How can we wire the Gear Tooth sensor?

[Alan Anderson]

The Gear Tooth Sensor output is a digital signal. Your code is expecting to see it on DIO 1. That's the first General Purpose I/O pin on the Digital Sidecar. Your connection drawing indicates that you have wired it to an analog input instead.

Remove it from the Analog Breakout and connect it to the Digital Sidecar.

I think the "bug" you saw was just a coincidence unrelated to the aluminum plate. It would happen if something was touching the DIO pin and causing it to pick up random noise, triggering the counter repeatedly.