How to Build a Secret Laser Tripwire Alarm with Arduino for Under $15

Building a laser tripwire alarm with Arduino is an easy and fun electronics project that can be built on a budget under $15. This alarm creates an invisible laser beam that triggers an alarm when broken. It's great for pranks, home security, or just experimenting with DIY circuits.

What You Will Need

Here is a list of components needed to build the alarm:

• Arduino Uno board - The brain of the alarm that controls everything. Can be purchased for around $5.

• Laser diode module - Generates the invisible laser beam. Can find these on Amazon for ~$2.

• Photoresistor - Detects the laser beam. Costs less than $1.

• Buzzer - Creates the alarm sound when the beam is broken. About $1.

• Resistors - Current limiting resistors for the laser and buzzer. Just a few cents each.

• Jumper wires - For connecting the components. Often come with Arduino starter kits.

• Breadboard - Makes it easy to prototype circuits without soldering. Around $5.

• 9V battery - Powers the Arduino. Rechargeable are best.

• Battery clip - Connects the 9V to the Arduino power jack.

So as you can see, all the components can be sourced for less than $15 total. The exact total cost will depend on where you source the parts.

How the Laser Tripwire Alarm Works

The alarm uses the laser diode to send an invisible beam of laser light to the photoresistor. The photoresistor changes resistance depending on how much light hits it.

When the laser beam is interrupted, less light reaches the photoresistor, which causes its resistance to increase. This resistance change is detected by the Arduino.

Once triggered, the Arduino activates the buzzer to create the alarm sound until the circuit is reset.

The laser and photoresistor are placed across the area you want to detect intrusions. When someone passes through the beam, they block the light beam and activate the alarm.

Building the Circuit

First, connect the power. Clip the 9V battery to the Arduino power jack. This gives power to the whole circuit.

Next, assemble the laser diode part of the circuit:

• Laser diode positive lead to Arduino 5V pin
• 220 ohm resistor between laser diode negative lead and Arduino pin 3

This powers the laser diode and lets us control it with pin 3.

Then assemble the alarm side:

• Buzzer positive lead to Arduino pin 11
• Buzzer negative lead to 220 ohm resistor
• Other side of resistor to Arduino GND

The buzzer is controlled by pin 11. The resistor limits current to protect the Arduino.

Finally hook up the photoresistor:

• Photoresistor from analog pin A0 to 5V
• 10K resistor from A0 to GND

This divides the voltage so the Arduino can read the changing resistance.

Upload the Arduino sketch to detect the tripwire breaks and activate the alarm.

Testing and Using the Tripwire Alarm

To test it out, open the serial monitor on the Arduino IDE. Then aim the laser across the room at the photoresistor. When you break the beam, the alarm should sound!

You should hear the buzzer and see the serial output change when the tripwire is broken.

Try adjusting the laser beam path and sensitivity. Get creative setting up laser maze courses or staging pranks on roommates.

Just remember to be safe. Never aim lasers at eyes or aircraft. And always switch the alarm circuit off when not in use.

This simple Arduino project teaches the basics of lasers, sensors, and alarm circuits. It's fun to build and experiment with. The skills learned can be applied to more advanced projects.

Let me know in the comments if you have any other questions! I'm happy to help explain any part of the build process.