This Arduino based Capacitive 3D Controller that Kyle McDonald built looks like it is way to simple to function. Have a look at the video to see the amazing type of accuracy this device is capable of. The possibilities are endless for this cheap setup, I can imagine a security pass code that is not a code but rather a gesture within the 3space that the sensor field can detect.

"The "circuit" is just two resistors per piece of aluminum. To understand why they’re there, it helps to know what we’re doing with the Arduino. What we’ll do with each pin, sequentially, is:

• Set the pin to output mode.
• Write a digital "low" to the pin. This means both sides of the capacitor are grounded and it will discharge.
• Set the pin to input mode.
• Count how much time it takes for the capacitor to charge by waiting for the pin to go "high". This depends on the values for the capacitor and the two resistors. Since the resistors are fixed, a change in capacitance will be measurable. The distance from ground (your hand) will be the primary variable contributing to the capacitance."