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Justway

Published February 20, 2025 © CC BY-NC

Alarm Protected Shelf

A shelf that rings an alarm when an item is removed

BeginnerWork in progress6 hours95

Things used in this project

Hardware components

SparkFun Load Cell Amplifier - HX711

Arduino UNO

Seeed Studio Grove - 12-Channel Capacitive Touch Keypad (ATtiny1616)

DFRobot I2C 16x2 Arduino LCD Display Module

Software apps and online services

Arduino IDE

Hand tools and fabrication machines

3D Printer (generic)

Story

Youtube video

I have a lot of rubik’s cubes. Some are big; some are small; some cost an absurd amount of money. Two years ago, my younger brother destroyed one of my cubes by throwing it on the ground, and since then I have spent $90 on security upgrades to prevent it from happening again. But even when he’s not destroying my cubes, he’s scrambling them when I have them on display. I just want to display my cubes without having them being touched. So, I spent a month building this shelf that rings an alarm whenever you remove an item from it.

Materials:

Strobe light + piezo buzzer
Arduino Uno
4x Load cells + HX711
I2C LCD
4x4 Keypad
Wires, breadboard, and transistor
2x 12V power supply

Optional if you are designing your own frame

Frame pieces
Screws and threaded inserts

Frame

I designed my frame in tinkercad and sent the files to Justway to produce.

Justway is an affordable online manufacturing service, with services such as 3d printing, cnc machining, and sheet metal forming. To place an order on Justway, click this link to open the menu page. Select the service you are using, and click upload your design.

This will take you to this page, where you can upload your design and pick your material, color, and other special requests. Once all selections have been made, submit your request and wait for it to be approved. Once approved, pay for the order and wait for the parts to arrive.

When I first used Justway for my 3d printed parts, I received an email shortly after the order about a design flaw in one of my models. The production team asked me if I wanted to continue with my original design or a modified version they had created. I opted for the original design because it wasn’t a major issue.

When I unboxed the parts, I was surprised by not only the quality and dimensional accuracy of the parts, but also the fact that I received extras for a bulk order presumably in case of shipping damage.

You can design your own frame, but if you want to use my frame, it is linked here.

Entire circuit

This is a diagram with all the wire connections. It's a bit overwhelming, so let's break it down.

KeypadandLCD

Using an I2C LCD, connect the ports as listed below and shown in the diagram

VCC to 5V(ideally through breadboard)
GND to GND(ideally through breadboard)
SDA to A4
SCL to A5

Using a 4x4 keypad, connect the ports as listed below and shown in the diagram

Row 1 to 5
Row 2 to 4
Row 3 to 3
Row 4 to 2
Column 1 to 9
Column 2 to 8
Column 3 to 7
Column 4 to 6

1 / 2

LoadCells

Tinkercad Circuits does not have a HX711, so I used a breadboard in the diagram.

Using four load cells and a HX711, connect the ports as listed below and shown in the diagram

Place four load cells in corners of shelf
Connect adjacent black and white wires
Connect two opposite(diagonal) red wires to A+ and A-
Connect other two opposite(diagonal) red wires to E+ and E-
DO NOT CONNECT B+ AND B-
VCC to 5V
GND to GND
DT to 12
SCK to 11

StrobeLight and Buzzer

Connect Piezo's + to pin 13 and - to GND

Connect 12V power supply to strobe light through a mosfet transistor

Gate to 10
Drain to ground of power supply
Source to negative of strobe light
power of power supply to power of strobe light

Code

I have included example code in this project. Note that the calibration factor is for my load cells. You will have to calibrate yours by yourself using the calibration example in the HX711_ADC library.

Code

Code

#include <Keypad.h>
#include <LiquidCrystal_I2C.h>
#include <HX711_ADC.h>
//include libraries needed

#define calibration_factor -22233.33
#define LOADCELL_DOUT_PIN  12
#define LOADCELL_SCK_PIN  11
int alarmPin = 10;
int buzzerPin = 13;
//define pins used

const byte ROWS = 4;
const byte COLS = 4;
char hexaKeys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

byte colPins[ROWS] = {9, 8, 7, 6};
byte rowPins[COLS] = {5, 4, 3, 2};
//define keypad


Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);
LiquidCrystal_I2C lcd(0x27, 16, 2);
HX711_ADC scale(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);  
//initialize keypad, lcd, and load cells

char resetKey = '*';


String code = "1234#";
String input = "";
bool alarmActive = false;
float oldWeight;
int timeOff = 60;
//changable values

void setup() {
  Serial.begin(9600);
  scale.begin();
  scale.start(2000); 
  scale.setCalFactor(calibration_factor);
  scale.tare();
  //initialize scale

  lcd.init(); 
  lcd.backlight();
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("SECURITY ON");
  //initialize LCD

  oldWeight = scale.getData(); 
  //define initial weight

  pinMode(buzzerPin, OUTPUT);
  //initialize buzzer
}

void loop() {
  scale.update();
  char customKey = customKeypad.getKey();
  //check for key presses
  if (customKey) {
    Serial.println(customKey);
    if (customKey == resetKey) {
      input = "";
      lcd.clear();
      //clear input if reset key pressed
    }else {
      input += customKey;
      //add key press to input
    }
    lcd.setCursor(0, 1);
    lcd.print(input);
  }
  if(input==code){
    stopAlarm();
    disableSecurity();
    input = "";
    //disable security if correct code entered
    
  }
  if (fabs(scale.getData() - oldWeight) > 0.2 && !alarmActive) {

    soundAlarm();
    //activate alarm if abnormal weight
  }
}

void soundAlarm() {
  alarmActive = true;
  digitalWrite(alarmPin, HIGH);
  tone(buzzerPin, 1200);
  //activate strobe light and buzzer

  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("ALARM ACTIVE");
  lcd.setCursor(0, 1);
  lcd.print(input);
  //print alarm active on lcd
}

void stopAlarm(){
  noTone(buzzerPin);
  alarmActive = false;
  digitalWrite(alarmPin, LOW);
  //turn off strobe light and buzzer
}

void disableSecurity() {
  if (input == code) {
    for (int i = timeOff; i > 0; i--) {  
      //if code is correct
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print("SECURITY OFF FOR");
      lcd.setCursor(0, 1);
      lcd.print(String(i) + " SECONDS");
      delay(1000);
      scale.update();
      oldWeight = scale.getData();
      //disable security and show countdown on LCD
    }
    scale.update();
    oldWeight = scale.getData();
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("SECURITY ON");
    //change oldWeight to after
  }
  
}