I may have solution, a battery powered TX which is claimed to have a year battery life ( D cells ) and will detect a 100 milli-second beam break.
The angled beam to a passive reflector would work and would be easier to install but the price for a reliable 18 metre reflective sysem is getting beyond the sensible budget.
CCTV or ANPR or a stills camera are in the frame.
A passive sign "ROOF" similar to a car number plate fitted to the wall and projecting out under the thatch has already detered one driver from risking a right turn and taking the left turn instead.
The angled beam to a passive reflector would work and would be easier to install but the price for a reliable 18 metre reflective sysem is getting beyond the sensible budget.
CCTV or ANPR or a stills camera are in the frame.
A passive sign "ROOF" similar to a car number plate fitted to the wall and projecting out under the thatch has already detered one driver from risking a right turn and taking the left turn instead.