Further comments on this. Chamberlain appear to have lost all their business
in the US. They were the number 1 supplier. The last time I looked, Sears,
Home Depot, Menards & Lowes had all moved over to Genie screw driven
systems. I was unable to buy Chamberlain chain drive spares off the
shelf.(Although they are still available.) Be aware that this may happen
here. My own opener went obsolete 2 years after buying it.

Most photo cell systems operate as normally closed or normally open constant
current switches. It is sometimes possible to add a sensor to an existing
door controller if the electronics can be fiddled appropriately, but it is
not easy. I had to do this, using a Chamberlain photocell system, powered
off a walwart and with the output logic reversed. I don't recommend it. In
the US, there have been a number of children crushed under garage doors when
closing and current practice is always to fit the photo cell units which are
supplied as part of the fixing kit.
Regards
Capitol