On Fri, 29 Nov 2019 10:34:48 +0000, Andy Burns
wrote:

T i m wrote:

I was going to investigate a(n invisible?) break-beam sort of device
that could be set higher than most ground dwelling creatures and below
the typical flight line of a bird.

dual (modulated) beams are supposed to be best for least false triggering.

Funnily enough, that's the sort of thing I built for BIL's
Arduino-automated Model Railway project, the sort that wouldn't be
'confused' if the were in view of another transmitter etc.

It's just that they were for very short distances, not sure if they
would cope over 5m or so (especially if reflecting from the other
end)?

I have seen such used across driveways where people might regularly
use them to turn round (near a school possibly) where they make an
audible alarm to indicate to people 'I know what you are doing'.

Cheers, T i m