Reading temperature sensor (LM35, TMP36) in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the LM35 or TMP36 sensor in Arduino projects?

They are temperature sensors that provide an analog voltage output proportional to the temperature, allowing Arduino to read and measure temperature.

beginner

How do you convert the analog reading from an LM35 sensor to temperature in Celsius?

Read the analog value (0-1023), convert it to voltage (0-5V), then multiply by 100 because LM35 outputs 10mV per °C. Formula: temperature = (analogValue * 5.0 / 1023) * 100

intermediate

What is the difference in output voltage between LM35 and TMP36 sensors?

LM35 outputs 0V at 0°C and 10mV per °C increase. TMP36 outputs 0.75V at 25°C and 10mV per °C increase, so TMP36 has an offset voltage of 0.5V at 0°C.

beginner

Write the Arduino code line to read the analog value from a temperature sensor connected to analog pin A0.

int sensorValue = analogRead(A0);

intermediate

How do you adjust the formula to convert TMP36 sensor readings to Celsius?

First convert analog reading to voltage, then subtract 0.5V offset, then multiply by 100. Formula: temperature = ((analogValue * 5.0 / 1023) - 0.5) * 100

What does the analogRead() function return when reading a temperature sensor on Arduino?

A Temperature in Celsius directly

B A value between 0 and 1023 representing voltage level

C Voltage in volts

D Digital HIGH or LOW

analogRead() returns a number from 0 to 1023 representing the voltage level on the analog pin.

Which sensor outputs 0.5V at 0°C as a baseline voltage?

A Neither LM35 nor TMP36

B LM35

C Both LM35 and TMP36

D TMP36

TMP36 outputs 0.5V at 0°C, while LM35 outputs 0V at 0°C.

If analogRead(A0) returns 512, what is the approximate voltage on the pin (assuming 5V reference)?

A 2.5V

B 5V

C 1V

D 0.5V

Voltage = (512 / 1023) * 5V ≈ 2.5V

How do you calculate temperature from LM35 sensor reading?

A Multiply voltage by 100

B Subtract 0.5V then multiply by 100

C Divide voltage by 10

D Add 0.5V then multiply by 10

LM35 outputs 10mV per °C with 0V at 0°C, so temperature = voltage * 100.

Which Arduino function reads analog voltage from a sensor?

A digitalRead()

B analogWrite()

C analogRead()

D digitalWrite()

analogRead() reads analog voltage levels from pins.

Explain how to read temperature from an LM35 sensor using Arduino, including the formula to convert the analog reading to Celsius.

Think about how the sensor voltage relates to temperature and how Arduino reads voltage.

Describe the difference in output voltage behavior between LM35 and TMP36 sensors and how that affects temperature calculation.

Focus on the baseline voltage each sensor outputs at zero degrees Celsius.

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