I've proved to my satisfaction that this little bit of magic can be made to work. Any ideas for or against will be received with thanks.
The idea is to have a PIR sensor in the motorhome. The one I bought to try the system is meant to work patio lights and it won't do. It works but it only works in the dark and there is no sensitivity adjustment on the sensor. PIR stands for Passive Infra Red. A motion sensor.
Here's it laid out on a board to check the theory using it to light up an LED strip.
It works fine – but only in the dark so this particular sensor won't do. But these might: I've ordered two.
Automatic 8A Infrared PIR Motion Sensor Detector Switch For LED light DC 12V-24V | eBay
The idea is to replace the LED strip with a relay. The PIR operates the relay and the relay operates a horn. I plan to use two circuits – two PIRs – to operate two relays to operate two horns. One in the cab to disorient and panic (I hope) thieves and one on the roof to attract attention – mine in the house and any neighbours too. Separate so that disabling one leaves the other operating. This idea can only be used while the van is unoccupied. No good while you're in there asleep.
The whole thing to be switched on by a key operated switch in the power supply – but outside the van – perhaps hidden in the gas locker or the loo cassette locker. Wiring to be hidden behind panels or inside cabinets.
MagiDeal Electric 2 Positions 1NO 1NC SPDT Keylock Switch, AC 250V, 5 Amp with 2 Keys 19mm: Amazon.co.uk: DIY & Tools
The weak point is, perhaps, the sensor. Break it and maybe it doesn't work – though maybe it fails safe in the on position if broken while on but I'm not yet ready to try that by breaking one. The internal horn to be hidden maybe behind a speaker grill so the thief can't simply silence it by ripping it off or under the seat as someone else mentioned and, maybe, the PIR sensor behind glass disguised maybe as an internal light for the same reason – more experimentation needed.
It would be easy to vary this idea and wire a horn to courtesy light door switches.
What do you think? If it's so simple why isn't it being used?
And ... no, we haven't replaced the van yet. Lotsl of trivial reasons but I think we're still worried about the next one being stolen too.