LESSON 09 – READING DIGITAL PINS

INTRO

In this lesson, we will learn how to read if a digital pin is high or low.

INPUT PIN

To read a pin, it needs to be an INPUT pin. We designate the pin as INPUT by using the pinMode() function.

Here is an example of making pin #10 an input pin:

pinMode(10, INPUT);

Note that we do not write to INPUT pins. They are used for reading.

READING A VALUE

To read if a pin is HIGH or LOW, we use the digitalRead() function. We do have to specify the pin’s number.

Here is an example of reading pin #8’s value:

int value = digitalRead(8);

Notice that we are storing the result of digitalRead(8) into a variable called value. We do this so that we can store and later use the reading.

POSSIBLE VALUES

The digitalRead() command gives either a 0 or a 1. The 0 represents a LOW state and the 1 represents a HIGH state.

In programming, instead of saying that a command gives a certain value, we say that it returns a certain value.

NO FLOATING PINS

If you try to read from a floating pin, you will get seemingly random results. The value of the read will switch back and forth between 0 and 1 with no predictablity. So we should never allow for a pin to be floating when we are using it to read.

PULL-UP & PULL-DOWN RESISTORS

In digital design, we (almost) always want a circuit to have either a state of 0 or a state of 1. We almost never want to leave a circuit open (or floating). Switches that either open or close a circuit unfortunately create the undesired floating state. The solution is to use a pull-up or pull-down resistor along with such switches to assure that the circuit is always 0 or 1.

The two circuit drawing below show two different scenarios where a switch could leave a circuit in a floating state.

On the left, when the switch is closed, the circuit gets a HIGH state inputted. When the switch is open, it gets a floating (unpredictable) state. It would be better for the circuit to get a LOW state instead.

Similarly, in the above image on the right, the circuit gets a LOW state when the switch is closed but a floating state when its open. It would be better for the circuit to get a HIGH state when the switch is open.

The images below show how we can add a resistor to the circuits from above to either pull-up the floating state to a HIGH state or pull-down the floating state to a LOW state.

The resistor’s resistance has to be high enough so that when the switch is closed, most of the current will ignore that branch with the resistor in it. In other words, when the switch is closed, the added pulling branch with the resistor doesn’t really affect anything.

When the switch is open however, the pulling branch provides the circuit either with a LOW state or a HIGH state (instead of a floating state).

TRY THIS…

PRACTICE PROGRAM/CIRCUIT 9.1

A-CIRCUIT

Create the following circuit.

B-DISCUSSION

Analyze the circuit to make sure you understand the explanation below:

When the switch is open, pin #2 is connected to GROUND (via a pull-down resitor). So pin #2 is low.

When the switch is closed, pin #2 is now connected to 5V. So it is high.

C-CODE

Write the code that will continuously read the digital value of pin #2 and output whether it is high or low to the serial monitor. You should add a delay of 500 milliseconds between checks.

D-TEST YOUR PROGRAM

Test your program. You should see the output change when the button gets pressed.