How can I make a laser alarm?

Question

How can I make a laser alarm? I'm just getting into electronics, so if there is a design that isn't too complicated that would be nice. By laser alarm, I mean those things that set of an alarm when you block the laser from hitting the receiver.

Answer 1

The design is trivial. You need the laser and power supply in one box and a detector, power supply, and noise source or connectors to a further alarm system. Only if you want the laser beam to pulse and the detector to only light with that pulse rate does it get more exciting.
To do the former, you need a photo detector that will react to the color of the laser, a short tube to block as much ambient light as possible, a low power reed relay to handle the higher power needs of the buzzer/bell/etc. The photo detector will come on when hit with more light, so you want a relay that is open when off, so the when the beam is interrupted, the relay loses power and the connection is made.
To do the latter, you will need a laser diode and a 555 timer or equivalent to pulse the laser. You will need a phase lock loop (PLL) on the photocell on the receiver and an adjustment rheostat to match the frequency. The PLL will have an on/off output when receiving the signal.

Answer 2

Might I suggest this book by Forest Mimms.