Store entry sensor

I need a door entry sensor and i cant find what i am looking for anywhere. I want it to be powered anyway (AC DC) EXCEPT BY bateries. I some inexpensive wireless IR units but I hate replacing batteries and one i did buy was activated by any movement in a 50 foot radius of the sensor. I just want an electronic eye beam that wwill send a signal to a bell unit when the beam is broken so i can tell when someone has come into my shop whenm I am downstairs. I figure I can create a using garage door sensors but am not familliar with the electric power neecessary to operate them. Can I hook it up to a power source and have it send a signal to my bell when the beam is broken? Any help here will be appreciated.

If it takes batteries, then you can always connect a DC power source to where the batteries are supposed to go. Get a wall adapter with the right voltage, pull the wires off the battery clips and viola, no more changing batteries.

Or just use an ultrasonic range sensor? Then put something on the door that is in range of said sensor, when the distance readings change too much (you'll have to give it some flex) then make it transmit a signal by using either a 433mhz radio transmitter or a 2.4Ghz transceiver (depending on range) to another arduino with a buzzer or something?

Doesn't have to be optical :3...

I would imagine that this is a solved problem. This has existed in stores for 40 years, probably 80 years now. If there aren't cheap modules on eBay to do exactly this with free shipping then you're just typing in the wrong search terms.

Unless you need it to do more than just ring a bell, I would not expect to use an Arduino.