Temperature Alarm

What You'll Build

This is a safety gadget. It watches the temperature and stays completely silent while everything is normal, but the moment things get too hot the red LED flashes and the buzzer sounds. Point it at a radiator, a greenhouse or a cup of hot chocolate.

What You Need

• ✓Arduino Uno
• ✓TMP36 or DHT11
• ✓Piezo buzzer
• ✓Red LED
• ✓220Ω resistor

How It Works

A TMP36 sensor outputs a voltage that rises steadily with temperature — 10 millivolts per degree, offset by 500 millivolts at 0 °C. The Arduino reads that voltage, converts it to degrees, and compares it with a limit. If the reading is over the limit, the alarm code runs; otherwise everything stays quiet.

Wiring

Step-by-Step Instructions

1. 1

   Wire the sensor the right way round

   A TMP36 plugged in backwards gets extremely hot within seconds. Check the flat face before powering up.

2. 2

   Convert volts to degrees

   Multiply the reading by 5.0 and divide by 1024 to get volts, then subtract 0.5 and multiply by 100 to get Celsius.

3. 3

   Check against a real thermometer

   Compare your value with a room thermometer. If it is consistently off by a degree or two, add a small correction.

4. 4

   Add the alarm

   Use tone() for the buzzer so you can choose the pitch, and flash the LED in the same rhythm.

5. 5

   Set your limit

   Pick a threshold that makes sense for your test. 30 °C is easy to reach with warm hands.

Arduino Code

const int SENSOR = A0;
const int BUZZER = 8;
const int LED = 7;
const float LIMIT = 30.0;   // degrees C

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

float readCelsius() {
  float volts = analogRead(SENSOR) * 5.0 / 1024.0;
  return (volts - 0.5) * 100.0;
}

void loop() {
  float t = readCelsius();
  Serial.println(t);

  if (t > LIMIT) {
    tone(BUZZER, 1200);
    digitalWrite(LED, HIGH);
    delay(250);
    noTone(BUZZER);
    digitalWrite(LED, LOW);
    delay(250);
  } else {
    noTone(BUZZER);
    digitalWrite(LED, LOW);
    delay(500);
  }
}

Understanding the Code

Challenges & Upgrades

Troubleshooting