After some time of thinking it would be fun to know exactly how fast or far I am riding, I picked up a computer for my old Peugeot. Did some research and decided on a wired PlanetBike Protege 9.0, (even springing for the temperature sensor for the extra $5). I have never had a bicycle computer before but find it rather temperamental. When it does work, which is most of the time truth be told, it is really neat.
Installed it and initially could not get it to work despite a few days of fidgeting and re reading the directions. Would not pick up the signal even touching the magnet to the sensor. Happened to have a small more powerful magnet stuck to the fridge and put that on the wheel sensor. After some repositioning, got it to work. Worked for a few days before it stopped picking up the signal. Had not touched the setup once I initially got it to work. Readjusted the sensor and magnet and got it to work once more.
Fast forward a few months and centrifugal force overcame the magnetism. Picked up a new more powerful magnet and glued it on this time.
Did I just happen to pick up a defective unit? Surprised it would not work at all without augmenting the magnet. Even then, the alignment seems like it needs to be just right. Am I just a noob when it comes to this particular matter?
It is possible the sensor or wire may be damaged. If the unit is powering consistently that's definitely where I'd start. Place a magnet by the sensor and test for continuity with a multimeter across the two contacts in the mount with the main module (aka head unit, display, computer or whatever else you might like to call it) removed... Now, try rotating the sensor... it's also possible that a specific alignment may be required as is the case with reed switch based sensors (which is what I am assuming here...).
I don't know about the specific technology, but I do know that the specified gap between the magnet and sensor varies widely. This particular PlanetBike model specifies 1-2 mm, which is really tight. CatEye seems to specify 5mm, and the Sigma Rox specifies 12mm.
I wouldn't knowlingly buy a bike computer that specified a gap as tight as the PlanetBike. It's just too tight a clearance to get and keep right.
Most kits come with ferrite magnets... they're quite weak in general. If I bought a computer which specified such a tight clearance I'd swap in a neodymium magnet of about the same size and open the gap a little.
Ballen, if you can't use a meter to test the sensor try this: take a small piece of wire with the two ends open. Set the computer to the odometer. Hold one end of the wire against one of the contacts on the bottom of the computer. Now tap the other end of the wire to the other contact rapidly. If you see the odometer reading change the problam is definitely the sensor. Depending on how small of an increment your odometer will report you may need to keep tapping the wire for a few moments (shouldn't take long though).
The gap was something I did not consider, having never used one before. Chalk that up to experience. I tried testing out the sensor and it does seem to work the way it should. The neodymium magnet I glued on works well. In the end, I suppose the sensor blends in with the oldness of the bike it is attached to. Thanks for the information all!
I've only ever had one bike when I just could not get the sensor close enough. Generally I find I've got a lot more room than they specify.
So you've got everything back up and running? Good. I'm surprised that companies cheap out on the magnet... it's not like the price difference is that much more; just another case of them trying to rake in every single penny.
How did you glue on the magnet and where? It might be good to post up a pic for anyone else who needs to follow your lead. I used epoxy and made a bead of it which wrapped around both magnet and spoke.
I went the simpler route with attaching the magnet. Was going to use expoxy or something hardcore but the glue I was going to use was all dried up in the tube. So instead, just found where the magnet was happy sticking to the original and put a dollop of wood glue and squished them together. Did not bother with a clamp as I figured the magnets pulling on each other would be sufficient. If it comes off, I have another 5 magnets and can upgrade the glue.
The additional magnet is just the thin silver circle on the top (top of the picture).