You could use a simple IR trip alarm. A 'stupid' IR modulator placed at a single location and a receiver module that looked for that IR signal. If something walks inbetween the transmitter and the receiver it would know and you could send the information using a wireless module to your wrist display. I would't recommend LEDs though you should be looking at the field not your hand, the motor from a pager or old cell phone are perfect, if you use a low voltage you can't hear anything but you'll be able to feel it, perfect silent alarm. Not a quick and easy project though, good to get to know micro controllers, simple RF modules from sparkfun can be pretty cheap. Worth looking into. More than one sensor would require more receivers and possibly more 'beacons' Although if you aimed the LEDs right you could use a single beacon like a star, where the recievers are the points. You could use the vibrate for a beam break alert and status LEDs for each of the sensors. It could get relativly complex.