What sensitivity of reed switch should I use?

Question

For my GCSE Engineering project, I am building a device which attaches to the frame of a bike, and uses a magnet (Attached to the wheel) to detect 1 turn of the wheel (I have also compensated for contact bounce etc.) The only problem I face is that I have never used reed switches before, and I don't know what sensitivity of reed switch to use.

Assuming the bike wheel is 2150mm circumference, the max speed is 13 m/s, the magnet diameter is 50mm and it will be 180mm from the centre of the wheel, I calculated that (hopefully) the smallest amount of time the switch will be directly passing the magnet is 0.0037 seconds.

I thought this seemed like a very small amount of time for a switch to close and open again before 1 rotation is over. A 1AT switch apparently closes at the north pole from the earth itself, so just to be safe i want to avoid disaster from it being too sensitive. Does anyone know what type/sensitivity of reed switch to use?

Answer 1

Somewhere around 25 AT will be more than enough to prevent accidental triggering from the earth's field, although you could go higher depending on your magnet and the proximity to the switch.

The trick is to get the alignment right. You want to align the magnet and reed switch so that the reed switch is magnetized along its length. See https://standexelectronics.com/wp-content/uploads/Reed-Switch-And-Magnet-Interaction.pdf .

If your magnet is 180 mm from the center (360 mm diameter), the magnet will pass in .0073 seconds, and a reed switch should be fine. They have operation times on the order of .0005 seconds.

Good luck!