When I first started trying to figure out how to build a tripwire i was able to find a lot of different ways to do it but they each had their flaws. Some were too complex, requiring ICs or Arduino. Some were simple but only stayed on while the beam was broken. Finally after experimenting with a variety of methods I came up with a simple laser tripwire that stayed on when the beam was broken.
Step 1: Materials
What You need:
4 100 ohm resistors
1 switching diode
3 transistors. I used 2n2222 but anything similar will work
1 photoresistor (LDR)
1 buzzer. you can also substitute an LED or anything else you want the Tripwire to set off
You will also need a breadboard, jumpers and a laser pointer for assembly
Sorry for the poor quality picture
Step 2: Assembly
Step 3: How It Works
The tripwire is divided into 2 parts. the part that detects the light and sends out the pulse and the "switch" that turns on the buzzer and keeps it on. The reason I have "switch in quotes is because it isn't something like a rocker or toggle switch that change the direction or allow current to pass. This switch simply keeps current flowing through it when it encounters a single pulse. The only way to turn it off is to disconnect it from power.
The first picture shows the Light Detector. When light (or a laser) is shining on the photoresistor the photoresistor alows current to pass. Since we want it to send a pulse when there is no light ( or when the laser beam is broken) we run it into an inverter or not gate. This makes it so that when light is shining there is no output current, but when there is no light there is current flowing through the output.
The second picture shows the switch. this is simply an arrangement of 2 not gates. when power comes into the first gate it stops current going to the second gate. the lack of power then causes the second gate to allow power to flow through to the load ( Buzzer LED ect.) and then acts as a second pulse at the first gate. This causes the cycle to start all over again thus keeping the circuit on.
Step 4: Your Done!
Just set it up so the laser is shineing on the photoresistor, conceal it, and sit back and relax as you watch people trip it.