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Reading inputs into an Arduino

We can pass an input to an Arduino in 2 ways. With one we ensure that the Arduino receives a high signal (5v) and with the other a low signal (0v).

A high signal (pulldown resistance):

When the switch U1 is closed, a current will flow through the resistor. This is usually 1k or 4k7 Ohm. Because we have a series connection, the component with the highest resistance will receive the most voltage and the component with the lowest resistance will receive the most voltage. The switch that has the least resistance will have about 5v across the switch. The input connection will therefore receive a voltage of 5v and therefore has a 'high signal'. When the switch is open, the Arduino pin will be grounded, so it will get 0v.

A low signal (grounding or pullup):

When the switch is open, there will be no voltage across the input of the Arduino , so it is not 'high' or 'low'. When you close the switch, the Arduino input will get a voltage of 0v. This is called a low signal.