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.