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Arduino Projects Book - Project 03: Love-O-Meter

In this project, I will play around with the temperature sensor. I learned a lot of built-in functions through this project. So, let’s prepare a cold hand and a warm hand for the sensor!






I used both ANALOG and DIGITAL in this project. I learned more about ANALOG and how to process it in Arduino IDE. Here is my schematic:


Below is my breadboard layout from Fritzing:


Both of them are available on my GitHub. And here is my circuit. Again, the anode of the LED should be connected to a DIGITAL pin, not the POWER:



There’re several built-in functions in this project, but I will try my best to explain them in my own way. As always, pay attention to cAsE sEnSiTiViTy.


This function opens a connection between the Arduino and the computer with a defined speed, baud_rate. There’re a lot of bit rates, such as 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, or 115200. I mostly use 9600 and 115200 as my baud rates.


Similarly to digitalRead(pin), analogRead(pin) will get the value from the particular pin number (from A0 to A5). However, this function will get the value of the pin, not just the state like digitalRead(). That means the value it reads in is a real number (float). The function’s range is between 0 and 1023.


It prints the value of variable from the Arduino to the serial monitor. We can view this on the Arduino IDE using the combination Ctrl + Shift + M.

Here are some formulas from this lab. They interpret how the sensor works and relationship between the sensor and the voltage:

float voltage = (sensorValue / 1024.0) * 5.0;

Voltage is assigned a real value between 0V and 5V. Since the sensor will read a value between 0 and 1024, we need to divide it by 1024 to get the ratio (or how much of 1024 the sensor reads in) and then multiply by 5 to get how much of 5V the voltage is going to have.

temperature = (voltage - 0.5) * 100.0;

For this TMP36 temperature sensor, every 10mV equals 1 degree Celsius.

See below for my full code for this project, or view it on GitHub.

 * Project Name: Arduino Projects Book - Project 03: Love-O-Meter
 * File Name: love_o_meter.ino
 * Description: Uses the temperature sensor to test how hot you really are!
 * Author: Phi Luu
 * Location: Portland, Oregon, United States
 * Created: September 30, 2015
 * Updated: June 22, 2017

// Required hardware I/O connections
const byte SENSOR_PIN = A0; // connect TMP sensor to A0
const byte LED_1      = 2;  // connect LED 1 to 2
const byte LED_2      = 3;  // connect LED 2 to ~3
const byte LED_3      = 4;  // connect LED 3 to 4

// Global constants
const float BASE_TEMP = 20.0;

// Global variables
unsigned int sensor_val;

void setup() {

    for (byte pin = LED_1; pin <= LED_3; pin++) {
        pinMode(pin, OUTPUT);
        digitalWrite(pin, LOW);

void loop() {
    // read & print temperature value
    sensor_val = analogRead(SENSOR_PIN);
    Serial.print("Sensor value: ");

    // map & print temperature value to voltage
    float voltage = (sensor_val / 1024.0) * 5.0;
    Serial.print("Volts: ");

    // from voltage, define temperature
    float temperature = (voltage - 0.5) * 100;
    Serial.print("Degrees C: ");

    // from temperature, indicate the LEDs
    if ((temperature >= BASE_TEMP)
            && (temperature < BASE_TEMP + 2)) {
        digitalWrite(LED_1, HIGH);
        digitalWrite(LED_2, HIGH); // level 1: all LEDs turned on
        digitalWrite(LED_3, HIGH);
    } else if (temperature < BASE_TEMP) {
        digitalWrite(LED_1, LOW);
        digitalWrite(LED_2, LOW); // level 0: all LEDs turned off
        digitalWrite(LED_3, LOW);
    } else if ((temperature >= BASE_TEMP + 2)
               && (temperature < BASE_TEMP + 4)) {
        digitalWrite(LED_1, HIGH);
        digitalWrite(LED_2, LOW); // level 2: LED_1 turned on, LEDs 2 & 3 turned off
        digitalWrite(LED_3, LOW);
    } else if ((temperature >= BASE_TEMP + 4)
               && (temperature < BASE_TEMP + 6)) {
        digitalWrite(LED_1, LOW);
        digitalWrite(LED_2, HIGH); // level 3: LED_2 turned on, LEDs 1 & 3 turned off
        digitalWrite(LED_3, LOW);
    } else {
        digitalWrite(LED_1, LOW);
        digitalWrite(LED_2, LOW); // level 4: LED 3 turned on, LEDs 1 & 2 turned off
        digitalWrite(LED_3, HIGH);


Place the cold hand on the temperature sensor, then place the hot hand. There will be some magical changes in the LEDs.


Today I have learned a bunch of new commands and cool things about the temperature sensor. I’m very tired now. I need to sleep. Thanks for reading!