Tachometer Part 2

After using my bicycle speedometer-tachometer for a while, I decide to test the Schwinn speedometer that I saw on Amazon for $10. Although it uses the same programming values that I show in the table from my earlier Low-cost Tachometer article, it has some differences that I thought I should point out.

  1. The maximum “speed” on the readout is limited to 99.9 (mph or kph). You can accommodate 100 RPM or 1000 RPM or whatever you want to program, but the resolution will remain 1 part in 1000. The earlier Bell Speedometer specifies a limit of 200.0 mph or kph and I assumed a max reading of 199.9.  It turns out to my surprise that the display has another digit and goes well past that. I’ve had mine up to 458.0. Even at the documented 200.0 mph or kph limit you have 1 part in 2000 resolution. So the Bell Speedometer gives a little better resolution if that is an issue.
  2. The update time on the Bell Speedometer is one second and the update time on the Schwinn speedometer is two seconds. This has plusses and minuses. If you’re trying to grab an RPM reading quickly as in the Flywheel Spin-down work measurement application, you might want the one second update. For most of my work I prefer the 2 second update because the longer period lets it read lower RPM and have less variation. Using 4 magnets and programming the units so I read 1 kph as 1 RPM the minimum speed on the Bell Speedometer is 15 RPM and on the Schwinn it is 8 RPM. The Schwinn speedometer also has a larger display that is easier to read at a distance although the Bell Speedometer display is quite usable.
  3. For either unit don’t plan on using the included magnet for mounting on a bicycle spoke (unless you’re using it for that application). Us a strong magnet and use larger magnet if possible so you can have more clearance between the magnet and the pickup. I use these units to measure RPM in a variety of temporary applications and the larger magnets let me locate the pickup about half an inch from the magnets. Smaller magnets typically require smaller clearances.