make it function using standard servo
pulses, it sounds like you prefer a
quick and straightforward solution,
probably using 12V if this is a real
wiper motor. If you can use three
microcontroller pins, then you can
read the potentiometer position using
an analog-to-digital converter (one pin
to ADC, built into most micros) and
use two pins to drive the motor via an
H-bridge. You can buy an L298N H-bridge for a few bucks on eBay or roll
your own if you like relays as much as
Figure 8 shows how to use two
SPDT relays in an H-bridge
configuration to drive a motor. The
right diagram allows power-off motor
coasting or dynamic braking.
Another approach to making a
big powerful servo would be to copy
ServoCity’s “power gearbox” format
shown in Figure 9. Put a pinion on a
servo to drive a much larger output
gear. In this case, you would modify
the servo to allow 360 degree
rotation, and you would connect the
servo electronics to an external
potentiometer on the output shaft.
This would be controlled exactly like a
regular servo, but keep your fingers
out of those gears!
More info is available at https://
Q. Back when I was in school (I rode my dinosaur up hill, in the snow, both ways!), we
had an Apple II computer hooked up
with a tether to a small robot called a
“Turtle” and it was programmed in a
language called “Logo.” I would like
to get my grandson such a robot to
help his technical education. Is there a
Logo programmable Turtle robot you
A. The Valiant Turtle was a “recent” LOGO based drawing robot, sold from 1983-2011.
They turn up on eBay occasionally. I
certainly want one for my vintage
collection! Honestly for your
grandson, you may prefer a more
modern expandable PC based robot.
Parallax’s line of “Scribbler” robots
(Figure 10) is fantastic.
They are not kits, but come ready
to roll with built-in programs to drive
around, avoid obstacles, seek light,
flash LEDs, make sounds, follow lines,
and more. They can all draw on paper
with a marker (Figure 11 — courtesy
Carol Hazlett) —
much like a
can do so much
Kids can learn
languages. Parallax has a fantastic
The green S3 is their latest robot
and has an entire STEM education
course built around it.
Check it out at https://www.
It’s hackable and expandable, and
can be programmed using either a
drag and drop GUI (great for kids),
BlocklyProp, C, or Spin, which is
Parallax’s own language for the
Propeller processor which runs the S3.
Highly recommended, I own one and
my seven year old twin girls can use
the GUI to drive the S3 around, flash
lights, and make sounds. It’s a great
value at $179; you’re getting a LOT of
robot plus a giant 4,000 mAh
rechargeable LiPo battery.
The red Parallax S2 is easily found
on eBay, and it is also a great
Propeller based robot platform which
can draw, and has many of the S3’s
other features. It can be programmed
using a simple GUI or Spin language. I
own several. The original blue
Scribbler is BASIC Stamp 2 based.
Also available on eBay/Amazon, it
can be programmed in BASIC or using
a GUI. It also draws, but with less
accuracy than the S2/S3 since it lacks
wheel encoders. Nonetheless,
“Old Blue” came first and
holds a special place in my
heart and collection. I own ...
you guessed it ... several.
That’s all for now, folks.
Like I said, I’m passionate
about robots so ask away if
you have questions. If I don’t
know, I’ll try to find out. See
you next month! SV
26 SERVO 01.2017