Up to this point we have focused entirely on outputs. It's time to get our Metro to listen, watch and feel. We'll start with a simple pushbutton.
Wiring up the pushbutton is simple. There is one component, the pull up resistor, that might seem out of place. This is included because a Metro doesn't sense the same way we do (i.e: button pressed, button unpressed). Instead it looks at the voltage on the pin and decides whether it is HIGH or LOW. The button is set up to pull the Metro's pin LOW when it is pressed, however, when the button is unpressed the voltage of the pin will float (causing occasional errors). To get the Metro to reliably read the pin as HIGH when the button is unpressed, we add a pull up resistor into the circuit.