Page 97 - Programming the Photon Getting Started With the Internet of Things

Figure 5.1 A push-button switch.

You are going to use a small push-button switch to make or break a connection
between 3V3 and one of the digital pins on the Photon, which you will configure in your
firmware program that you will upload to the Photon. If you just connect the switch

directly to the Photon board, this creates a problem when the switch is not closed, as the
input pin is not connected to anything. This is referred to as floating, and could easily give
you false readings when reading the state of the switch. When creating a switch circuit,
you need your readings to more precise, and the way to do this is using what is called a

pull-down resistor.

Now think about what happens when the button is not pressed with the pull-down
resistor in the circuit. The input pin will be connected through a 10-K resistor to ground.
Although the resistor will restrict the flow of current, there is still enough to ensure that

the input pin will read a LOW logic value. 10 K is a common pull-down resistor value.
The value of the resistor must be low enough to make it resistant to any electrical
interference, but also at the same time it must be high enough so that it prevents excessive
current drain when the switch is closed. When the button is pressed, the input pin is
directly connected to 3V3 through the button. In this circuit the current has two options:

It can flow through a zero resistance path to 3V3.

It can flow through a high-resistant path to ground.