In article ,
Owain writes:
JB wrote:
Are there any suppliers of such devices?

Standard intruder alarm PIR, with a 12V relay (car headlight or starter
motor relay?) to switch the high-current load.

Intruder alarm PIR will only latch on for a second or two.
You would probably need some extra logic to latch on for
a reasonable time period. Also, might want some logic
to prevent it operating unless it's dark.

--
Andrew Gabriel