I found this circuit using 12vdc and considering the Hall Sensor output voltage is equal to operating voltage does the base of the transistor not require a resistor?
Actually the A3144 output is open collector, giving a choice of 'operating voltage' and requiring a pull-up resistor.
So the BC547 in the post #1 pic would be redundant if the LED needed less than 25mA.
BTW, the A3144 is obsolete.
A3144 3144E OH3144E Hall Effect Sensor is how it was advertised and they have been shipped so I hope to get them soon.
I slipped a 10k resistor onto the base of the transistor and it seems to be working fine through the transistor without the Hall sensor.
I don't see the need for the transistor when the Hall Sensor is the switch.