Crazy! I just bought an arduino to work on this exact project. I’m pysched there is someone else interested in this project!
Background:
With a local vegetarian advocacy group, I manage a handful of free literature stands around my city. We use this model of stand: http://www.goplastics.com/ss-5c.shtml In them we offer free vegetarian starter kit magazines, with recipes, health info, reasons why people become veg (actually, like this one: http://www.goveg.com/ORDER.asp).
To track distribution numbers I bought a cheap digital scale on ebay. I measure the weight of the remaining magazines before I refill the stand, then weight the stack again afterwards. Take the difference to know how many I added and divide that by the weight of one magazine to get the number of mags.
About the actual circuit:
I think weight sensors are the best option. For my application, just measuring once a week when I show up to fill the stand would be sufficient–no need to have a tweeting stand or take measurements daily to get a more precise rate. But knowing the daily rate would be kinda neat too… Anyway, I’ve been looking at the flex sensors: http://www.sparkfun.com/commerce/categories.php?c=143 Not sure what other kinds of weight sensors exist.
I do not think a height sensor would work well because the magazines often do not stay in a perfect stack. Being outside, they often get bumped and moved. Sometimes they get crumbled or folded. The stacks don’t stay vertical very well. I guess I could mod the stand with wood to make the magazines stay more orderly. Seems like using weight is less obtrusive.
Another thing I foresee being an issue is taring. If the weight sensor is placed permanently under the newspapers/magazines the circuit may not have a zero starting point. Not sure how to address that.