/*
Platinenbelichter V0.1 (UV Exposure Unit)

Acts as a timer for PCB exposure. The leds are controlled by a FET.
Furthermore a 16x2 Character LCD and three push buttons are connected.

The circuit:
* LCD RS pin to digital pin 12
* LCD Enable pin to digital pin 11
* LCD DO4 pin to digital pin 5
* LCD DO5 pin to digital pin 4
* LCD DO6 pin to digital pin 3
* LCD DO7 pin to digital pin 2
* LCD R/W pin to ground
*Push button to digital pin8 / 9 / 10

Copyright: Alexander Kroth 2014

*/

//include LCD library
#include <LiquidCrystal.h>

//include Debounce library
#include <Bounce2.h>

//LCD initilization
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

//Button pin definition
const int MinButtonP = 8;
const int SecButtonP = 9;
const int StartButtonP = 10;

//FET pin definition
const int FetP = 13;

//Timer variable
unsigned long current_time;
unsigned long termination_time;

int Min_Set = 0;
int Sec_Set = 0;
int Min_Cur = 0;
int Sec_Cur = 0;

boolean running;

//Initialize button objects
Bounce MinButton = Bounce();
Bounce SecButton = Bounce();
Bounce StartButton = Bounce();

void setup() {
  
  //Pin Setup
  
  pinMode(MinButtonP, INPUT);
  digitalWrite(MinButtonP, HIGH);
  
  pinMode(SecButtonP, INPUT);
  digitalWrite(SecButtonP, HIGH);
  
  pinMode(StartButtonP, INPUT);
  digitalWrite(StartButtonP, HIGH);
  
  pinMode(FetP, OUTPUT);
  
  //Button debouncing using Bounce 2 from thomasfredericks
  
  MinButton.attach(MinButtonP);
  MinButton.interval(100);
  
  
  SecButton.attach(SecButtonP);
  SecButton.interval(100);
  
  
  StartButton.attach(StartButtonP);
  StartButton.interval(100);
  
  
  
  //Row and columns setup of LCD
  lcd.begin(16,2);
  //Startup Screen
  lcd.print("Platinen-");
  lcd.setCursor(0,1);
  lcd.print("Belichter");
  lcd.setCursor(0,0);
  delay(2000);
  
  //Prepare Display
  lcd.print("Dauer           ");
  lcd.setCursor(0,1);
  lcd.print("Min. 00 Sec. 00 ");
  
}
  
void loop() {
  
  //Button Updates
  MinButton.update();
  SecButton.update();
  StartButton.update();
  
  running = false;
  
  
  
  //Info Screen
  if(MinButton.read() == false && SecButton.read() == false && StartButton.read() == false) {
    lcd.setCursor(0,0);
    lcd.print("Hackaday.io     ");
    lcd.setCursor(0,1);
    lcd.print("Alexander Kroth ");
    delay(2000);
    
    lcd.setCursor(0,0);
    lcd.print("Dauer           ");
    lcd.setCursor(0,1);
    lcd.print("Min. 00 Sec. 00 ");
    
    Min_Set--;
    Sec_Set--;
    running = true;
    
    if(Min_Set < 10) {
      
      lcd.setCursor(6,1);
      lcd.print(Min_Set);
    }
    else {
      lcd.setCursor(5,1);
      lcd.print(Min_Set);
    }
  
    if(Sec_Set < 10) {
      
      lcd.setCursor(14,1);
      lcd.print(Sec_Set);
    }
    else {
      lcd.setCursor(13,1);
      lcd.print(Sec_Set);
    }
  }
  
  
  //Reset time
  if(MinButton.read() == false && SecButton.read() == false) {

    
    Min_Set = -1;
    Sec_Set = -1;
    lcd.setCursor(0,1);
    lcd.print("Min. 00 Sec. 00 ");

  } 
  
  
  //Increment minutes
  //Check wether exposure is running and rising edge was detected
  if(MinButton.rose() == true) {
    //Increase minutes to expose
    Min_Set++;
    
    //Maximum of 99 minutes, seriously, no one needs more?
    if(Min_Set > 99) {
      Min_Set = 99;
    }
    
    //Move cursor to position the numbers
    if(Min_Set < 10) {
      
      lcd.setCursor(6,1);
      lcd.print(Min_Set);
    }
    else {
      lcd.setCursor(5,1);
      lcd.print(Min_Set);
    }
  }
  
  
  
  
  //Increment seconds
  //Check wether exposure is running and rising edge was detected
  if(SecButton.rose() == true) {
    Sec_Set++;
    
    //Seconds overflow and Cursor correction
    if(Sec_Set > 59) {
      Sec_Set = 0;
      lcd.setCursor(13,1);
      lcd.print("00");
    }
    
    //Move cursor to position the numbers
    if(Sec_Set < 10) {
      
      lcd.setCursor(14,1);
      lcd.print(Sec_Set);
    }
    else {
      lcd.setCursor(13,1);
      lcd.print(Sec_Set);
    }
  }
  
  
  //Start exposure
  if(running == false && StartButton.rose() == true) {
    //Calculate end time of exposure
    termination_time = millis()/1000 + Min_Set * 60 + Sec_Set;
    
    //Inform user of exposure
    lcd.setCursor(0,0);
    lcd.print("Belichtung      ");
    lcd.setCursor(0,1);
    lcd.print("00:00 / 00:00    ");
    if(Min_Set < 10) {
      
      lcd.setCursor(9,1);
      lcd.print(Min_Set);
    }
    else {
      lcd.setCursor(8,1);
      lcd.print(Min_Set);
    }
  
    if(Sec_Set < 10) {
      
      lcd.setCursor(12,1);
      lcd.print(Sec_Set);
    }
    else {
      lcd.setCursor(11,1);
      lcd.print(Sec_Set);
    }
    
    
    
    
    //Switch the leds on
    digitalWrite(FetP, HIGH);
    
    //Wait for PCB to be exposed
    while( millis()/1000 < termination_time) {
      
      //Still read the StartButton to get free
      StartButton.update();
      
      //Abort exposure
      if(StartButton.rose() == true) {
        break;
      }
      
      //Calculate time to display
      Min_Cur = int(long(termination_time-(millis()/1000))/60);
      Sec_Cur = long(termination_time-millis()/1000) - (int(long(termination_time-(millis()/1000))/60)*60);
      
      
      //Update display
      if(Min_Cur < 10) {
        lcd.setCursor(0,1);
        lcd.print(0);
        lcd.setCursor(1,1);
        lcd.print(Min_Cur);
      }
      else {
        lcd.setCursor(0,1);
        lcd.print(Min_Cur);
      }
  
      if(Sec_Cur < 10) {
        lcd.setCursor(3,1);
        lcd.print(0);
        lcd.setCursor(4,1);
        lcd.print(Sec_Cur);
      }
      else {
        lcd.setCursor(3,1);
        lcd.print(Sec_Cur);
      }
    }
    digitalWrite(FetP, LOW);
    
    //Restore former display
    lcd.setCursor(0,0);
    lcd.print("Dauer           ");
    lcd.setCursor(0,1);
    lcd.print("Min. 00 Sec. 00 ");
    
    if(Min_Set < 10) {
      
      lcd.setCursor(6,1);
      lcd.print(Min_Set);
    }
    else {
      lcd.setCursor(5,1);
      lcd.print(Min_Set);
    }
  
    if(Sec_Set < 10) {
      
      lcd.setCursor(14,1);
      lcd.print(Sec_Set);
    }
    else {
      lcd.setCursor(13,1);
      lcd.print(Sec_Set);
    }
  }


  
}
  

Lacking a Github Account I have to post the entire code here.

The 16x2 LCD is controlled by the standard LCD library, that comes with the Arduino IDE. The button debouncing is taken care by Bounce2 by Thomas Fredericks.

There's one button to increase the minutes and one to increase the seconds. If both are pushed at the same time, the time is reset to zero. The third button is there to start and stop the exposure. Once the button is pushed, the Led-Stripe is switched on and the display starts to count down.

If all three buttons are pushed at the same time, my name and Hackaday.io is displayed.

The electronics are working as intended, but the casing needs a redesign....

Any suggestions regarding the code?