I built one of those DIY cadence meters (http://www.instructables.com/id/Very...-Your-Bike-12/). For those who wish to try this, a word of warning. I tried two different types of magnets that can be purchased at your local Xmart in the "craft" departments (one type round, another much thicker and square), and neither one was powerful enough to trigger the sensor. I ended up using a strong magnet from a magnetic latch from H-depot (around $2). You have to carve the magnet out, it's about 1" square with a small hole in the middle. I split it into 4 even pieces with a chisel (magnets are very hard and brittle). I attached the magnets with two-sided heavy-duty mounting tape to a very clean chainwheel, and put the sensor on the seat tube (with a triple, that's the only option). Took it on a 25-miler (1/2 gravel roads) this weekend and it worked great!
86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.
I picked up my magnets at Radio Shack. $2/5. PN 64-1883.
That's after buying a pack of "fridge" magnets that DIDN'T work either!
They seem to work fine, doing a brief test. I used a "thick" drop of RTV to secure them. They seem to "stick" decently, but I think a hard jolt may dislodge them.
I found a spot to mount the sensor on the down tube, basically butted up against the BB.
I still have to lengthen the wire though so I can mount the head where I can see it with my bad eyes. Waiting for a friend to stop by that does electronic assembly, since I don't have a soldering iron.
BTW, I used 3 magnets and set the computer to 5555. Gotta save weight!