EMS1EP Lecture 5Digital Outputs

Dr. Robert Ross

Overview (what you should learn today)

• Hardware Connections• What the pins on the LArduino board do• Setting pin directions• digitalWrite()

Hardware

• Microcontrollers need to interact with the real world (sensing, computing and outputting)

• Today we look at how the microcontroller can control or output data

• There are lots of things we might want to control:– LED– Motor– Servo– Linear Actuator– Relay– Speaker

Connecting hardware to the microcontroller

• The pins on the microcontroller can only supply a small amount of current (<20mA)

• Indicator LEDs are relatively low current (~5-15mA) and so they can be driven directly – larger current devices need another way to drive them

Driving LEDs from an Arduino

Datasheet gives current and voltage for LED

For example:• Vf = 1.9V

• If = 10mA

Use KVL and ohms law to find the value of R

Driving LEDs from an Arduino

• Vf = 1.9V

• If = 10mA

• If Vcc = 5V• R = 310Ω

f

f

i

VVccR

Class Quiz

• As Vcc, Vf and If may change depending on the circuit, what current limiting resistor is needed for the following:

• Vcc = 3.3V• Vf = 2.1V

• If = 15mA

f

f

i

VVccR

Driving higher currents

• Use a transistor as a switch

• Transistor has small base current but much larger collector current – so it can switch a higher load

• For even higher loads use FETs as they have a lower on-resistance

Pins on the LArduino

Select between USB supplied power and power on V+ pin

Reset switch

Analog input pins for ADC

GND, 5V and V+

Digital I/O PinsPins with a ‘P’ can be used for pulse width modulation

Driving a LED with the LArduino

• Use 5V pin as Vcc• Use R = 330Ω Resistors• Set power to USB

Driving a LED with the LArduino// PIN ASSIGNMENTSint ledPin = 10; // LED connected to digital pin10

// The setup() method runs once on startupvoid setup() {

// initialize pin as output:pinMode(ledPin, OUTPUT);

}

// Infinite loopvoid loop() { digitalWrite(ledPin, HIGH); // LED OFF delay(1000); // Wait 1S

digitalWrite(ledPin, LOW); // LED ON delay(1000); // Wait 1S}

Digital Inputs and Outputs

• The next two lectures will focus on the digital pins – the analog pins will come later

• Before using the digital pins we need to set them up as either inputs or outputs

• This is called setting the direction of the pins (input direction or output direction)

• To do this we use the pinMode() function• Use pin assignments to refer to pins by name

rather than number

pinMode() function

• Syntax:pinMode(<pin_number>, <direction>)

• Normally some pins will be assigned at the top of your code

• e.g. – int LED1Pin = 10;– int switchPin = 11;

• These assignments are used for the pin number• Direction should be OUTPUT or INPUT• e.g.

– pinMode(LED1Pin, OUTPUT);– pinMode(switchPin, INPUT);

Class Quiz

• Pins 5 and 9 have LEDs connected to them• Pin 7 has a speaker connected to it• Pins 6 and 12 have switches connected to them

• Write code to assign names to these pins and then code to set the direction of these pins inside the setup() function

digitalWrite()

• After the direction of the pins has been setup the pins can either be set to high (5V) or low (0V)

• digitalWrite() is used to set the value of each pin• Syntax:– digitalWrite(<Pin_Number>, <HIGH/LOW>);

• e.g. //Sets pin to 5V – LED off digitalWrite(LED1Pin, HIGH);//Sets pin to 0V – LED on digitalWrite(LED1Pin, LOW);

(0 and 1 can also be used to assign low and high respectively)

Class Quiz

• Use the digitalWrite() function to do the following:– Set the pin LED1Pin to 5V– Set the pin Buzzer1Pin to a logical low– Set the pin LED3Pin to High– Turn on the LED at LED2Pin (the LED is in a sinking

arrangement as shown earlier)

Class Challenge

• Write a full sketch to alternately flash two LEDs (like a railway crossing)

• LED flash period should be 1.5 seconds• LEDs on pins 10 and 12• Write:– Pin assignments– Setup Function – Loop function

Summary(What you learnt in this session)

• How to interface an LED with an Arduino

• What the different pins on the LArduino do

• Setting up pins and inputs and outputs

• Setting pins high and low