The Hall Effect Sensors I am using are Allegro A1301. The reason I chose these is that the voltage swing from North to South Pole is almost supply voltage. The quiescent voltage is half supply voltage. When using these with the picaxe with a 5 volt supply it is very easy to set up the ADC. The usual Hall Sensor, UGN3503U,  when supplied with 5 volts gives a voltage swing from North to South Pole of 1 volt. About 1v South, 1.5v quiescent, and 2v North Pole. These sensors would also be perfect when used with my OpAmp decoder, as the OpAmps will detect minute voltage changes and output the OpAmp supply voltage. The only change required is to set the adjustable reference voltages to a lower level.

Note there are 2 sets of 3 Hall Sensors. Each set is mounted on its own acrylic (perspex) plate. The plates are adjustable relative to each other, and the whole assembly is adjustable to the coils for fine tuning. The secondary set of sensors allow for over length magnets, and the decoding board takes care of this. Power savings may be possible by adjusting the length of the on cycle for each step.