Cheap human-sensing from ceiling - recommendations please!

Dan Williams wrote 04/05/2015 at 19:30 1 point

I'm working on an interactive art project. I need to sense people's movement around the gallery (the artwork, hung from the ceiling in a series of nodes, will respond to it).
I don't need high resolution, but I need to cover an area about 10x20m, with 6' tall dividers in it - the ceiling is high, but I can't guarantee covering it with one camera.
Obvious options include camera(s) mounted high up, or a network of PIRs or infrared proximity detectors hung above the space.
I'd like something reasonably cheap (low total cost, preferably < £50, or low cost per sensor, preferably < £2 - assume I've already got a raspi & arduinos or similar, so don't worry about those), something that won't require too much calibration, and preferably something that will spot children as well as adults (so proximity type sensors would need to have the range to spot someone 2' tall, as well as 5-6' adults.
Before I embark on building and testing a few options, I'm sure someone else must have some experience? Big thanks if anyone can share any ideas, recommendations, or even "this failed" type stories.