Introduction: Laser Tripwire

About: Chef with an interest in how the world functions

This project will create two units, a laser transmitter unit and a receiver.

For the schematic picture, I could not find the symbols for a buzzer or a switch.... So the switches and the buzzer are represented by diodes but are named accordingly.

You will need:
2 Battery packs
Laser Module
Light Dependent Resistor
NPN Transistor (low voltage)

Solder Iron 

WARNING: My computer messed up some of the photos so they might not be in the correct order or from the correct instructable. Do not rely only on the photos. They are a rough guide of what it could look like.

Step 1: The Laser

The schematic for laser unit says that you will need:
A Battery
A Switch
A Laser Module

First of all, solder the negative wire of the battery pack to one of the terminals of the switch and the negative wire from the module to the other terminal. Then solder the LED to the positive wire for the battery and the laser module and it is complete.

Step 2: The Buzzer Receiver

You will need:
Battery pack
Light Dependent Resistor
NPN Transistor (low voltage)

For the resistor you need to work out the resistance with the voltage divider sum. I used a 6V battery pack, and my LDR read 230Ω in laser light and 31,000Ω in ambient light.

For the sake of an example I chose 1KΩ for the resistor and it worked rather well.

So the equations were:
6 x 230 / (230+1000)= 1.121V          and 
6 x 31,000 / (31,000=1,000) = 5.8125V                         In this case I could power a 6V buzzer when the light did not shine on the LDR. (the buzzer was quiet but it still worked)

First, solder the switch onto the negative battery wire. Then make a three-way connection between one of he parts of the LDR and the negative wire of the buzzer.     Solder the positive buzzer wire to terminal C of the transistor. Solder the LDR's free wire to the B terminal. Connect your resistor (do the voltage divider sum for resistance required) across B and E. 
Solder the LED to the positive wire of the battery and the transistor on terminal E.

Terminals B and E  should all be three-way connections. B= LDR+resistor+transistor     E=resistor+LED+transistor

The buzzer should sound when the laser beam does not shine on the LDR. When the laser is shining on the LDR the buzzer is silent.

Step 3: Setting Up the Alarm

If you are planning on protecting your staircase with the tripwire, then it is best to set it up like so. 

Place both units at the top of the stairs (that is if you are up there) a slight distance between them and preferably one of them higher than the other.

Point the laser at a mirrored object at the bottom of the stairs and align it with the LDR in the receiver.

This way both the units are out of reach and only you can activate / deactivate them. and if the mirror is moved slightly it will redirect the beam, missing the LDR and activating the buzzer.

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