need to construct a digital circuit to switch from ‘0 to 1’, so to speak, when a certain resistance threshold is crossed. A comparator will do the job, but I used a program on the Arduino and a Max patch. You will need to experiment to determine the appropriate threshold as it depends upon the properties of the ice/water (minerals, air-bubbles), the distance between the exposed wires in the ice, and the shape of the popsicle (which will determine the manner in which water presents itself as a conductor).
I implemented a threshold on the Arduino with a simple ‘>’ operator: when the voltage reached a certain threshold (voltage fluctuates inversely with respect to resistance), I ‘toggled a bit’. I also implemented the logic in a Max patch using the same strategy. See below a diagram for the simple circuit I used. This circuit can be found in the Getting Started with Arduino by Massimo Banzi.
Full documentation can be found at my class website.
