Aaron wrote.

> Can anyone help
>
> I am wanting to control a motor from my pic 16c84.
> I want to be able to vary the speed of the motor.

A simple one directional speed control can be achieved with a logic
level mosfet.
I dont know the current consumption of the motor or its supply
voltage but for a 12 volt motor at 1 amp or so you could use an
Nchannel logic level mosfet such as RFD14N05 (can get from Farnell)
which would easily handle most punishment typical inflicted upon such
a driver. No heatsink should be needed with such a motor.
The source pin goes to ground, gate to the PIC i/o you wish to use to
control it, and drain to the motor -ve, the motor +ve goes to
+12volts (in this example). You will need a "flywheel" or back EMF
diode across the motor to stop back EMF damaging the mosfet, a 1N4001
would connect anode to motor -ve, cathode to +12v.

Now the motor will go full speed if a high or 'one' is presented
continually to the motor SO to vary the speed switch the mosfet on
and off with a varying duty cycle. This will vary the speed.
The frequency will be chosen at set up and then not varied, only the
pulse width or duty cycle will be varied.
I have found that 400Hz is a good frequency for your average little
DC motor. Serious ineffiencies start to creep in the higher the
frequency goes with motor inductance being one problem, the worst
problem at very high frequencies, which the PIC is quite capable of
doing, is the large-ish gate capacitance of the mosfet starts to load
the PIC i/o pin to the point that 20mA will be exceeded if the
frequency gets too high.
PWM routines can be found at many sites including www.microchip.com,
the PICLIST archives etc.

Hope this has been some help.


_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
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