How to make Hall Sensor

First thing, what is Hall Sensor?
Hall sensor is mechanism to measure parameters like rotation, pulse, speed, flow rate etc. using Hall effect.

Then What is hall effect?
the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current.

discovered by Edwin Hall in 1879

Edwin Herbert Hall (1855-1938)
Born	November 7, 1855 Gorham, Maine, USA
Died	November 20, 1938 (aged 83) Cambridge, Massachusetts, USA

Now To the point?

What will we make today?

We'll make a rotation counter.

Take a permanent magnet, and attach it to any portion of your bicycle wheel (maybe with spoke). Then Make a wire coil with a soft iron core. Then attach this coil with stationary part of your bicycle, so that the permanent megnet rotating with wheel passes by the coil, not touching but close. And must align the iron core inside the coil paerpendicular.

Then when the permanent magnet come close to the coil, according to hall effect you get a pulse of hall voltage.

here a graphing calculator shows the pulses through picks. And we also see the time required for rotation.

Now just you need to count the pulse.