Reading temperature sensor (LM35, TMP36) in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Arduino code reading LM35?

Given the following Arduino code snippet that reads an LM35 sensor connected to analog pin A0, what will be the printed temperature in Celsius if the analogRead returns 512?

Arduino

int sensorPin = A0;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int reading = analogRead(sensorPin);
  float voltage = reading * (5.0 / 1023.0);
  float temperatureC = voltage * 100.0;
  Serial.println(temperatureC);
  delay(1000);
}

A 512.0

B 2.5

C 250.0

D 25.1

💡 Hint

Recall that LM35 outputs 10mV per degree Celsius and analogRead returns values from 0 to 1023 for 0-5V.

The analog reading 512 corresponds to voltage ≈ 512 * (5.0 / 1023.0) ≈ 2.50 V. The code calculates temperatureC = voltage * 100.0 ≈ 250.0 °C, which is printed.

❓ Predict Output

intermediate

What temperature does this TMP36 reading code print?

Consider this Arduino code reading a TMP36 sensor on analog pin A1. If analogRead returns 400, what temperature in Celsius will be printed?

Arduino

int sensorPin = A1;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int reading = analogRead(sensorPin);
  float voltage = reading * (5.0 / 1023.0);
  float temperatureC = (voltage - 0.5) * 100.0;
  Serial.println(temperatureC);
  delay(1000);
}

A 0.0

B 40.0

C 35.0

D 145.5

💡 Hint

TMP36 outputs 0.5V at 0°C and 10mV per degree Celsius above that.

For reading=400, voltage ≈ 400 * (5.0 / 1023.0) ≈ 1.955 V. temperatureC = (1.955 - 0.5) * 100.0 ≈ 145.5 °C, which is printed.

🔧 Debug

advanced

Why does this LM35 reading code print wrong temperature?

This Arduino code reads LM35 sensor but prints wrong temperature values. What is the main bug?

Arduino

int sensorPin = A0;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int reading = analogRead(sensorPin);
  float voltage = reading / 1023.0 * 5.0;
  float temperatureC = voltage * 10.0;
  Serial.println(temperatureC);
  delay(1000);
}

A The temperature calculation uses wrong multiplier; should be 100.0 not 10.0.

B The voltage calculation uses integer division causing zero voltage.

C The analogRead pin is not initialized in setup.

D Serial.begin baud rate is too low for correct printing.

💡 Hint

Check the LM35 sensor output voltage to temperature conversion factor.

LM35 outputs 10mV per degree Celsius, so temperatureC = voltage * 100.0, not 10.0. Using 10.0 underestimates temperature by factor 10.

📝 Syntax

advanced

Which option fixes the syntax error in this TMP36 reading code?

Identify the option that corrects the syntax error in this Arduino code snippet:

Arduino

int sensorPin = A1
void setup() {
  Serial.begin(9600);
}
void loop() {
  int reading = analogRead(sensorPin);
  float voltage = reading * (5.0 / 1023.0);
  float temperatureC = (voltage - 0.5) * 100.0;
  Serial.println(temperatureC);
  delay(1000);
}

A Change 'void setup()' to 'void setup(void)'

B Add semicolon after 'int sensorPin = A1;'

C Replace 'Serial.begin(9600)' with 'Serial.begin(9600);'

D Add curly braces around analogRead line

💡 Hint

Check for missing semicolons in variable declarations.

The line 'int sensorPin = A1' is missing a semicolon at the end, causing a syntax error.

🚀 Application

expert

How to convert TMP36 voltage to Fahrenheit in Arduino?

You read a TMP36 sensor voltage on analog pin A2. Which code snippet correctly converts the voltage to temperature in Fahrenheit?

A

float voltage = analogRead(A2) * (5.0 / 1023.0);
float tempC = (voltage - 0.5) * 100.0;
float tempF = tempC * 9.0 / 5.0 + 32.0;

B

float voltage = analogRead(A2) / 1023.0 * 5.0;
float tempF = (voltage - 0.5) * 100.0 * 9.0 / 5.0 + 32.0;

C

float voltage = analogRead(A2) * (5.0 / 1023.0);
float tempF = (voltage - 0.5) * 100.0 * 1.8 + 32.0;

D

float voltage = analogRead(A2) * (5.0 / 1023.0);
float tempF = (voltage - 0.5) * 100.0 + 32.0 * 9.0 / 5.0;

💡 Hint

First convert voltage to Celsius, then convert Celsius to Fahrenheit using F = C * 9/5 + 32.

Option A correctly calculates voltage, then Celsius temperature, then converts Celsius to Fahrenheit with the right formula.

Practice

1. What does the analogRead() function do when reading from an LM35 temperature sensor?

easy

A. It sets the sensor's output voltage to a fixed value.

B. It converts the temperature directly to Celsius.

C. It sends data to the sensor to start measuring temperature.

D. It reads the voltage level from the sensor's analog output pin.

Solution

1. Step 1: Understand analogRead() function

   The analogRead() function reads the voltage level on an analog pin and returns a number between 0 and 1023 representing that voltage.

2. Step 2: Relate to LM35 sensor output

   The LM35 outputs an analog voltage proportional to temperature, so analogRead() reads this voltage level.

3. Final Answer:

   It reads the voltage level from the sensor's analog output pin. -> Option D

4. Quick Check:

   analogRead() reads voltage level = A [OK]

Hint: Remember: analogRead() reads voltage, not temperature directly [OK]

Common Mistakes:

• Thinking analogRead() converts voltage to temperature
• Assuming analogRead() sends commands to sensor
• Confusing analogRead() with digitalRead()

2. Which of the following is the correct way to convert the analog reading from an LM35 sensor to voltage in Arduino code?

easy

A. float voltage = analogRead(sensorPin) / 5.0;

B. float voltage = analogRead(sensorPin) * 1023.0 / 5.0;

C. float voltage = analogRead(sensorPin) * (5.0 / 1023.0);

D. float voltage = analogRead(sensorPin) * 5.0;

Solution

1. Step 1: Understand analog to voltage conversion

   The analog reading ranges from 0 to 1023 for 0 to 5 volts. To get voltage, multiply reading by (5.0 / 1023.0).

2. Step 2: Check each option

   float voltage = analogRead(sensorPin) * (5.0 / 1023.0); correctly applies the formula. Others either divide incorrectly or multiply by wrong factors.

3. Final Answer:

   float voltage = analogRead(sensorPin) * (5.0 / 1023.0); -> Option C

4. Quick Check:

   Voltage = reading * (5/1023) [OK]

Hint: Use (5.0 / 1023.0) to convert analog reading to voltage [OK]

Common Mistakes:

• Dividing by 5 instead of multiplying
• Using 1024 instead of 1023 in denominator
• Multiplying by 5 without dividing by 1023

3. What will be the output on the Serial Monitor if the following Arduino code reads an analog value of 250 from an LM35 sensor?

int sensorPin = A0;
int reading = 250;
float voltage = reading * (5.0 / 1023.0);
float temperatureC = voltage * 100;
Serial.println(temperatureC);

medium

A. 122.0

B. 12.2

C. 0.25

D. 1.22

Solution

1. Step 1: Calculate voltage from reading

   voltage = 250 * (5.0 / 1023.0) ≈ 1.22 volts.

2. Step 2: Calculate temperature in Celsius

   temperatureC = 1.22 * 100 ≈ 122 °C. Serial.println displays approximately 122.19, closest to 122.0.

3. Final Answer:

   122.0 -> Option A

4. Quick Check:

   Voltage ≈1.22V, Temp = voltage*100 = 122 [OK]

Hint: Multiply voltage by 100 to get Celsius for LM35 [OK]

Common Mistakes:

• Forgetting to multiply voltage by 100
• Using wrong analog to voltage conversion
• Confusing TMP36 formula with LM35

4. Identify the error in this Arduino code snippet for reading TMP36 temperature sensor:

int sensorPin = A0;
int reading = analogRead(sensorPin);
float voltage = reading / 1023 * 5.0;
float temperatureC = (voltage - 0.5) * 100;
Serial.println(temperatureC);

medium

A. The voltage calculation divides before multiplying, causing integer division error.

B. The sensorPin should be declared as float, not int.

C. The temperature formula is incorrect for TMP36 sensor.

D. Serial.println() cannot print float values.

Solution

1. Step 1: Analyze voltage calculation

   reading / 1023 * 5.0 uses left-to-right precedence: first reading / 1023 (int / int = integer division, truncates), then * 5.0, yielding wrong voltage.

2. Step 2: Rule out other options

   A: Formula (voltage - 0.5)*100 correct for TMP36. B: Pin declaration int is fine. D: Serial.println prints floats fine.

3. Final Answer:

   The voltage calculation divides before multiplying, causing integer division error. -> Option A

4. Quick Check:

   Use float divisor to avoid integer division [OK]

Hint: Use float numbers in division to avoid integer division [OK]

Common Mistakes:

• Using integer 1023 instead of float 1023.0
• Misunderstanding operator precedence
• Thinking Serial.println() can't print floats

5. You want to read temperature from a TMP36 sensor connected to analog pin A1 and print the temperature in Celsius every second. Which Arduino code snippet correctly implements this?

hard

A. int sensorPin = A1;
void loop() {
int reading = analogRead(sensorPin);
float voltage = reading / 1023 * 5;
float temperatureC = voltage * 100;
Serial.println(temperatureC);
delay(1000);
}

B. int sensorPin = A1;
void loop() {
int reading = analogRead(sensorPin);
float voltage = reading * (5.0 / 1023.0);
float temperatureC = (voltage - 0.5) * 100;
Serial.println(temperatureC);
delay(1000);
}

C. int sensorPin = A1;
void loop() {
int reading = analogRead(sensorPin);
float voltage = reading * (5 / 1023);
float temperatureC = (voltage - 0.5) * 100;
Serial.println(temperatureC);
delay(1000);
}

D. int sensorPin = A1;
void loop() {
int reading = analogRead(sensorPin);
float voltage = reading * (5.0 / 1023.0);
float temperatureC = voltage * 100;
Serial.println(temperatureC);
delay(1000);
}

Solution

1. Step 1: Confirm TMP36 formula

   Temperature = (voltage - 0.5) * 100.

2. Step 2: Check voltage conversion

   Correct: reading * (5.0 / 1023.0). Avoid integer division like reading / 1023 * 5 (truncates) or 5 / 1023 (zero).

3. Step 3: Verify loop structure

   A1 pin, analogRead, Serial.println, delay(1000) must all align with correct math.

4. Final Answer:

   int sensorPin = A1; void loop() { int reading = analogRead(sensorPin); float voltage = reading * (5.0 / 1023.0); float temperatureC = (voltage - 0.5) * 100; Serial.println(temperatureC); delay(1000); } -> Option B

5. Quick Check:

   TMP36 temp = (voltage - 0.5)*100 with float math [OK]

Hint: Use (voltage - 0.5)*100 for TMP36 temperature [OK]

Common Mistakes:

• Using LM35 formula for TMP36 sensor
• Integer division in voltage calculation
• Using integer math like 5 / 1023 resulting in zero