This image shows the signal between the garage door thru beam sensors and the garage door opener head unit (head unit is what I call the main part that actually opens the door). To probe the thru beam sensors, I just added some extension wires on the terminal strip where the normal wires, coming from the sensors, are terminated. I wanted to see what happens when something blocks the electric eyes. Sure enough, the signal goes flat-line when something is blocking the eyes. Only when the path is clear does the signal return to normal (as shown in the main picture on this page).
So, for a normal “not blocked” signal from the thru beam eyes, the signal looks like 6Vdc, with short 1/2 millisecond pulses spaced every 6 1/4 milliseconds. Makes sense, I guess, the thru beam eyes steal / store power from the signal between pulses (as stored energy in a big capacitor) – then, the eyes generate short pulses to ground while using the stored energy from the storage capacitor that is within the sensor eye itself. Well, the sensor generates those pulses if the thru beam is clear. If the beam path is block the sensor simply stops generating pulses. This makes it possible to operate both eyes (one on each side of the garage door near the ground – one eye generating IR light and the other eye receives) using only two wires. Using only two wires keeps the cost down.