SS443A Unipolar HALL sender

It senses magnetic field (B=8..18 mT), can detect fast changes (unlike a reed relay, for example). Usual as crank and camtriggers, wheel speed sensors. It's for automotive and industrial use. Often packaged into easy to mount units that usually sell for 29 Euro. This one needs to be mounted using epoxy glue or silicone, use your mechanical experience.
Click this to see how Mik mounted his
It's small, about 3mm, but it has 3x20mm pins, see SS443A datasheet (click this) (HALL SENSOR, Honeywell, -40 .. +150C, DIGITAL temperature compensated 75-180 GAUSS, COHS compliant, NOPB)
Technical hints:

• unipolar HALL sensor, so the magnetic field direction (north / south) matters.
• We always used capacitor (100-220nF) between GND and +5V, near the pins of the chip.
• Note that pullup resistor is necessary (eg. 2k7 resistor between output and +5V). The sensor cannot by itself output a positive voltage, it can only pull down it's output to GND. This is often called "open collector" signal.The signal must be pulled up by external means. If output is constant 0V, consider this: forgetting the pullup resistor is the most common mistake. Luckily it doesn't cause damage. Just apply the resistor and it should work.

See also TLE4935L item, but that one is bipolar, which must see once north, once south direction magnetic field.
The unipolar is preferred by most installers for easier mounting (less magnets needed). Actually, the packaged Honeywell 1GT101DC sensor (see sensors) is better choice in most cases, much easier to mount. Choose that unless you have very restricted mounting space.
some general information about unipolar HALL sensors