Do you know anything about programming such a device (although I suspect there will be someone who has made a program for what you want, all sorts of things available.)
It won't be Rocket Science - I was machine code programming in the mid '70's. But it's that sensor board that clinches it. A 3x3 axis sensor array is a wonderful thing. plus compass - so one can tie Yaw and compass changes into the tilt equation. So it'll light going round corners and not when it falls over.
If I can tie in a GPS receiver we'll have "dead reckoning" too
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Guys - FYI the Raspberry Pi SENSE HAT is now available - horribly expensive - A whole 23 pounds.
The Pi solution is cumbersome BUT cheap - just about anything ancillary you can think of is available.
I will be pricing and ordering this month. But I have to speak to a Pi expert first - Yes he's a 16 year old.
How about a fingerprint reader ignition switch for example?
Ian - the problem with the old cellphone is getting it to switch relays.