Adapting garage door sensors

[DenZelle]

Pretty much a novice here. I want to set up a light that will come on to tell me when the garage door is blocked, then go off when I've backed in far enough for the door to close (leaving maximum room behind the vehicle). I have an extra set of sensors that I can use, if needed, in addition to the safety sensors already installed. Or maybe I can somehow connect the relay to the existing safety sensors (???).

What I don't know -- and haven't been able to find -- is how the sensors actually work. I mean, both of the safety sensors (sending and receiving units) are plugged in to the same places on the door opener. So if it is powering both of them, how does it know when the beam is broken? What happens to tell the door opener that there is an obstruction? I haven't even been able to find the power requirements for these sensors (Craftsman/Chamberlain) so that I can try to play with it to try and figure it out.

What I need help with: (a) How can I figure out the power requirements for the sensors? (b) What do the sensors actually do to signal that the beam is broken (or not)? If I knew these two things, I should be able to connect a relay that powers a light bulb.

 

[MaxHeadRoom78]

I don't have the actual circuitry, but the one with the amber LED is the transmitter emitting a train of pulses, the one with green LED is the receiver.

 

[DenZelle]

So the door opener is sensing pulses coming through those connectors (???).

 

[rjenkinsgb]

So the door opener is sensing pulses coming through those connectors (???).

Yes, from what I can find they are fed via a resistor and when the light beam is not blocked, the receiver momentarily shorts the connections at regular intervals.

More info here; someone using a pair stand-alone:

Garage Door Opener Electric Eye

If yours works the same, it may be possible to use a similar detector circuit but without the power feed resistor, as that exists in the door operator?