Hi Folks,

Does anybody have any hints on driving motors with the PIC?  I'm still
trying to get my I/R Robot working and my biggest (and hopefully last)
problem is with the motors.

I originally tried to copy the circuit presented in the "Runabout Robot" in
Electronics Now, but I've reached a stumbling block.  The motor driver is an
"H" transistor Driver (a PNP 2N4403 from 3 Volts (battery) to a motor
terminal to an NPN 2N4401 dropping the circuit to Ground).  The Transistor
bases are connected to a PIC driver via a 750 Ohm resistor.

The mechanical pieces I used is from two Tamiya tracked vehicle kits (one
motor and gearbox to drive each set of tracks).  Actually, if you are
interested in building a simple device that runs along the floor and uses
differential motors for steering, Tamiya has the tracked vehicle kit (which
I used two for the motors and gearboxes) and the "wall hugging mouse", which
uses two motors in a similar fasion to the Runabout Robot.

I originally set up the circuit as shown in the article (with an individual
PIC Pin driving a PNP/NPN combination).

Two problems arose from this.

The first was that the PIC was locking up intermittantly when a Motor was
energized.  The second was that the transistor circuit was not driving
enough current for the Motors to run (I measured the motors as requiring
roughly 400 mA - The circuit would only provide 300 mA).

I went after the second problem, thinking that I had a flakey connection
causing the locking up.  I decided to drop the in line resistor value to
increase the current - a 220 Ohm resistor was used.

In doing this, the PIC locked up right from application of power (making the
first problem a lot worse).  Going through the Datasheets (I am using a
'C84), I found that I was exceeding the PIC's Source/Sink currents and this
seemed to be causing the lockup.  I removed some of the load from the PIC
Port (I was using PORTA to drive the Transistors) and the locking up has
gone away.

Now, doing the math, it's pretty obvious that the way I'm going is the wrong
one.  I was going to split out the PNP/NPN pin controls to two PIC Pins from
one, but that will help the Pin Current Requirements, but not the PORT POWER
PROBLEMS (which is what seemed to be causing the PIC to Lock-Up).

I would like to use Darlington Pair Transistors (I've used NPN arrays such
as the ULN2003A in the past to drive relays) to drive the motors.  This
should eliminate the Port Power consumption problems.  Before I jump in and
do this, I have a few questions:

1.  Does anybody have experience with PNP Darlington Pairs?

2.  Can I replace the "top" PNP with a NPN and make the motor terminal drive
like a totem pole output?

3.  What PIC Pin to Transistor Base Resistor Value should I use?  Assuming
an Hfe of 5K for the Darlington NPN Array and current requirements of 600 mA
(for some guardband):

  C-E Current = Hfe x B-E Current ("BEI")
  600mA       = 5K (for example) x BEI
  BEI         = 600mA/5K
              = 0.12 mA

  Resistor    = PIC Output Voltage / BEI
              = 4.8 Volts / 0.12mA
              = 40K

So, from these calculations, a 40K resistor should give me approximately 600
mA through a Darlington Pair NPN Transistor driven by a PIC Pin.  Is this
correct?

4.  Is is possible to do away with the in-line resistor all together?  When
I've used the ULN2003A in the past, I've driven the Darlington pairs
directly from TTL/CMOS.

Ideally, I would like to use four NPN Darlington Pair Transistors driven
directly from the PIC.

Sorry for the long note, I appreciate any answers or ideas you can give me.
And before I start adding parts, I would like to see if what I want to do
has any merit.

Thanx,

Myke

Do you ever feel like an XT Clone caught in the Pentium Pro Zone?