How to Build a Temperature-Based LED Bar with TMP36 and LCD Display

In this upgraded version of the LED temperature meter, we’ll display the current temperature in real-time on a 16×2 LCD alongside the LED bar. The LEDs light up based on temperature level, while the LCD gives you the exact number in °C.

Table of Contents

Components Required

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TMP36 Pinout and Wiring

Facing the flat side of TMP36 with the text facing you:

TMP36 Pin	Connects to	Description
1 (VCC)	5V	Power supply
2 (VOUT)	A0	Analog output to Arduino
3 (GND)	GND	Ground

LCD I2C Wiring

I2C LCD Pin	Arduino Pin
GND	GND
VCC	5V
SDA	A4
SCL	A5

LEDs: Connect to digital pins D2 through D7, each with a 220Ω resistor.

Arduino Code


#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int tempPin = A0;
const int ledPins[] = {2, 3, 4, 5, 6, 7};
const int ledCount = 6;

LiquidCrystal_I2C lcd(0x27, 16, 2); // Adjust I2C address if needed

void setup() {
  lcd.init();             // Use lcd.init() instead of lcd.begin() for Tinkercad
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Temp: --.- C");

  for (int i = 0; i < ledCount; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  int sensorValue = analogRead(tempPin);
  float voltage = sensorValue * (5.0 / 1023.0);
  float temperatureC = (voltage - 0.5) * 100.0;

  // Display temperature on LCD
  lcd.setCursor(6, 0);
  lcd.print(temperatureC, 1);
  lcd.print(" C ");

  // Map temperature to LED level (20–40°C → 0–6 LEDs)
  int level = map(temperatureC, 20, 40, 0, ledCount);
  level = constrain(level, 0, ledCount);

  for (int i = 0; i < ledCount; i++) {
    digitalWrite(ledPins[i], i < level ? HIGH : LOW);
  }

  delay(500);
}

Visual Temperature Bar with LCD Display

This enhanced version of the temperature bar adds an I2C LCD screen to display the temperature value in real-time while lighting up LEDs as a visual indicator.

Step 1: Include libraries and define pins


#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int tempPin = A0;
const int ledPins[] = {2, 3, 4, 5, 6, 7};
const int ledCount = 6;

LiquidCrystal_I2C lcd(0x27, 16, 2); // Adjust I2C address if needed

– Wire.h and LiquidCrystal_I2C.h are needed to control the I2C LCD.
– lcd(0x27, 16, 2) initializes the LCD with I2C address 0x27 and a 16×2 layout.

Step 2: Setup LCD and LED pins


void setup() {
  lcd.init();             // Use lcd.init() instead of lcd.begin()
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Temp: --.- C");

  for (int i = 0; i < ledCount; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

– Initializes the LCD and turns on the backlight.
– Displays a placeholder temperature on the first line.
– Configures all LED pins as OUTPUT.

Step 3: Read temperature and update display


int sensorValue = analogRead(tempPin);
float voltage = sensorValue * (5.0 / 1023.0);
float temperatureC = (voltage - 0.5) * 100.0;

– Reads the analog value and converts it into voltage.
– Applies TMP36 conversion to get temperature in Celsius.

Step 4: Display temperature on LCD


lcd.setCursor(6, 0);
lcd.print(temperatureC, 1);
lcd.print(" C ");

– Moves the cursor to overwrite the temperature section.
– Displays the temperature with 1 decimal place followed by “°C”.

Step 5: Map temperature to LEDs


int level = map(temperatureC, 20, 40, 0, ledCount);
level = constrain(level, 0, ledCount);

– Translates temperature from 20–40°C into 0–6 LED levels.
– Ensures the number of LEDs stays within the valid range.

Step 6: Light up LEDs based on temperature


for (int i = 0; i < ledCount; i++) {
  digitalWrite(ledPins[i], i < level ? HIGH : LOW);
}

– Turns on LEDs corresponding to the current temperature level.
– Creates a simple LED thermometer effect.

Step 7: Add delay to stabilize output


delay(500);

– Adds a short delay between readings to prevent flickering.

How It Works

The TMP36 sensor outputs an analog voltage that varies linearly with temperature. At 0°C, the output is approximately 0.5V. Each 1°C change increases the output by 10mV.
Arduino reads this voltage through pin A0 and converts it to temperature in Celsius using the formula (voltage - 0.5) * 100.0.
The measured temperature is then displayed on the 16×2 LCD using the I2C interface, which simplifies wiring by using only 2 pins (SDA and SCL).
At the same time, the temperature value is mapped to a number of LEDs to be lit up. For example:
- At 20°C → 0 LEDs on
- At 50°C → ~3 LEDs on
- At 100°C or above → All LEDs on
This combination provides both a visual scale (LED bar) and a precise numeric display (LCD) for temperature monitoring.

🛠️ Optional Upgrades

Add buzzer if temperature goes above 35°C.
Use RGB LED instead of single-color bar.
Add humidity sensor for combined temperature + humidity display.
Display °F alongside °C by converting: F = C * 1.8 + 32

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