Digital In Out

In this lab, you'll connect a digital input circuit and a digital output circuit to a microcontroller. Though this is written for the Arduino microcontroller module, the principles apply to any microcontroller.

for this lab you'll need:

Prepare the breadboard

Conect power and ground on the breadboard to power and ground from the microcontroller. On the Arduino module, use the 5V and any of the ground connections:

If you're using an Arduino breadboard shield, there is a row of sockets connected to 5V on the analog in side of the breadboard, and a row connected to ground on the digital in side of the board:

Add a Digital Input (a switch)

Connect a switch to digital input 2 on the Arduino. The switch shown below is a store-bought momentary pushbutton, but you can use any switch. Try making your own with a couple of pieces of metal.

Breadboard version	Shield version

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Add Digital Outputs (LEDs)

Connect a 220-ohm resistor and an LED in series to digital pin 3 and another to digital pin 4 of the Arduino.

Breadboard version	Shield version

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Program the Arduino

Connect the microcontroller to your computer via USB. Assuming you've installed the Arduino software environment and the USB-to-serial drivers correctly, you'll find a new serial port in the Tools-->Serial Port menu. in OSX, the name will begin with /dev/tty.usbserial-. In Windows it will start with COM like all the other serial ports.

Here's a program that reads the digital input on pin 2. Then it turns on the LED on pin 3 if the input is high (i.e. the switch is on), or turns on the LED on pin 4 is the input is low (the switch is off):

// declare variables:
int switchPin = 2;      //  digital input pin for a switch
int yellowLedPin = 3;   //  digital output pin for an LED
int redLedPin = 4;      //  digital output pin for an LED
int switchState = 0;    // the state of the switch

void setup() {
  pinMode(switchPin, INPUT);       // set the switch pin to be an input
  pinMode(yellowLedPin, OUTPUT);   // set the yellow LED pin to be an output
  pinMode(redLedPin, OUTPUT);      // set the red LED pin to be an output
}

void loop() {
  // read the switch input:
  switchState = digitalRead(switchPin);

  if (switchState == 1) {
    // if the switch is closed:
    digitalWrite(yellowLedPin, HIGH);    // turn on the yellow LED
    digitalWrite(redLedPin, LOW);       // turn off the red LED
  } 
  else {
    // if the switch is open:
    digitalWrite(yellowLedPin, LOW);   // turn off the yellow LED
    digitalWrite(redLedPin, HIGH);     // turn on the red LED
  }
}

Click the Verify button to compile this code. Then press the reset button on the Arduino module and click the Upload button to upload the program to the module. After a few seconds, the following message will appear in the message pane to tell you the program was uploaded successfully.

Binary sketch size: 5522 bytes (of a 7168 byte maximum)

Atmel AVR ATmega8 is found.
Uploading: flash
Firmware Version: 1.18
Firmware Version: 1.18

Press the switch and watch the LEDs change until you get bored. That's all there is to basic digital input and output!

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Get Creative

Many projects can be made with just digital input and output. For example, a combination lock is jsut a series of switches that have been switched in a particular sequence. Consider the cymbal-playing monkey below:

His cymbals can be turned into a switch by lining them with tin foil and screwing wires to them:

Those wires can be run to a breadboard and used as a switch. Then the microcontroller could be programmed to listen for pattern of cymbal crashes, and if it sees that pattern, to open a lock by switching on a digital output.

Come up with your own physical interface for a combination lock.