what is the name of this device?

[sonaiko]

i want to do this project.. an electronic circuit that when someone comes through the door the alarm starts... like an alarm system i mean..

what is the name of the device i need to use that can give a signal or smthng to the circuit when someone crosses the door! i see them doing that using a laser beem in movies but i guess there is smthng more real to do that!!! :lol:

can u help me guys? hope im clear!

 

[chemelec]

Here is a Laser Alarm that I designed.
Although this one is for made for LONG Distances, Short distances are also easier.

**broken link removed**

 

[LordError]

ooopss

nice design. ill try to assemble

 

[Teletech]

It's called a photo-electric beam.

 

[Oznog]

what is the name of the device i need to use that can give a signal or smthng to the circuit when someone crosses the door! i see them doing that using a laser beem in movies but i guess there is smthng more real to do that!!! :lol:

can u help me guys? hope im clear!

Also called "electronic eye", or whatever they want to market it as.

They don't use lasers except in the movies. It's unnecessary and obviously could be seen.

They normally use a lens-focused infrared emitter and a lens-focused detector. They could be on separate sides, more commonly, they put a piece of retroreflective material on the far side so the emitter and detector can be in one place.

It uses an AC signal because changes in lighting conditions can make the detector see higher/lower levels than the emitter is responsible for. But if you create a 5KHz square wave for the transmitter and a 5KHz bandpass filter on the receiver, you can be sure nothing but the signal the transmitter created will come through.