SS541 Hall Effect Sensor
From CunningWiki
The SS541 is a surface mount hall effect sensor. When a magnetic field is present the sensor will trip and output a high signal.
Spec Sheet: http://sccatalog.honeywell.com/pdbdownload/images/ss5.series.chart.1.pdf
Vin : 3.8 V ~ 30 V Current - Output (Max) 20mA Output Type Digital, Open Collector
Package SOT-89B
Alternative 3 lead package in SS441