Whether you’re building a kitchen timer, a darkroom exposure controller, or a timed charging station, a programmable relay timer is a classic “must-have” project. This guide breaks down the code and hardware for a professional-grade countdown timer with an LCD interface and EEPROM memory—which means it remembers your last set time even if you unplug it!
The Components: What’s Under the Hood?
To build this project, you need a few specific parts. Here is why each one is vital:
| Component | Purpose |
| Arduino Uno / Nano | The “brain” that processes the logic and counts the seconds. |
| I2C LCD (16×2) | Displays the countdown and the setup menu. The I2C module on the back saves you from wiring 12+ pins, reducing it to just 4. |
| Relay Module (5V) | Acts as a smart switch. It allows the low-power Arduino to turn high-power devices (like a lamp or motor) ON and OFF. |
| Active Buzzer | Provides an audible alert once the timer reaches zero. |
| Push Buttons | Used for user input (Set, Up, Down, and Start/Reset). |
The Wiring Guide
Connecting the components is straightforward if you follow the I2C and Analog pin assignments.
- LCD Display: Connect SDA to A4 and SCL to A5. Power it via the 5V and GND pins.
- Input Buttons: Use pins A0, A1, A2, and A3. These are configured as
INPUT_PULLUP, so you only need to wire the button between the pin and GND. - Relay & Buzzer: Connect the Relay signal to Pin 5 and the Buzzer to Pin 6.
How the Code Works
The code uses three main libraries to keep things running smoothly:
LiquidCrystal_I2C.h: Manages the screen via the I2C protocol.Countimer.h: A specialized library that handles the “ticking” logic in the background so the screen doesn’t flicker.EEPROM.h: This is built into Arduino. It saves yourHH:MM:SSsettings to the chip’s permanent memory.
Key Features of the Script:
- Setting the Time: By tapping the Set button, you cycle through Seconds, Minutes, and Hours. Use Up/Down to adjust the values.
- Memory Save: Every time you adjust the time, the code calls
eeprom_write(). This ensures that if you lose power, your timer doesn’t reset to zero. - The Relay Trigger: When the timer is active (
flag2 == 1), the relay is closed. When the timer hits $00:00:00$, the relay opens and the buzzer triggers a “beep-beep-beep” sequence. - The “Long Press” Reset: We added a custom logic that monitors how long you hold the Set button. If held for more than 1.5 seconds, it wipes the timer back to zero.
Arduino Code
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include "Countimer.h"
#include <EEPROM.h>
// Try 0x27 if 0x3F didn't work
LiquidCrystal_I2C lcd(0x27, 16, 2);
Countimer tdown;
#define bt_set A3
#define bt_up A2
#define bt_down A1
#define bt_start A0
int time_s = 0;
int time_m = 0;
int time_h = 0;
int set = 0;
int flag1=0, flag2=0;
int relay = 5;
int buzzer = 6;
// For Reset Logic
unsigned long buttonPressTime = 0;
bool isPressing = false;
void setup() {
Serial.begin(9600);
pinMode(bt_set, INPUT_PULLUP);
pinMode(bt_up, INPUT_PULLUP);
pinMode(bt_down, INPUT_PULLUP);
pinMode(bt_start, INPUT_PULLUP);
pinMode(relay, OUTPUT);
pinMode(buzzer, OUTPUT);
digitalWrite(relay, LOW);
digitalWrite(buzzer, LOW);
lcd.init();
lcd.backlight();
lcd.clear();
lcd.setCursor(0,0);
lcd.print(" Timer Ready ");
tdown.setInterval(print_time, 999);
eeprom_read();
delay(1000);
lcd.clear();
}
void print_time(){
if(time_s > 0 || time_m > 0 || time_h > 0){
time_s--;
if(time_s < 0) { time_s = 59; time_m--; }
if(time_m < 0) { time_m = 59; time_h--; }
}
}
void loop() {
tdown.run();
// --- RESET LOGIC ---
if(digitalRead(bt_set) == LOW) {
if(!isPressing) {
buttonPressTime = millis();
isPressing = true;
}
// If held for 1.5 seconds, Reset to Zero
if(millis() - buttonPressTime > 1500) {
time_s = 0; time_m = 0; time_h = 0;
set = 0;
flag2 = 0;
tdown.stop();
digitalWrite(relay, LOW);
eeprom_write();
lcd.clear();
lcd.print(" Timer Reset! ");
delay(1000);
}
} else {
// Normal Mode Switching on short release
if(isPressing && (millis() - buttonPressTime < 1500)) {
set++;
if(set > 3) set = 0;
delay(100);
}
isPressing = false;
}
// Up Button
if(digitalRead(bt_up) == 0) {
if(set == 0) { tdown.start(); flag2 = 1; }
if(set == 1) { time_s++; if(time_s > 59) time_s = 0; }
if(set == 2) { time_m++; if(time_m > 59) time_m = 0; }
if(set == 3) { time_h++; if(time_h > 99) time_h = 0; }
if(set > 0) eeprom_write();
delay(200);
}
// Down Button
if(digitalRead(bt_down) == 0) {
if(set == 0) { tdown.stop(); flag2 = 0; }
if(set == 1) { time_s--; if(time_s < 0) time_s = 59; }
if(set == 2) { time_m--; if(time_m < 0) time_m = 59; }
if(set == 3) { time_h--; if(time_h < 0) time_h = 99; }
if(set > 0) eeprom_write();
delay(200);
}
// Start Button
if(digitalRead(bt_start) == 0) {
flag2 = 1;
digitalWrite(relay, HIGH);
tdown.restart();
tdown.start();
}
// Display logic
lcd.setCursor(0,0);
if(set == 0) lcd.print(" Timer ");
else if(set == 1) lcd.print(" Set Timer SS ");
else if(set == 2) lcd.print(" Set Timer MM ");
else if(set == 3) lcd.print(" Set Timer HH ");
lcd.setCursor(4,1);
if(time_h <= 9) lcd.print("0"); lcd.print(time_h); lcd.print(":");
if(time_m <= 9) lcd.print("0"); lcd.print(time_m); lcd.print(":");
if(time_s <= 9) lcd.print("0"); lcd.print(time_s); lcd.print(" ");
// End of Timer
if(time_s == 0 && time_m == 0 && time_h == 0 && flag2 == 1) {
flag2 = 0;
tdown.stop();
digitalWrite(relay, LOW);
for(int i=0; i<3; i++) {
digitalWrite(buzzer, HIGH); delay(300);
digitalWrite(buzzer, LOW); delay(200);
}
}
if(flag2 == 1) digitalWrite(relay, HIGH);
else digitalWrite(relay, LOW);
delay(1);
}
void eeprom_write() {
EEPROM.write(1, time_s);
EEPROM.write(2, time_m);
EEPROM.write(3, time_h);
}
void eeprom_read() {
time_s = EEPROM.read(1);
time_m = EEPROM.read(2);
time_h = EEPROM.read(3);
// Prevent EEPROM garbage values (255) from breaking the timer
if(time_s > 59) time_s = 0;
if(time_m > 59) time_m = 0;
if(time_h > 99) time_h = 0;
}Troubleshooting Tips
- Blank Screen: If the backlight is on but no text appears, turn the blue potentiometer on the back of the I2C module to adjust the contrast.
- Buttons Unresponsive: Ensure the buttons are connected to GND. Since we use
INPUT_PULLUP, the pin needs to be pulled to ground to register a press. - Relay Not Clicking: Make sure your power source provides enough current (mA). If the Arduino is powered by a weak laptop USB port, the relay might not have enough juice to flip the internal switch.
