digitalRead() for input reading in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the digitalRead() function do in Arduino?

It reads the value from a specified digital pin, returning HIGH if the voltage is high (usually 5V) or LOW if the voltage is low (0V).

beginner

How do you use digitalRead() to check if a button is pressed?

You connect the button to a digital pin and use digitalRead(pin). If it returns LOW or HIGH depending on your wiring, you know if the button is pressed or not.

beginner

Why should you use pinMode(pin, INPUT) before using digitalRead()?

Setting the pin mode to INPUT tells Arduino to prepare the pin to read signals from sensors or buttons, ensuring digitalRead() works correctly.

beginner

What values can digitalRead() return?

It returns either HIGH or LOW, representing the voltage level on the pin.

beginner

If digitalRead(7) returns LOW, what does it mean?

It means the voltage on digital pin 7 is low (close to 0 volts), which often means a button connected to that pin is pressed or the circuit is connected to ground.

What must you do before using digitalRead() on a pin?

A Set the pin mode to INPUT using <code>pinMode(pin, INPUT)</code>

B Set the pin mode to OUTPUT using <code>pinMode(pin, OUTPUT)</code>

C Call <code>digitalWrite(pin, HIGH)</code>

D Nothing, just call <code>digitalRead()</code>

You must set the pin as INPUT to read signals correctly with digitalRead().

What does digitalRead() return when the pin voltage is high?

A HIGH

B LOW

C 1

D 0

digitalRead() returns HIGH when the voltage is high (usually 5V).

Which of these is a valid use of digitalRead()?

A Changing the baud rate

B Setting a pin to HIGH

C Reading a sensor connected to a digital pin

D Writing a value to an LED

digitalRead() reads the state of a digital pin, useful for sensors or buttons.

If a button is connected to pin 2 and digitalRead(2) returns LOW, what might that mean?

A Button is not pressed

B Button is pressed

C Pin is set as OUTPUT

D Pin is disconnected

Often, buttons connect the pin to ground when pressed, so digitalRead() returns LOW.

What type of signal does digitalRead() read?

A Serial data

B Analog voltage levels

C PWM signals

D Digital HIGH or LOW signals

digitalRead() reads digital signals that are either HIGH or LOW.

Explain how to use digitalRead() to detect if a button is pressed.

Think about how the button connects the pin to ground or voltage.

What are the possible return values of digitalRead() and what do they mean?

Consider the two states of a digital signal.

Practice

1. What does the digitalRead() function do in Arduino?

easy

A. Sets a digital pin to HIGH or LOW

B. Reads the voltage level (HIGH or LOW) from a digital input pin

C. Configures a pin as an output

D. Reads analog voltage from a pin

Solution

1. Step 1: Understand the purpose of digitalRead()

   The function digitalRead() checks the voltage level on a digital pin and returns HIGH or LOW.

2. Step 2: Differentiate from other functions

   Unlike digitalWrite() which sets pin output, digitalRead() only reads input state.

3. Final Answer:

   Reads the voltage level (HIGH or LOW) from a digital input pin -> Option B

4. Quick Check:

   digitalRead() reads pin state [OK]

Hint: digitalRead() always reads input pin state [OK]

Common Mistakes:

• Confusing digitalRead() with digitalWrite()
• Thinking digitalRead() sets pin output
• Mixing digitalRead() with analogRead()

2. Which of the following is the correct syntax to read a digital pin 7 in Arduino?

easy

A. digitalReadPin(7);

B. digitalRead = 7;

C. digitalRead(7);

D. readDigital(7);

Solution

1. Step 1: Recall the correct function syntax

   The correct function call is digitalRead(pinNumber); where pinNumber is the pin to read.

2. Step 2: Check each option

   Only digitalRead(7); matches the correct syntax digitalRead(7); others are invalid function calls or assignments.

3. Final Answer:

   digitalRead(7); -> Option C

4. Quick Check:

   Correct function call [OK]

Hint: digitalRead(pinNumber) reads pin state correctly [OK]

Common Mistakes:

• Using assignment instead of function call
• Wrong function name like readDigital()
• Adding extra words like digitalReadPin()

3. What will be the output on the Serial Monitor if the button connected to pin 2 is pressed (assuming HIGH when pressed)?

void setup() {
  pinMode(2, INPUT);
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(2);
  Serial.println(buttonState);
  delay(500);
}

medium

A. 1

B. Error: pinMode missing

C. 0

D. Random values

Solution

1. Step 1: Analyze pinMode and digitalRead usage

   Pin 2 is set as INPUT, so digitalRead(2) reads the button state correctly.

2. Step 2: Understand button press state

   When the button is pressed, the pin reads HIGH which is 1, so Serial.println prints 1 repeatedly.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Pressed button = HIGH = 1 [OK]

Hint: Pressed button reads HIGH (1) on digitalRead() [OK]

Common Mistakes:

• Assuming pressed button reads 0 instead of 1
• Forgetting to set pinMode to INPUT
• Confusing analogRead with digitalRead

4. Identify the error in this code snippet that reads a button state on pin 4:

void setup() {
  Serial.begin(9600);
}

void loop() {
  int state = digitalRead(4);
  Serial.println(state);
  delay(1000);
}

medium

A. delay() cannot be used with digitalRead()

B. digitalRead() cannot be used in loop()

C. Serial.begin() should be in loop()

D. Missing pinMode(4, INPUT) in setup()

Solution

1. Step 1: Check pin configuration

   The code does not set pin 4 as INPUT using pinMode(), which is required before reading.

2. Step 2: Verify other parts

   digitalRead() can be used in loop(), Serial.begin() must be in setup(), and delay() is allowed.

3. Final Answer:

   Missing pinMode(4, INPUT) in setup() -> Option D

4. Quick Check:

   Always set pinMode before digitalRead() [OK]

Hint: Always set pinMode(INPUT) before digitalRead() [OK]

Common Mistakes:

• Forgetting pinMode() setup
• Moving Serial.begin() to loop() incorrectly
• Thinking delay() breaks digitalRead()

5. You want to detect if a switch connected to pin 3 is pressed, but the switch is wired so it connects the pin to GND when pressed (active LOW). Which code snippet correctly reads the switch state and prints "Pressed" or "Not Pressed" accordingly? A)

pinMode(3, INPUT);
if (digitalRead(3) == HIGH) {
  Serial.println("Pressed");
} else {
  Serial.println("Not Pressed");
}

B)

pinMode(3, INPUT);
if (digitalRead(3) == LOW) {
  Serial.println("Pressed");
} else {
  Serial.println("Not Pressed");
}

C)

pinMode(3, OUTPUT);
if (digitalRead(3) == LOW) {
  Serial.println("Pressed");
} else {
  Serial.println("Not Pressed");
}

D)

pinMode(3, INPUT_PULLUP);
if (digitalRead(3) == HIGH) {
  Serial.println("Pressed");
} else {
  Serial.println("Not Pressed");
}

hard

A. Code snippet B

B. Code snippet A

C. Code snippet C

D. Code snippet D

Solution

1. Step 1: Understand active LOW switch wiring

   The switch connects pin 3 to GND when pressed, so digitalRead(3) returns LOW when pressed.

2. Step 2: Check pinMode and condition

   pinMode must be INPUT (not OUTPUT). The condition to detect press is digitalRead(3) == LOW.

3. Final Answer:

   Code snippet B -> Option A

4. Quick Check:

   Active LOW switch reads LOW when pressed [OK]

Hint: Active LOW means pressed when digitalRead() returns LOW [OK]

Common Mistakes:

• Using OUTPUT mode instead of INPUT
• Checking for HIGH instead of LOW for active LOW switch
• Using INPUT_PULLUP but checking wrong logic