Arduino LM35 Temperature Sensor with Alarm LED

hardSensorsArduino (C++)

The LM35 outputs 10 mV per °C; the Arduino reads that voltage and converts it into a temperature.

The question

An LM35 temperature sensor is on A2. Convert the reading to °C with temp = reading × 500.0 / 1024, print it at 9600 baud, and turn ON an LED on pin 7 when the temperature is above 30 °C (OFF otherwise).

The sketch

sketch.ino

void setup() {
  Serial.begin(9600);
  pinMode(7, OUTPUT);
}

void loop() {
  int reading = analogRead(A2);
  float temp = reading * 500.0 / 1024;
  Serial.println(temp);
  if (temp > 30) {
    digitalWrite(7, HIGH);
  } else {
    digitalWrite(7, LOW);
  }
  delay(1000);
}

Open the Arduino simulator »

Wiring

LM35: +Vs → 5V, GND → GND, Vout → A2. LED → 220Ω → pin 7.

How it works

1analogRead() gives 0–1023 for 0–5 V.
2temp = reading × 500.0 / 1024 converts it to °C (5 V = 500 °C at 10 mV/°C).
3Print the temperature, then turn the LED on if it is above the limit.

Common mistakes

Writing 500 / 1024 with whole numbers — the division gives 0. Use 500.0.
Connecting the LM35 back to front, which makes it heat up.

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