> I want to occasionally control a ~1.5V DC motor from a PIC circuit
> running from 3V.  The speed isn't critical.  Power consumption, both
> while the motor is running and when not, is important (it's battery
> powered, and this will be where the bulk of the energy goes), and space
> could be important later.  It's a one-off hobby project, so I'm not
> worried about saving every last penny in components.
>
> It seems like overkill including a full SMPS.  Would it work to feed a
> pulsed 3V to the motor through a transistor at a suitable (possibly
> empirically determined) duty?  I'm a bit worried about (a) ruining the
> motor, and (b) wasting lots of current in the protection diode during
> the off periods.
>
> I'm intending to use a 12F629 or 675 which only has one spare pin,
> although it's possible I could free up another for a comparator/ADC.

Simple PWM output should work fine.  If you don't care that much about the
motor speed and only want to not overdrive the motor, then 50% duty cycle
from your 3V supply should be good enough.  All you need is a square wave
of a few 100Hz, which can be easily produced in software to any output
pin.  The only gotcha is that 3V is a bit low to turn on most FETs, so you
can either spend more money on a low gate voltage FET, or spend a little
extra current driving a bipolar transistor.  Don't forget the flyback
diode.


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