motion detector alarm for doorway w/no door?

[caedmyn]

I need a motion-detector alarm for a doorway that does not have a door. I've looked for some on Amazon but I'm not sure any are quite what I need. They all seem to have an angle range where they detect motion, and I need one that won't go off til someone is in the doorway. This doorway is between our kitchen and landing. I need the alarm to let me know when my boys decide to escape, but I don't want something that will detect any motion in the kitchen. Does anyone know of a product that would work for this? Or some way to rig up an alarm for this doorway?

[hornblower]

I think you'll want a system that uses an IR beam between two sensors, rather than standard motion detectors. They go off when something crosses the IR beam & interrupts the signal. 

[Catalytic]

The only problem with what hornblower suggests is that the boys may play with them which will knock them out of alignment.  (If you've ever had a garage door, you probably understand what I'm talking about)

 

I wonder if something like this would work, if you mounted the sensor in the top center of the doorway, facing down to the floor.

[caedmyn]

That's exactly what I need, something like the garage door sensors. Maybe DH could rig one of those up for me. The doorway space is a couple of feet deep, so maybe they could be installed at the far side of it which would make it more difficult to mess with.

 

I've looked at the wireless doorbell things but they all say something like that one, 140 degree range x 40 feet so I'm thinking they would trigger off motion in the kitchen.