I found this sensor in a treadmill [picture attached]. There is a pulley attached to the motor shaft which has a small magnet. The sensor sees the magnet every revolution, I guess.

What kind of sensor is this? How does it work? Seems like a hall effect magnetic sensor. But it has only two wires, which got me confused.

 

Probably a reed switch

 

Either a reed switch as mentioned and there are some proximity switches that are available with 2 conductors.
Max.

 

Certainly. Thank you. Do you know if this is a reliable method of measuring speed?

 

A reed switch does not have very high operation capability so only used on relatively slow applications, if high speed is needed use the prox variety.
Also switch bounce is more prevalent on the reed ver.
Max.

 

I think it is unlikely to be a reed switch if the end of the tube is oriented toward the magnet.

I think it is probably just a pickup coil - wire wound on a ferromagnetic (probably just iron) core. Each time the magnet passes a pulse is generated - typically something rather like a single sine wave.

A variable reluctance sensor works in a similar way, except the coil is wound on a magnet and it will sense a passing ferromagnetic object. They are often used for things where multiple pulses per revolution are wanted and using multiple magnets isn't practical. They can detect things like the teeth of a large steel gear.

 

If you search Digikey for Omron E2E DC 2-Wire these are the Prox type that detect a ferrous object.
Max.

 

Probably a hall effect sensor. They are often used to detect magnets for tracking the speed of spinning things.

 

You can also pick up these hall effect gear sensors for an Auto wrecker, they are also made by Honeywell and used for crank gear tooth timing/sensing.
Max.