Automatic Siren

This program creates a siren that automatically rises and falls according to your code. To control the noise, you'll use a for loop- a counting loop within the void loop.

Code

/* * Play a tone ascending, then descending, on a speaker using 'for' loops */ void setup() { pinMode(A5, OUTPUT); //speaker } void loop() { //starting at 40, increase pitch by 1 until it equals 1999 for (int pitch = 40; pitch < 2000; pitch++) { tone(A5, pitch); //play the pitch delay(5); //time before the next pitch is played } //starting at 2000, decrease pitch by 1 until it equals 41 for (int pitch = 2000; pitch > 40; pitch--) { tone(A5, pitch); //play the pitch delay(5); } } // (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!

Change the second 'for' loop to start with (int pitch = 1000... to see how the 'for' loops are working together.

Change the variable 'pitch' to 'note' in all four places of the first 'for' loop. See that the 'for' loops are using two different variables; the name doesn't matter.

Change one of the 'for' loops to delay(0). Upload that experiment.
Now change the other delay(5) to delay(0); .

Try to add two LED lights- one that is on when the tone is rising and one that is on when the tone is falling.
You'll need pinMode and digitalWrite commands.

Adjust the delays and tones to get the perfect police siren sound.

Videos

Read the Code Walkthrough Text (click to open)

This program requires only a speaker in the setup.

Inside the void loop(), there are two more loops: for loops.

The first for loop creates a variable called pitch and gives it a value of 40. This variable was created inside the for loop and it exists only inside this for loop. After a semicolon, there is a condition: pitch < 2000; . As long as this condition is true, the for loop continues to run. The third condition of the for loop is "what should happen to the variable each time the for loop runs?". In this case, the pitch will increase by 1 each time the for loop runs.

After the opening curly brace are the actions that occur each time the for loop runs. In this case, a tone of 40 hertz is played and there is a delay of 5 milliseconds. At the closing curly brace, the code checks the for loop again. "Is the pitch less than 2000?" Yes, so the pitch variable is increased by one and the for loop runs again.

The for loop continues without checking any code around it until completes the condition of the for loop. This for loop will run 1,959 times (1999 - 40) and should take about 9.8 seconds (1,959 * 5 millisecond delays). When the for loop is complete, the computer goes down to the next line of code and runs it.

The second for loop picks up right where the first left off.

This for loop follows the same pattern as the first: it creates a variable pitch and gives it the condition that as long as the pitch is greater than 40, run the for loop and subtract one from pitch.

It is important to understand that this variable pitch is not the same variable as the variable pitch in the previous for loop. These variables exist only in the for loop.

To see an example of this, you can type tone(A5, pitch); at the very end of void loop, before the closing curly brace. The code won't run! This is because of the idea of scope discussed earlier- some parts of a program cannot interact with each other.

These mini-loops are a great way to do something a set number of times and then stop, but remember that once you start a for loop, it will run until it stops, not checking any other statements or code while it runs.

Concepts

New Concept: For Loops

The for loop repeats some code a set number of times, then completes and exits. Every for loop has three parts: a variable with a value, a condition based on that variable, and a way to modify that variable each time the for loop runs.

Think of the for loop as answering these questions: "What is my starting point?"- the variable. "How do I know I'm finished?"- the conditional. "How do I get there?"- the modifier statement. Finally, "What do I do each time I run?"- the code that repeats each time the for loop runs.

Take for example a tone that should increase from 0 to 500 hertz, one hertz at a time, then stop increasing. You need to start the tone at 0 by making a variable with a value of 0. You need to set a condition so that as long as the variable is less than 501, the variable increases in value. And finally, you need to define that the variable should increase by one (not two or ten) as it ascends.

The for loop only modifies the variable and keeps track of it. In this example, you still need to include the tone commands and the delays between tones. You could also include an LED blink or any other code. The for loop's job is to repeat from the starting point you gave it to the ending point you gave it and follow the rule for how to change along the way.

Keep in mind that once the for loop starts, it will repeat without checking other code around it. For example, if you have an if statement that is supposed to stop the for loop from running, that if statement has to be inside the for loop. Otherwise, it won't be checked by the computer until the for loop has ended.

Quiz

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

1. Which of the following is not a part of creating a'for' loop?

Variable with starting value.
Condition to test each time the 'for' loop runs.
Code to modify the variable each time the for loop runs.
Empty parentheses ().

2. How many times will a 'for' loop run?

An infinite number of times.
Until it satisfies the condition you give it as one of the arguments.
100 times.
One time.

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