input to comparators 24 26 respectively in driver circuits for the odd and even open coils 12 respectively the resistors r 1 r 4 when activated by their associated transistors t 1 t 4 are in parallel with resistor r a in the upper half of the voltage divider circuit 20 the resistors r 5 r 8 when activated by their associated transistors t 5 t 8 are in parallel with resistor r a in the upper half of the voltage divider circuit 22 thus it can be seen that by selectively turning on or off selective combinations of the transistors t 1 t 4 selective combinations of the resistors r 1 r 4 are changing in this case raising the voltage in the voltage divider circuit 22 and consequently the resulting voltage input to comparator 24 similarly by selectively turning on or off combinations of the transistors t 5 t 8 selective combinations of the resistors r 5 r 8 are provided to the voltage divider circuit 20 and selectively provide a voltage level input to the comparator 26 preferably although not necessarily the resistors r 1 r 4 are all of different values thus providing sixteen different possible combinations of resistors and thus sixteen possible voltage inputs to the comparator 24 preferably the resistance values of resistors r 5.