Randy wrote:
My shop has no windows so on nice days I like to keep the overhead
door open. Well some kid watched me welding for awhile then decided
it was safe - snuck in and stole my laptop computer that was connected
to my CNC mill.

Got him on video, filed a police report.

I need some kind of photocell and beam across the doorway to alert me
if anyone walks in. Can garage door safety beams be used? I have a
couple of friends that can wire up or build me a circuit to make them
work, I just don't know how the work, is there a NO or NC output on
those? I was told you cannot just jumper them out, so I'm thinking
it's more like a signal output rather than a switch closure output.

Thank You,
Randy

I know one place that uses an am or fm radio. When a person activates a trigger (Opens
the door and finds themselves inside a small cubicle) the radio blares out at about 120
db. - in the shop but well heard in the cubicle.

You can use many different triggers. The beam works fine but leaves a visible trail in
dusty or smoky conditions. You may want to install one to give them something to dance
around while they activate another one. :-)

A very simple motion sensor works well. You can rig a flood light sensor so that a bulb
comes on or you can screw in a plug and have it activate the above mentioned radio or
anything you can plug into that outlet. Sensor does not have to be mounted on the flood
bulb set.

Personally, I'd use a couple different switches so they never know what or which triggered
the notice or you may make them progressive. The first one could turn on a light or
sound a small chime in the location you expect to be in. This one could figuratively say,
"Someone just rounded the corner of the building." The next one could sound a different
alarm and let you know their progression. The time lapse in between the two would tell
you their approx. speed (sneaking, running, etc.) When they walk past a yellow lined
do-not-enter area (or whatever you choose) you could make it sound like all hell has
broken loose! This way they'd know that you knew! Bells, whistles, lights, the works! :-)

It can be as simple or complex as you wish to make it. First, You need to know. Second,
They need to know that you know. They could be casing the place for a return when you're
not around. You can get auto dialers to call you or anyone else if the place is entered
during certain periods of time. I know of one auto dialer that calls several different
numbers and announces that a high level alarm has been triggered or that there seems to
have been a bearing failure in a lift station - giving the identity of the station.