Thanks for the help, but I obviously wasn't overly clear in my OP. I have a standard DC motor with requires a small amount of power (about a watt), I can control its speed using pwm via an npn transistor, however this motor has a tachometer that I need to read in my sketch, I do this using the interrupt RISING functionality. My main issue is that when I try to use both the PWM and read the tacho at the same time the ticks from the PWM are read as rising on the tachometer cable thus rendering useless my RPM counter (If I hold the motor in place on one of the spots where the tachometer is high, the RPM reads as arduino PWM frequency as each time the arduino's voltage rises the tachometers voltage rises sending out a false count).
I there fore need a way to get an adjustable voltage from the arduino to this motor, it obviously needs as little ripple as possible so that I can avoid triggering the rising interrupt. I have not been able to achieve this with an RC low pass filter, or similar (1 watt load seems to be the stumbling block)...
So does anyone have any ideas,
Many Thanks