Cycle LED Colors with Button

For your code to use a single input to affect multiple outputs, you need to use a variable between them to ‘hold’ the value. In this code, the button press affects the variable pressCount, then the pressCount affects the state of the LED light.

Code

/* * Use a button to cycle through three LEDs or a multi-color LED. * Hold down the button to strobe! */ int pressCount = 0; //How many times the button has been pressed. void setup(){ pinMode(9, OUTPUT); //Set up your LED pins as outputs pinMode(10, OUTPUT); pinMode(11, OUTPUT); pinMode(A5,INPUT_PULLUP); //button } void loop(){ //If the button is pressed, increase the counter variable by 1 if(digitalRead(A5)==LOW){ pressCount++; //Delay before next button press is sensed. Lower delay = faster strobe delay(250); } //Use the pressCount to determine which light is on. if(pressCount==1){ digitalWrite(10, LOW); //turn off the light on pin 10 and... digitalWrite(11, HIGH); //turn on the light on pin 11 } else if(pressCount==2){ digitalWrite(11, LOW); digitalWrite(9, HIGH); } else if(pressCount==3){ digitalWrite(9, LOW); digitalWrite(10, HIGH); //No matter how many LEDs you add, the last "if" sets pressCount to 0 pressCount=0; } } // (c) 2017 Let's Start Coding. License: www.letsstartcoding.com/bsdlicense

Challenges

How many can you complete? Change the code and hardware according to the challenges below. Upload your code to see the effect when you're finished. Complete a challenge? Check it off the list!

"Comment out" the line containing delay(250); by putting two slashes (//) in front of it. Now the computer won't run this line.

Add another multi color LED to three new ports on Carrier Board. Try to make the program light up a different color on two LEDs with each press.

Add more 'else if' statements and mix the colors by using digitalWrite(11,HIGH); and digitalWrite(12,HIGH); at the same time, for example.

How would you add another button and another LED so that the new button cycles the colors of the new LED?

Videos

Read the Code Walkthrough Text (click to open)

The pressCount variable starts with a value 0 above the setup and the loop. As long as it stays 0, nothing will happen because there are no conditions based on pressCount == 0.

The code is still running through the loop hundreds of times per minute, but there just isn’t anything for it to do.

As soon as the digitalRead function receives a LOW signal, the pressCount variable increases by 1. Now the first ‘if’ statement is true: pressCount == 1. The effect is that the code between the { and } runs, turning pin 10 LOW and pin 11 HIGH.

Why do you ensure that pin 10 is turned LOW? When the code loops back through from pressCount = 3 to pressCount = 1, pin 10 will be HIGH, since that’s which LED is on if pressCount = 3. If you don’t ensure that 10 is LOW, you’ll end up with a mixed color when pressCount = 1.

The loop continues to run, checking the if statements each time through (hundreds of times per second). As soon as digitalRead(A5)==LOW again, the pressCount increases by one. Now that pressCount is equal to 2, the first if statement in the loop is false.

The computer now checks the first else-if statement. Is pressCount equal to 2? Yes, so the code between that statements curly braces will run.

Remember that at this point, the code has never checked the condition of the second if-else statement. Once the code finds a true statement, it runs that and skips the other else-if statements. That is what makes else-if different from if. Every if statement is checked, but not every else-if statement is checked.

When the button is pressed for a third time, pressCount now equals 3 and that else-if statement is true, so the last light will turn on.

Notice that the last else-if sets pressCount back to 0 as a part of its action. Because pressCount ==0 doesn't do anything, the light for the third else-if statement stays on until pressCount equals 1 again.

You can see in the ‘if digitalRead(A5) ==LOW” statement that there is a delay of 250 milliseconds. This keeps the code from running too quickly and increasing pressCount by 2,3, or more each time you press the button. Holding the button for more than 250 milliseconds, though, will increase the pressCount variable by more than one. Play with that delay and see how it changes the strobing effect between the colors.

Concepts

New Concept: Combining if and else-if Statements

You can combine if and else-if statements to define a wide number of 'paths' for a program to take. Each else-if is another decision for the code to make. You can also use else-if statements to prioritize decisions. Think of deciding where to eat as you walk down a street. You may think "If pizza is available, I'll have that. If not, I'll look for Greek. If that's not available, I'll look for Chinese food.". This set of decisions prioritizes pizza over Greek and Greek over Chinese.

Now imagine the same decision using only if statements. Your decision would be "If pizza is available, I'll have that. If Greek is available, I'll have that. If Chinese is available, I'll have that." You would be extremely full!

Quiz

If you're having trouble, try to run a program that can help you find the answer.

1. What is the maximum number of "else if" statements you can string together?

Only one 'else if' statement per 'if' statement.
Unlimited 'else if' statements per 'if' statement- string together as many as you want.
Up to ten 'else if' statements per 'if' statement.

2. When does an 'else if' statement get run?

Every time the loop runs.
Only when the 'if' statement it is paired with is false.
Only when the 'if' statement it is paired with is true.

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