More good info: It turns out that there are two connections on the ceiling unit. They are marked for the cells.
It turns out my photo cells look the same. They're not. One is a receptor, the other is a transmitter. Both have small lites on them, and they should shine brightly, as long as the units "see" each other. So, we have one receptor, with two leads. We have one transmitter with two leads. These may be infrared--not sure.
I'm wondering which leads on the ceiling unit go to the receptor and transmitter?
I guess this thing works by the light beam interruption breaks a circuit, which causes the door to not come on down.
Still not sure how it breaks the circuit. Maybe with ac as crutshow explains.