something like this ?

I'm assuming you want the detector to be operated by the IR emitted by Mrs G or a vehicle, and then to send a radio signal to the shed where a receiver engages mains powered floodlights, cameras, watch towers or whatever  and the increased current draw on the outbuilding supply alerts you to put the kettle on back at G-HQ ?

The sort of thing already  linked to will do it, though an alternate thought is that if you don't want other folk  and or random wild animals to turn on the lights,  could you save a bit of installation time and use one of several types of  wireless keyfob switches?

Mike.

Exactly so Mike, although perhaps not the second part of the paragraph.

Unless I've misunderstood the scant instructions the link shows some wild eyed strigiforme plugged into a house socket to warn you of imminent arrival. Perhaps with a twit or a woo depending on the guest.

I'm frustrated because I installed exactly the system I want at a massive country pile to light up their rather grand approach steps but due to my rapidly declining cognative abilities I can't remember the manufacturer. Although I think Wilts were the supplier.

Mind you, it was fraught with problems and I ended up digging up Sir's flowerbeds and laying cables. He said "you  can't beat a bit of wire, can you?"

I was rather hoping technology had advanced a bit since then.

This kind of thing? https://www.tlc-direct.co.uk/Products/QUPIR.html

(not tried myself, so can't recommend, but looks reasonable on paper). There's also Mike's keyfob option: https://www.tlc-direct.co.uk/Products/QUFB1.html

  - Andy.

Thanks Andy, I did look at that but, apart from the aesthetics, it only has a range of 20m.

Legal radio links that can be installed anywhere and don't need a license are power limited to avoid causing interference to other similar uncontrolled systems.  This does tend to mean that the useful range is a bit hit and miss - and actually I'd take any quoted distance figure with a large pinch of salt - a great many things affect radio propagation and there is a tendency of the marketing types to quote the best performance ever seen when measured in ideal conditions as if it was typical.

That said getting antennas as high as possible, and oriented the radiating elements parallel at both ends of the link and away from obstructions can go a long way to making things more reliable.

Mike

https://www.tlc-direct.co.uk/tcl/search?query=easyswitch&sort_order=&category_id=&sub_category_id=&manufacturer_ids=

I have not used this range but they have been sold for a long time by tlc, HTH

Thanks for that, looks promising.

Cheers.