I'm hoping that someone can appreciate what I'm trying to accomplish.
My son and I have been discussing safety and one of the things we
discussed was the safety sensors on the garage door that cause it to
reverse when something is in its path. He wants me to use a spare set
of sensors I purchased to help him build a science fair project to
demonstrate this in class using a simple electrical circuit with a
light connected say via a relay so that each time the beam is broken
the light goes on or off and a buzzer will sound.

Here's the problem, I'm pretty good with electricity and wiring but I
don't get the wiring for the sensors. There are only 2 screws on the
transmitter and 2 screws on the receiver with nothing identifying a
voltage or polarity. I also don't see how a connection is being broken
or made when I was expecting to see a contact closure.

For the sake of keeping dad hero image thing going, help.

Many thanks

JE