HALL SENSOR USING SS49E #16

Hall sensors basically senses the magnetic field. What's Hall voltage is when magnetic field is passed perpendicular to the conductor carring a current in certain direction will produce a voltage accross its body. What google says is

1. The Hall effect is 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. It was discovered by Edwin Hall in 1879.

Now what hall sensor does is when magnetic field is detected hall voltage is generated within the sensor and voltage is given as output.
Hall sensors are available as digital , analog and some very complex progarmmable.
For example SS49E by honeywell is cheap analog sensor.
TLE4997 by Infineon is programmable hall sensor.
MLX90242 of Melexis is a Hall switch.
Besides there are many other types. You can check it out in digikey list http://www.digikey.com/product-search/en/sensors-transducers/magnetic-sensors-hall-effect-digital-switch-linear-compass-ics/1967232?k=hall%20sensor

Hall sensors are availabel mostly in SOT23 packages and SIP packages, besides there are other packages but not used mostly.
One of the common use of Hall sensor in RPM measurement in CPU fan.

The LD1117 ,here, is SOT223 package adjustable voltage regulator. SS49E is 5V device but it can be used for lower voltage. 

The no field output voltage is VCC/2. When field is detected according to polarity of the field its voltage will rise or fall. 

The sensitivity is 1.2mG/mV.

The sensor can be used for the sensing current. According to direction of the current the output voltage will rise or fall.

The PCB is some what like