Using sensor libraries in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Arduino sensor reading code?

Consider this Arduino sketch that reads a temperature sensor value and prints it. What will be printed on the Serial Monitor?

Arduino

int sensorPin = A0;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int sensorValue = analogRead(sensorPin);
  float voltage = sensorValue * (5.0 / 1023.0);
  float temperatureC = (voltage - 0.5) * 100;
  Serial.println(temperatureC);
  delay(1000);
}

A A raw sensor value between 0 and 1023 printed

B A voltage value between 0 and 5 printed directly

C A constant value 0 printed every second

D A temperature value in Celsius around 200.0 if sensorValue is about 512

💡 Hint

Think about how analogRead maps sensor values to voltage and then to temperature.

The code converts the analog sensor reading to voltage, then to temperature in Celsius using the sensor's formula. If sensorValue is about 512, voltage is about 2.5V, so temperatureC is about (2.5 - 0.5)*100 = 200°C, but typical sensor outputs vary. The code prints temperatureC, not raw values or voltage.

🧠 Conceptual

intermediate

Which library function initializes the sensor correctly?

You want to use a DHT11 temperature and humidity sensor with the DHT library. Which function call correctly initializes the sensor in your Arduino sketch?

A DHT dht(DHT11, 2);

B DHT dht(2, DHT11);

C DHT dht(11, 2);

D DHT dht(2);

💡 Hint

Check the order of parameters: pin number first, then sensor type.

The DHT library constructor takes the pin number first, then the sensor type constant. So DHT dht(2, DHT11); means sensor on pin 2 of type DHT11.

🔧 Debug

advanced

Why does this sensor reading code fail to compile?

This Arduino code uses the Adafruit_Sensor library but fails to compile. What is the cause?

Arduino

#include <Adafruit_Sensor.h>
#include <DHT.h>
#define DHTPIN 4
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);
void setup() {
  Serial.begin(9600);
  dht.begin();
}
void loop() {
  float h = dht.readHumidity();
  float t = dht.readTemperature();
  Serial.print("Humidity: ");
  Serial.println(h);
  Serial.print("Temperature: ");
  Serial.println(t);
  delay(2000);
}

A Serial.begin(9600); must be called after dht.begin();

B DHT dht(DHTPIN, DHTTYPE); should be DHT dht(DHTTYPE, DHTPIN);

C Missing #include <DHT_U.h> which is required for Adafruit_Sensor compatibility

D The delay(2000); is too long and causes a timeout error

💡 Hint

Check if all required library headers are included for sensor compatibility.

The Adafruit_Sensor library requires including DHT_U.h for the DHT sensor to work properly. Without it, the code may fail to compile due to missing definitions.

📝 Syntax

advanced

Which option correctly reads and prints sensor data using a library?

Given a sensor library with a read() method that returns a float, which code snippet correctly reads and prints the sensor value?

A float val = sensor.read(); Serial.println(val);

B float val = sensor.read; Serial.println(val);

C float val = sensor.read(); Serial.print(val); Serial.println();

D float val = sensor.read; Serial.print(val);

💡 Hint

Remember to call functions with parentheses () to execute them.

Option A correctly calls the read() method with parentheses and prints the value. Option A and D miss parentheses causing errors or wrong values. Option A prints value but adds an extra println() which is valid but not asked.

🚀 Application

expert

How many sensor readings are stored in the array after this code runs?

This Arduino code reads a sensor 5 times and stores values in an array. How many valid readings does the array contain after the loop?

Arduino

float readings[5];
int i = 0;
void setup() {
  Serial.begin(9600);
  while(i < 5) {
    readings[i] = analogRead(A0) * (5.0 / 1023.0);
    if(readings[i] > 0) {
      i++;
    }
  }
}
void loop() {}

A 5 readings, because the loop increments i only when reading is positive

B 0 readings, because analogRead returns 0 and i never increments

C Infinite loop, because i never increments if readings[i] is 0

D Less than 5 readings, because some readings might be zero and skipped

💡 Hint

Think about how the loop increments i only when readings[i] > 0.

The loop continues until i reaches 5. It only increments i when the reading is positive. Since analogRead returns values from 0 to 1023, and multiplied by (5.0/1023.0), readings are between 0 and 5 volts. Zero is rare but possible. The loop waits until 5 positive readings are stored.

Practice

1. What is the first step when using a sensor library in an Arduino sketch?

easy

A. Include the sensor library at the top of the sketch

B. Read sensor values in the loop() function

C. Create a sensor object inside the loop()

D. Start the sensor in the loop() function

Solution

1. Step 1: Understand the library usage

   Before using a sensor, you must include its library so Arduino knows how to communicate with it.

2. Step 2: Identify correct placement

   Including the library is done at the top of the sketch, before setup() and loop().

3. Final Answer:

   Include the sensor library at the top of the sketch -> Option A

4. Quick Check:

   Include library first = A [OK]

Hint: Always include libraries before setup() [OK]

Common Mistakes:

• Trying to create sensor object before including library
• Starting sensor in loop() instead of setup()
• Reading sensor before initializing it

2. Which of the following is the correct way to create a sensor object for a DHT sensor using its library?

easy

A. DHT sensor = DHT(2);

B. DHT sensor = new DHT(2, DHT11);

C. DHT sensor(2, DHT11);

D. DHT sensor = DHT11(2);

Solution

1. Step 1: Recall object creation syntax in Arduino C++

   Objects are created by calling the constructor directly, like ClassName objectName(parameters); without '=' or 'new' keyword.

2. Step 2: Match correct syntax for DHT library

   The DHT library uses: DHT sensor(pin, type); so DHT sensor(2, DHT11); matches this exactly.

3. Final Answer:

   DHT sensor(2, DHT11); -> Option C

4. Quick Check:

   Object creation = ClassName object(params); [OK]

Hint: Create objects without '=' or 'new' in Arduino [OK]

Common Mistakes:

• Using '=' sign when creating objects
• Using 'new' keyword which is not typical in Arduino sketches
• Confusing sensor type and pin order

3. Given the code below, what will be printed if the sensor reading fails?

#include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

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

void loop() {
  float h = dht.readHumidity();
  if (isnan(h)) {
    Serial.println("Failed to read humidity!");
  } else {
    Serial.print("Humidity: ");
    Serial.println(h);
  }
  delay(2000);
}

medium

A. Failed to read humidity!

B. Humidity: 0.00

C. Humidity: NaN

D. No output

Solution

1. Step 1: Understand the isnan() check

   The code checks if the humidity reading is 'NaN' (not a number), which means the sensor failed to read.

2. Step 2: Determine output when reading fails

   If isnan(h) is true, the code prints "Failed to read humidity!" to Serial.

3. Final Answer:

   Failed to read humidity! -> Option A

4. Quick Check:

   isnan true prints error message [OK]

Hint: Check isnan() to detect sensor read failure [OK]

Common Mistakes:

• Assuming sensor returns 0 on failure
• Printing raw value without checking for NaN
• Not calling dht.begin() in setup()

4. Identify the error in this Arduino sketch snippet using a sensor library:

#include <DHT.h>
DHT dht(2, DHT11);

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

void loop() {
  dht.begin();
  float t = dht.readTemperature();
  Serial.println(t);
  delay(1000);
}

medium

A. readTemperature() returns int, not float

B. dht.begin() should be called in setup(), not loop()

C. Missing #define for sensor pin

D. Serial.begin() should be called in loop()

Solution

1. Step 1: Locate sensor initialization

   dht.begin() initializes the sensor and should be called once in setup() to avoid repeated initialization.

2. Step 2: Identify why calling dht.begin() in loop() is wrong

   Calling dht.begin() repeatedly in loop() can cause sensor communication issues and slow down the program.

3. Final Answer:

   dht.begin() should be called in setup(), not loop() -> Option B

4. Quick Check:

   Initialize sensor once in setup() [OK]

Hint: Initialize sensors once in setup() [OK]

Common Mistakes:

• Calling sensor start functions repeatedly in loop()
• Confusing Serial.begin() placement
• Assuming readTemperature() returns int

5. You want to read temperature and humidity from a DHT11 sensor and turn on an LED if humidity is above 70%. Which code snippet correctly uses the sensor library and controls the LED?

hard

A. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (!isnan(h) || h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

B. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

C. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (h < 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

D. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (!isnan(h) && h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

Solution

1. Step 1: Check sensor initialization and pin setup

   #include <DHT.h>
   #define DHTPIN 2
   #define DHTTYPE DHT11
   #define LEDPIN 13
   DHT dht(DHTPIN, DHTTYPE);
   
   void setup() {
     Serial.begin(9600);
     dht.begin();
     pinMode(LEDPIN, OUTPUT);
   }
   
   void loop() {
     float h = dht.readHumidity();
     if (!isnan(h) && h > 70) {
       digitalWrite(LEDPIN, HIGH);
     } else {
       digitalWrite(LEDPIN, LOW);
     }
     delay(2000);
   }
   

   correctly calls dht.begin() in setup() and sets LED pin as output.

2. Step 2: Verify humidity check and LED control logic

   #include <DHT.h>
   #define DHTPIN 2
   #define DHTTYPE DHT11
   #define LEDPIN 13
   DHT dht(DHTPIN, DHTTYPE);
   
   void setup() {
     Serial.begin(9600);
     dht.begin();
     pinMode(LEDPIN, OUTPUT);
   }
   
   void loop() {
     float h = dht.readHumidity();
     if (!isnan(h) && h > 70) {
       digitalWrite(LEDPIN, HIGH);
     } else {
       digitalWrite(LEDPIN, LOW);
     }
     delay(2000);
   }
   

   checks if humidity is not NaN and greater than 70 before turning LED on, which prevents errors from failed reads.

3. Final Answer:

   The code snippet with !isnan(h) check and && h > 70 -> Option D

4. Quick Check:

   Initialize sensor + check isnan + control LED = A [OK]

Hint: Always check isnan() before using sensor values [OK]

Common Mistakes:

• Not calling dht.begin() in setup()
• Missing isnan() check causing wrong LED behavior
• Using || instead of && in humidity condition