Instructions | Android smartphone speedometer

1

wiring diagram
2

cut up some headphones with 4-pin plug

If they have a mic+buttons box, you can pry it open and check if all 4 pins are connected. if they don't have a box, they're probably not.
3

obtain a reed switch

My one is from a cheap speedometer, so it's got a case and a magnet included.
4

salvage a 1M resitor

the resistor is 1M, but I didn't find any specification for the range (it's there to supply for the mic, which is just a fancy resistor for our purposes anyway)
5

connect it all together

I didn't find svg reed switch, but any kind of switch can be used in this way
6

software

once I make a usable hardware version, I will start writing a program to poll /dev/input/... and calculate RPM. Hopefully it will be fast enough, and no kernel driver will be needed *fingers_crossed*

Android smartphone speedometer