Miniature Low-rate Motor Speed Controller             by Keith Walker
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This miniature speed controller was designed specifically for Speed 400
pylon racing. It is very small, and without the motor and battery leads,
only weighs about 8 grams. It will work on any number of cells, from one
to eight (probably even as many as 12 or more - I haven't tested it yet).
It drops 0.28 Volts with a motor current of 10 Amps, and will work with
higher currents, if you can afford the extra voltage drop.

It is a low rate controller, but this is no problem with small pylon
racers because low throttle is rarely used, they are either on or off.
I tested the prototype on a small plane with an AP-29 motor, running on
six 800 mAh cells, at about 12 Amps. It flew better than it had with a
larger controller having lower on resistance. The reduction in weight
made the difference.

The circuit is very simple. It uses a 385 [should be 358 -- EJB] dual
op-amp for the pulse integrator and comparator. A transistor amplifies
the rectangular wave output, and drives the MOS FET transistor on and
off at the frame rate of the transmitter. The six volts required to
turn the gate of the MOS FET fully on is achieved by using the sum of
the radio battery and the motor battery. In this way even a single cell
motor battery can be controlled effectively.

Don't be intimidated by the small size of this project. The reason it is
so tiny is because it uses so few components. It really is no more
difficult to make than the previous circuits that have been published
here. If you don't want to etch a circuit board, you can build it on
circuit board that is pre-drilled on 0.1" spacing, and has a solder pad
for each hole. Just use the foil side circuit board diagram as a guide
for wiring the pads together, using the components' leads.

If you prefer to make a printed circuit, but don't have facilities for
printing the resist photographically, you can do it by hand. First cut
a piece of single sided circuit board to size, and thoroughly clean the
copper side, using steel wool or fine emery. Use the drilling detail as
a guide to mark the location of each hole on the copper side of the board.
Using plastic model enamel, paint a circular pad around each marked
location on the copper [etch-resist pads from Radio Shack or any other
supplier will give neater results -- EJB]. Join the pads, using the foil
side diagram as a guide. When the paint is dry, check it for broken or
touching lines. Separate any shorts with the point of a modeling knife,
and patch up any breaks with a bit more paint.

You don't need to use very thick wire for the motor and battery leads.
Remember, the motor will only be drawing about 10 amps, so 18 or 20 gauge
will be fine as long as you don't make the leads more than a couple of
inches long. Drill the holes for the battery and motor leads large enough
for the wire you use. The M+ wire can be soldered right on to the middle
lead of the MOS FET rather than through the board if you prefer. Use the
cable and connector off an old servo to connect to the radio, or if you
don't have one, use a servo extension lead.

There is only one adjustment. Set it up without a prop on the motor. Even
little motors can bite! The range from off to on is fairly small, so it
should easily set up for any radio with a standard frame rate of 50 or 60
Hz. If you find you don't have enough range on the throttle stick, check
your transmitter. There may be stops on the stick that stop it from
moving all the way up and down. (Futaba and JR both do this). The stops
are easily removable, and will give at least an extra five degrees up and
down on the stick.

After it has been adjusted, the assembly can be insulated with a piece of
heat-shrink sleeving. Don't cover the metal tab on the MOS FET transistor
or it may overheat. The motor can be soldered directly to the wires [that
is, PCB traces? -- EJB]. This will eliminate the extra weight and
resistance of a pair of connectors. In small planes, a few grams can make
a very big difference in performance.