$2 Conductivity Probe

This is a simple conductivity probe using electronics from Lectrify http://lectrify.it and copper tape.

The cost of the LED and battery is $2 when purchased in bulk.

Since Lectrify components are designed to fit onto standard Lego bricks, designing for them is easy: simply make 5mm posts 2-3mm high 8mm on center from each other...

Teachers who are looking to extend the learning from this are encouraged to have their students design their own probes based on what they want to analyze.