Energia Reference - Tutorials

Examples > Control Structures

Switch (case) Statement, used with sensor input

Switch statements allow mulitple choices in which sections of code to run for a given conditional.

Hardware Required

MSP430 LaunchPad
(1) photocell, or analog sensor
(1) 10k ohm resistors
breadboard
hook-up wire

Relevant Groundwork

An if statement allows you to choose between two discrete options, TRUE or FALSE. When there are more than two options, you can use multiple if statements, or you can use the switch statement. Switch allows you to choose between several discrete options. This tutorial shows you how to use it to switch between four desired states of a photo resistor: really dark, dim, medium, and bright.

This program first reads the photoresistor. Then it uses the map() function to map its output to one of four values: 0, 1, 2, or 3. Finally, it uses the switch() statement to print one of four messages back to the computer depending on which of the four values is returned.

The photoresistor is connected to analog in pin A3 using a voltage divider circuit. A 10Kilohm resistor makes up the other side of the voltage divider, running from analog in A3 to ground. The analogRead() function returns a range of about 0 to 600 from this circuit in a reasonably lit indoor space.

Circuit

image developed using Fritzing. For more circuit examples, see the Fritzing project page

Schematic:

Code Explanation

None.

Code

/*
  Switch statement
 
  Demonstrates the use of a switch statement.  The switch
  statement allows you to choose from among a set of discrete values
  of a variable.  It's like a series of if statements.
 
  To see this sketch in action, but the board and sensor in a well-lit
  room, open the serial monitor, and and move your hand gradually
  down over the sensor.
 
  The circuit:
  * photoresistor from analog in 0 to +3V
  * 10K resistor from analog in 0 to ground
 
  created 1 Jul 2009
  by Tom Igoe 
  modified 30 Apr 2013
  by Adrian Fernandez 
 
  This example code is in the public domain.
 
*/

// these constants won't change. They are the
// lowest and highest readings you get from your sensor:
const int sensorMin = 0;      // sensor minimum, discovered through experiment
const int sensorMax = 600;    // sensor maximum, discovered through experiment

void setup() {
  // initialize serial communication:
  Serial.begin(9600);  
}

void loop() {
  // read the sensor:
  int sensorReading = analogRead(A3);
  // map the sensor range to a range of four options:
  int range = map(sensorReading, sensorMin, sensorMax, 0, 3);

  // do something different depending on the 
  // range value:
  switch (range) {
  case 0:    // your hand is on the sensor
    Serial.println("dark");
    break;
  case 1:    // your hand is close to the sensor
    Serial.println("dim");
    break;
  case 2:    // your hand is a few inches from the sensor
    Serial.println("medium");
    break;
  case 3:    // your hand is nowhere near the sensor
    Serial.println("bright");
    break;
  } 
  delay(1);        // delay in between reads for stability
}

Working Video

(Insert Video Here)

Try it out:

- use a switch case to integrate with a button