Humidity and temperature (DHT11/DHT22) in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading temperature from DHT11 sensor

What will be the output on the Serial Monitor when this Arduino code runs with a DHT11 sensor connected correctly?

Arduino

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

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

void loop() {
  float temp = dht.readTemperature();
  if (isnan(temp)) {
    Serial.println("Failed to read from DHT sensor!");
  } else {
    Serial.print("Temperature: ");
    Serial.print(temp);
    Serial.println(" *C");
  }
  delay(2000);
}

A Failed to read from DHT sensor!

B Temperature: 23.00 *C

C Temperature: 23 *F

D Error: DHT library not found

💡 Hint

The code reads temperature in Celsius and prints it if the sensor reading is valid.

The code uses dht.readTemperature() which returns temperature in Celsius. If the sensor reading fails, it prints an error message. Assuming the sensor works and temperature is 23°C, option B matches the output.

❓ Predict Output

intermediate

Humidity reading with DHT22 sensor

What will be printed on the Serial Monitor if the humidity reading is 55.5% and the sensor works correctly?

Arduino

#include <DHT.h>
#define DHTPIN 4
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

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

void loop() {
  float humidity = dht.readHumidity();
  if (isnan(humidity)) {
    Serial.println("Error reading humidity");
  } else {
    Serial.print("Humidity: ");
    Serial.print(humidity);
    Serial.println(" %");
  }
  delay(1500);
}

A Humidity: 55.50 %

B Humidity: 55.5

C Error reading humidity

D Humidity: 55 %

💡 Hint

The code prints humidity with two decimals and a percent sign if reading is valid.

The dht.readHumidity() returns a float with decimal precision. The code prints the value followed by a space and percent sign. Option A matches exactly.

🔧 Debug

advanced

Identify the error in DHT sensor initialization

What error will this Arduino code produce when compiling or running?

Arduino

#include <DHT.h>
#define DHTPIN 5
#define DHTTYPE DHT11
DHT dht(DHTPIN, "11");

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

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

A Runs correctly and prints temperature

B Runtime error: sensor not found

C Compilation error: invalid argument for DHT constructor

D Compilation error: missing semicolon

💡 Hint

Check the second argument passed to the DHT constructor.

The DHT constructor expects a constant like DHT11 or DHT22, not a string like "11". Passing "11" causes a compilation error due to invalid argument type.

📝 Syntax

advanced

Find the syntax error in this DHT sensor reading code

Which option correctly fixes the syntax error in this Arduino code snippet?

Arduino

#include <DHT.h>
#define DHTPIN 3
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

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

void loop() {
  float h = dht.readHumidity()
  if (isnan(h)) {
    Serial.println("Error");
  } else {
    Serial.println(h);
  }
  delay(2000);
}

A Add a semicolon after 'float h = dht.readHumidity()'

B Change 'isnan(h)' to 'isNan(h)'

C Remove the parentheses from 'dht.readHumidity'

D Replace 'Serial.println(h);' with 'Serial.print(h);'

💡 Hint

Look carefully at the end of the line where the variable h is assigned.

The line 'float h = dht.readHumidity()' is missing a semicolon at the end, causing a syntax error. Adding the semicolon fixes it.

🚀 Application

expert

Calculate heat index from DHT22 readings

Given temperature in Celsius and humidity from a DHT22 sensor, which code snippet correctly calculates and prints the heat index in Celsius?

Arduino

#include <DHT.h>
#define DHTPIN 6
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

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

void loop() {
  float t = dht.readTemperature();
  float h = dht.readHumidity();
  if (isnan(t) || isnan(h)) {
    Serial.println("Sensor error");
    delay(2000);
    return;
  }
  // Calculate heat index here
  float hi;
  // ???
  Serial.print("Heat index: ");
  Serial.print(hi);
  Serial.println(" *C");
  delay(3000);
}

A hi = t * h / 100;

B hi = 0.5 * (t + 61.0 + ((t-68.0)*1.2) + (h*0.094)); hi = (hi + t) / 2.0;

C hi = t + (0.33 * h) - (0.7 * windSpeed) - 4.0; // windSpeed undefined

D hi = -8.784695 + 1.61139411*t + 2.338549*h - 0.14611605*t*h - 0.012308094*t*t - 0.016424828*h*h + 0.002211732*t*t*h + 0.00072546*t*h*h - 0.000003582*t*t*h*h;

💡 Hint

Use the full formula for heat index in Celsius involving temperature and humidity.

Option D uses the full Rothfusz regression formula adapted for Celsius to calculate heat index accurately. Option D is a simplified formula for Fahrenheit. Option D uses undefined variable. Option D is a wrong formula.