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.