Ever since I started with Arduino I wanted to create a cash register for my girl.
My requirements:

• A “scanner” that triggers a buzzer and displays a (random) value on a LCD.
• A keyboard where numbers can be pressed and calculated.

For the scanner I thought it would be a good idea to use a LDR (Photo resistor). Initially I hard coded a threshold value and when the value returned from the LDR was over that value, it triggered a buzzer. I then realized that working with the LDR depended a lot on the light – meaning it really mattered if I tested at day or at night with no daylight. I decided to add a potentiometer with which I can fine tune the threshold value later instead of uploading new code to the Arduino just to change a constant.
I also added a LED to the scanner that is on most of the time but turns off when the LDR value goes over the threshold value, ie when it is dark.

Scanner complete

As a keypad I chose a 16 Key Membrane Switch Keypad (4×4 Matrix) which can be found cheap on ebay.

I was using my Arduino Uno during the prototype phase and decided to use the Arduino Nano for the finished project because it provides all the functionality I needed but it’s just lots smaller (and also cheaper) than the Uno. I ordered it also from ebay but I must have had the same bad luck as Wai Hung because I could not program it over USB. But instead of soldering and putting a lot of energy into solving the problem I just used my Uno as ISP like I did it in my Christmas Tree project.
I put the Nano onto a 170 tie point self adhesive breadboard.

Arduino Nano on a self adhesive breadboard

For the “wooden hardware” I used plywood and MDF and painted it.

The code goes here:

// include the library code:
#include <LiquidCrystal.h>
#include <Keypad.h>
#include <stdio.h>

// These constants won't change.  
// They're used to give names to the pins used:
const int analogInPin = A3;       // Analog input pin that the ldr is attached to
const int thresholdPin = A5;      // Analog input pin that the threshold poti is attached to
const int buzzer = 13;            // Analog output pin that the buzzer is attached to
const int led = 18;               // scanner LED

int sensorValue = 0;              // value read from the ldr
int lightThreshold = 0;

boolean hasBeeped = false;        // has beeped?
const int buzzerValue = 200;      // frequency
const int beepDuration = 250;     // beeps 250ms

// used for calculation of values entered by the keypad
String keyPadValue;
float keyValue = 0.0;
float keySum = 0.0;
boolean isKeypadInput = false;

// keypad
const byte ROWS = 4; // Four rows
const byte COLS = 4; // columns
// Define the Keymap
char keys[ROWS][COLS] = {
{'1','2','3','A'},
{'4','5','6','B'},
{'7','8','9','C'},
{'*','0','#','D'}
};
// Connect keypad ROW0, ROW1, ROW2 and ROW3 to these Arduino pins.
byte rowPins[ROWS] = { 12, 11, 10,  9 };
// Connect keypad COL0, COL1 and COL2 to these Arduino pins.
byte colPins[COLS] = {  8,  7,  6,  5 };

// Create the Keypad
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd( 16, 4, 3, 2, 14, 15 );

// for the greeting message
byte heart[8] = {
        B00000,
        B01010,
        B11111,
        B11111,
        B01110,
        B00100,
        B00000,
        B00000
};

void setup() {
  pinMode(14, OUTPUT);
  pinMode(15, OUTPUT);
  pinMode(16, OUTPUT);

  lcd.createChar(0, heart);

  // set up the LCD's number of columns and rows: 
  lcd.begin(16, 2);
  // Print a greeting message to the LCD.
  lcd.write((uint8_t)0);
  lcd.print(" Hoi Laura! ");
  lcd.write((uint8_t)0);

  pinMode(led, OUTPUT);  
  digitalWrite(led, HIGH);  

  // initialize serial communications at 9600 bps:
  Serial.begin(9600); 

  // delay it by 3sec so we are able to see the message
  delay(3000);
  lcd.clear();
  lcd.setCursor(0, 1);
}

void loop() {
  char key = keypad.getKey();

  if (key != NO_KEY){

    beep();

    int len=0;

    switch(key)
    {
      case '*':
        // treat it like a '+'
        keySum += keyValue;
        len = keyPadValue.length();
        // write from right to left
        lcd.clear();
        lcd.setCursor(16 - (len+9), 0);
        lcd.print("Fr. ");

        lcd.setCursor(16 - (len+3), 0);
        lcd.print(keySum);
//        lcd.print(printf("%00.00d", keySum ));

        keyPadValue = "";
        keyValue = 0.0;

        break;
      case '#':
        keySum += keyValue;

        // treat it like a '='
        lcd.clear();
        lcd.setCursor(0, 0);
        lcd.print("Total:");    
        lcd.setCursor(0, 1);
        lcd.print("Fr. ");    
        lcd.print(keySum);    

        keyPadValue = "";
        keyValue = 0.0;

        break;
      case 'A':
      case 'B':
      case 'C':
      case 'D':
          // ignore
        break;
      default:
        if (!isKeypadInput)
        {
          lcd.clear();
          printKeySum(keySum);
        }
        isKeypadInput = true;
        // append new key input
        keyPadValue += key; 
        len = keyPadValue.length();

        // write from right to left
        lcd.setCursor(16 - len, 1);
        lcd.print(keyPadValue);    

        // store the string into a float                
        char buffer[100];   // didn't work wit len+1 so i took 100...
        keyPadValue.toCharArray(buffer, len+1);
        keyValue =  atof(buffer);  // convert char array to float
    }

  }

    // read the analog in value:
  sensorValue = analogRead(analogInPin);   
  lightThreshold = analogRead(thresholdPin);   

  Serial.print("sensor = ");                       
  Serial.print(sensorValue);                       
  Serial.print("\t threshold = ");                       
  Serial.println(lightThreshold);                       

  if (sensorValue > lightThreshold) 
  {
    // dark
    digitalWrite(led, LOW);  
    // change the analog out value:
    if (hasBeeped == false)
    {
      beep();
      hasBeeped = true;

      int d1 = random(1,20);
      int d2 = random(0,10);
      int d3 = random(0,2);
      if (d3==1)
      {
        d3 = 5;
      }

      char buf[6];
      sprintf(buf,"%d.%d%d",d1,d2,d3);   // to get a string in the format of ##.#5 or ##.#0

      int len=4;
      // write from right to left
      if (d1 >= 10)
      {
        len = 5;
      }
      else
      {
        lcd.clear();
      }

      lcd.setCursor(16 - (len+4), 1);
      lcd.print("Fr. ");

      lcd.setCursor(16 - len, 1);
      lcd.print(buf);

      printKeySum(keySum);

      keyValue = atof(buf);
      keySum += keyValue;
    }

  }
  else
  {
    // light
    digitalWrite(led, HIGH);  
    hasBeeped = false;
  }

  // wait 2 milliseconds before the next loop
  // for the analog-to-digital converter to settle
  // after the last reading:
  delay(2);                     
}

void printKeySum(float keySum)
{
  if (keySum > 0)
  {
    // sum
    int len = 4;
    if (keySum >= 10 && keySum < 100)
    {
      len = 5;
    }
    else if (keySum >= 100 && keySum < 1000)
    {
      len = 6;
    }
    else if (keySum >= 1000 && keySum < 10000)
    {
      len = 7;
    }
    else if (keySum >= 10000 && keySum < 100000)
    {
      len = 8;
    }

    lcd.setCursor(16 - (len+4), 0);
    lcd.print("Fr. ");

    lcd.setCursor(16 - len, 0);
    lcd.print(keySum);
  }
}

void beep(){
  analogWrite(buzzer, buzzerValue);
  // beep 250ms
  delay(beepDuration);  
  analogWrite(buzzer, 0);
}