Keep those suggestions coming folks! I'm perfectly
happy to provide a forum for the exchange of ideas.
Now, on to our questions ...
Q. Do you remember the name of the robot that
had moving eyes and mouth, and answered
the phone?
— John S.
Dayton OH
A. I remember something about what you are asking,
but my memory told me that it wasn't human (you
didn't specify) but some kind of an animal that would
signal the user that he/she had a call. If I'm not completely
off base here, I believe that you are referring to Stephan
Marti’s Cellular Squirrel. This was a project whose design
criteria was to have an inoffensive object — in this case,
a stuffed squirrel — signal the cell phone owner of an
incoming call. It would blink its eyes, move, and generally
provide a non-offensive visual cue. You can find information
about Stephan's project at http://web.media.mit.edu/
~stefanm/phd/cellularsquirrel/
Figure 1 shows the squirrel and Figure 2 shows how
much technology is stuffed inside it. (Photos courtesy of
Stephan Marti.)
Q. I usually read your article first when I get my
magazine. I have a question about using a voice
coil actuator for small quick motions — such as
eyelid blinking on a puppet or similar action — and
controlling it with a PICAXE-20M. Would the outputs
from the PICAXE have to be coupled through a Darlington
transistor or something to protect it from possible voltage
spikes from the coil? Also, how does one figure out how
many windings and of what gauge wire to make the coils
for five or twelve volts? I was thinking about using some
small rare earth magnets due to their size and high power.
Would using voice coils for some motions save space and
be simpler? The article on the servos in the puppet head
was very informative.
— Dale G.
New Jersey
A. Thanks for your support! I'll answer the easy
question first: in no case, should you drive an
inductive load directly from a microcontroller I/O line.
The surge currents could quickly damage the port. You
could perhaps protect against the voltage spikes, but it is
far safer to use a driver transistor (Darlington not required)
to drive the coil. Size your transistor to handle the current
that you are expecting and use a drive resistor between the
I/O pin and the base of the transistor; a 1K, 1/4 W resistor
is usually fine for this task. The PICAXE is based on the
Microchip PIC microcontroller whose I/O lines can typically
drive 20 ma, which is more than enough to control your
driver transistor. Do make sure that you guard against the
Figure 1. The Cellular Squirrel.
Figure 2. Servos and boards and wires!
inductive kickback (CEMF) from the solenoid with a diode
just as you would for driving a relay. See Figure 3 for an
example circuit.
I would not think that a speaker voice coil would have
the power to do what you want. You could create your
own solenoid using a magnet and winding your own coil,
but really this is a topic for an entire book. Magnetics isn't
exactly voodoo, but it is complex. I suggest that you use a
solution that someone else has already created, especially if
it is inexpensive. Use a small solenoid, for example. I found
one at Jameco ( www.jameco.com); PN 1919203 that
SERVO 02.2009
15
