Cheap Laser Tripwire Home Security Alarm System

This is a simple and cheap home security system that costs only $ 15, to ward off nosy siblings and burglars alike. It comprises of a laser used as a 'tripwire' and a photosensor to detect whether this laser beam is blocked or not. If it is blocked an intruder, an 'alarm' will sound off using the piezo buzzer. The alarm I chose is the beautiful sounds of Pachelbel's Canon in D major.

I was inspired to make this from spy movies with high-end home laser security systems where the protagonists sprays aerosol and navigate around a complex laser setup.

Step 1: BoM

Arduino 101 or Arduino Uno -- $7
Photoresistor or LDR -- $1
10kΩ resistor -- $0.2
Breadboard -- $4
Laser -- $2
Jumper Wire $1

Step 2: Circuitry

Connect the LED to the Arduino by connecting the positive pin of the LED to the 3.3V pin Arduino in series through a 100Ω resistor. This is shown as the red wire.
Connect the flat edge pin of the LED to ground, shown as the pin connected to the blue rail on the breadboard. This blue rail is then connected to the Arduino's common GND pin.

Connect one of the pins on the LDR to VCC using the red rail of the breadboard. Then, connect this red rail to the 3.3V pin on the Arduino.
Wire the LDR signal to the Arduino by connecting the other pin to A0 on the Arduino. This is the white wire is on the board.
Connect the other LDR pin to a 10kΩ resistor through to ground shown as the blue rail on the breadboard.

Step 3: Music Header File

The header file defines all the constants needed for the tone and the music that will be planes when the alarm rings.

/*************************************************<br> * Public Constants  *************************************************/

#define B0  31 #define C1  33 #define CS1 35 #define D1  37 #define DS1 39 #define E1  41 #define F1  44 #define FS1 46 #define G1  49 #define GS1 52 #define A1  55 #define AS1 58 #define B1  62 #define C2  65 #define CS2 69 #define D2  73 #define DS2 78 #define E2  82 #define F2  87 #define FS2 93 #define G2  98 #define GS2 104 #define A2  110 #define AS2 117 #define B2  123 #define C3  131 #define CS3 139 #define D3  147 #define DS3 156 #define E3  165 #define F3  175 #define FS3 185 #define G3  196 #define GS3 208 #define A3  220 #define AS3 233 #define B3  247 #define C4  262 #define CS4 277 #define D4  294 #define DS4 311 #define E4  330 #define F4  349 #define FS4 370 #define G4  392 #define GS4 415 #define A4  440 #define AS4 466 #define B4  494 #define C5  523 #define CS5 554 #define D5  587 #define DS5 622 #define E5  659 #define F5  698 #define FS5 740 #define G5  784 #define GS5 831 #define A5  880 #define AS5 932 #define B5  988 #define C6  1047 #define CS6 1109 #define D6  1175 #define DS6 1245 #define E6  1319 #define F6  1397 #define FS6 1480 #define G6  1568 #define GS6 1661 #define A6  1760 #define AS6 1865 #define B6  1976 #define C7  2093 #define CS7 2217 #define D7  2349 #define DS7 2489 #define E7  2637 #define F7  2794 #define FS7 2960 #define G7  3136 #define GS7 3322 #define A7  3520 #define AS7 3729 #define B7  3951 #define C8  4186 #define CS8 4435 #define D8  4699 #define DS8 4978

Step 4: Arduino Code

# include "pitches.h"

int melody[] = {A6, FS6, G6, A6, FS6, G6, A6, A5, B5, CS6, D6, E6, FS6, G6, FS6, D6, E6, FS6, FS5, G5, A5, B5, A5, G5, A5, FS5, G5, A5, G5, B5, A5, G5, FS5, E5, FS5, E5, D5, E5, FS5, G5, A5, B5, G5, B5, A5, B5, CS6, D6, A5, B5, CS6, D6, E6, FS6, G6, A6};

//int melody[] = {'A6', 'FS6', 'G6', 'A6', 'FS6', 'G6', 'A6', 'A5', 'B5', 'CS6', 'D6', 'ES6', 'FS6', 'G6', 'FS6', 'D6', 'E6', 'FS6', 'FS5', 'G5', 'A5', 'B5', 'A5', 'G5', 'A5', 'FS5', 'G5', 'A5', 'G5', 'B5', 'A5', 'G5', 'FS5', 'E5', 'FS5', 'E5', 'D5', 'E5', 'FS5', 'G5', 'A5', 'B5', 'G5', 'B5', 'A5', 'B5', 'CS6', 'D6', 'A5', 'B5', 'CS6', 'D6', 'E6', 'FS6', 'G6', 'A6'};

int noteDurations[] = {8, 16, 16, 8, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 8, 16, 16, 8, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 8, 16, 16, 8, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 8, 16, 16, 8, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16};

const int sensorPin = 0; int lightCal; int lightVal;

void setup() {   // iterate over the notes of the melody:   pinMode (9, OUTPUT);   lightCal = analogRead(sensorPin);

void loop() {    lightVal = analogRead(sensorPin);

  //if lightVal is less than our initial reading (lightCal) minus 50 it is dark and         //turn pin 9 HIGH. The (-50) part of the statement sets the sensitivity. The smaller       //the number the more sensitive the circuit will be to variances in light.   if(lightVal < 800){

    for (int i = 0; i < 56; i++) {          // to calculate the note duration, take one second       // divided by the note type.       //e.g. quarter note = 1000 / 4, eighth note = 1000/8, etc.       int noteDuration = 2500 / noteDurations[i];       tone(9, melody[i], noteDuration);          // to distinguish the notes, set a minimum time between them.       // the note's duration + 30% seems to work well:       int pauseBetweenNotes = noteDuration * 1.30;       delay(pauseBetweenNotes);     }    }