A contact sensor uses conductivity to create a digital input. When two pieces of conductive material, like copper, touch, electricity can flow through them to make a circuit. . In this card, the computer measures any changes to the current. When it is connected to ground (GND), it creates a change. In your sensor, how do you close the circuit? Where is the computer (the CPX) measuring the change in current?
Button and Switch Basics (click the image to go to Sparkfun’s tutorial)