The laser switch is a very easy to make and a very awesome way of, well, switching things on and off (or a tripwire, whatever you prefer). It's cheap and reliable if you have a reliable power source.
You will need:
1. Connect the one end of the relay coil to the collector of the transistor.
2. Connect the other end of the coil to +V end of your power source.
3. Connect your resistor and LDR to the base of the transistor.
4. Connect the resistor to +V and the LDR to ground.
5. Connect the emitter to ground.
It's that simple. Now just build the rest of your circuit like you normally would and connect the relay as a switch. See the schematic or video below for help with connecting everything.