Design a temperature warning system.

This project combines the thermistor voltage divider, Serial Monitor, variables, comparisons and several LED outputs. The focus is the design process: turn a brief into a circuit, plan the logic, build it, code it and prove that it works.

The customer brief

Design a simple temperature warning unit for an equipment enclosure. A green lamp should show normal conditions, amber should warn that the enclosure is becoming warm, and red should show an over-temperature condition.

Requirements

Use a thermistor in a manual voltage divider.
Show one clear status at a time: green, amber or red.
Print the raw sensor value to Serial Monitor while calibrating.
Allow threshold values to be edited in the sketch.
Use straightforward Level 1 code only.

▶ YOUTUBE DESIGN WALKTHROUGHSuggested title: “Arduino Temperature Warning System — Design, Build, Code and Test”

1. Identify the input and outputs

10 kΩ NTC thermistor divider→Arduino A0→Compare with limits→Green / Amber / Red LEDs

HOT

WARM

NORMAL

2. System plan diagram

Temperature→Thermistor resistance changes→Divider voltage changes→A0 reading→Threshold decision→Status LED

3. Circuit diagram

Direction check: with the NTC at the top of the divider as shown, warming the thermistor should increase the A0 reading. Verify this first. If you wire the divider the opposite way, your readings will move in the opposite direction and your comparison signs must change.

4. Breadboard build plan

Thermistor divider
5V → 10 kΩ NTC → A0 junction → 10 kΩ → GND

Green LED
Pin 8 → resistor → LED → GND

Amber LED
Pin 9 → resistor → LED → GND

Red LED
Pin 10 → resistor → LED → GND

Keep all LED cathodes on the common ground rail. Before adding the decision logic, upload a tiny sketch that prints A0 and warm the thermistor gently between your fingers. Record a few typical readings.

5. Pseudocode

START
Set green, amber and red pins as outputs
Start Serial Monitor
Set two threshold values

REPEAT FOREVER
Read thermistor value
Print value

IF value is below warm limit
  Green ON, amber OFF, red OFF
ELSE IF value is below hot limit
  Green OFF, amber ON, red OFF
ELSE
  Green OFF, amber OFF, red ON
END IF
Wait briefly
REPEAT

6. Arduino code

const int thermistorPin = A0;
const int greenLed = 8;
const int amberLed = 9;
const int redLed = 10;

int warmLimit = 500;
int hotLimit = 700;

void setup() {
  pinMode(greenLed, OUTPUT);
  pinMode(amberLed, OUTPUT);
  pinMode(redLed, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int temperatureValue = analogRead(thermistorPin);
  Serial.println(temperatureValue);

  if (temperatureValue < warmLimit) {
    digitalWrite(greenLed, HIGH);
    digitalWrite(amberLed, LOW);
    digitalWrite(redLed, LOW);
  }
  else if (temperatureValue < hotLimit) {
    digitalWrite(greenLed, LOW);
    digitalWrite(amberLed, HIGH);
    digitalWrite(redLed, LOW);
  }
  else {
    digitalWrite(greenLed, LOW);
    digitalWrite(amberLed, LOW);
    digitalWrite(redLed, HIGH);
  }

  delay(200);
}

Important: the example threshold numbers are not universal temperatures. They are raw ADC values used to teach the system. Calibrate your own warm and hot limits by observing Serial Monitor readings. Converting a thermistor reading into degrees Celsius can be taught later once the learner understands the basic divider and decision process.

7. Test plan

Test	Input / condition	Expected result
1	Sensor value below warmLimit	Green only
2	Sensor value exactly at warmLimit	Amber only
3	Value between warmLimit and hotLimit	Amber only
4	Sensor value exactly at hotLimit	Red only
5	Sensor value above hotLimit	Red only
6	Warm thermistor gradually	Status progresses green → amber → red
7	Allow thermistor to cool	Status returns red → amber → green

8. Evaluation questions

Were the chosen warm and hot limits sensible?
Did only one LED illuminate at a time?
Were the status changes repeatable?
What happens if the thermistor is disconnected?
Would a real product need an alarm, display or fail-safe state?