servos and the other for the rest of the circuit) and placing
optical isolators between the servos and the control signals.
In this configuration, the grounds of both power supplies are
not tied together.
A simple schematic for isolating the servos from the rest
of the circuit is shown in Figure 1. The 470 Ω resistor is used
to limit the current to the LED. The 1K resistor is used as a
pull down resistor to ensure that the control signal to the
servo is at zero volts when the LED is off. Any optical isolator
should work in this circuit. The PS2501-2 from NEC has two
separate optical isolators and works well in this application.
Using a circuit like this between each one of your servos
should solve the noise problems you are having. If not, the
noise is coming from a different source.
Q.I’m writing in regard to the Hexatron robot plans
you featured in your November 2003 issue of
SERVO. First of all, I realize that none of this is your
problem or your fault, but I’m hoping you can help.
I’m new to robotics, but very excited about it. I decided
to build the Hexatron robot — my first non-toy robotic
project. For about two months now, I’ve been trying everything I know of to find the parts for it. This includes Emailing
the author and many other people. Though I’ve had to go to
dozens — if not hundreds — of sites, I’ve found all but the
4.55K potentiometer. Would you have any idea where I
might find this part? I thought I had it at www.
Stampbuilder.com but I was wrong.
The problems I have been going through to find these
parts are very disheartening. I can’t help but wonder how
many people new to the robotics hobby have turned away
due to such problems. I’m tempted to start my own robot
supply business after this experience, but I can’t even do that
until I find a source for the remaining part I need. The author
of the article — Karl Williams — did a fabulous job on the
instructions and pictures. It’s a shame that he did not give
enough information on the parts to allow a novice hobbyist
to build the robot. For instance, there are dozens of
variations of semiconductors that have “2N4403 PNP general
purpose” as part of the description.
Thanks for reading my gripes. If I can just find this last
part, I’m sure the project will be a blast.
— Richard Alexander
Broken Arrow, OK
A.Welcome to the fun — and sometimes frustrating —
world of robotics. You are not alone here; every electronic hobbyist in the world experiences the same type
of frustration that you are experiencing now. One of the nice
things about electronics is that every part can be substituted
with a different one and perform the same function. Often,
these parts are not even remotely similar to one another.
The trick is learning how to pick the right parts to substitute
with the parts you can’t find. Actually, there is no trick or
black magic here; all that is required is to figure out what the
part is being used for in the circuit. Understanding basic
electronic fundamentals helps you, along with experience.
First off — if you cannot find the electronic parts at
Digikey ( www.digikey.com) or Mouser Electronics
( www.mouser.com) — then this should tell you that
they are not everyday, commonly used parts; hence, they
will be more difficult to find. When it comes to searching for
difficult to find semiconductors, I use FindChips.com
( www.findchips.com). If FindChips doesn’t locate them,
then they will be very hard to find.
Now, let’s take a look at the Hexapod circuit as an
example to learn how to substitute parts. The 4.5K Ω
potentiometer is connected between the +5V power supply
and the ground. The potentiometer’s center tap is connected
directly to the analog-to-digital converter (ADC) port on the
PIC. Since the ADC port is going to draw almost no current
from the potentiometer, the potentiometer is going to act
like a variable voltage divider. Thus, depending on the
rotational position of the potentiometer, the ADC port on the
PIC will see an analog voltage that will range between 0 to 5
volts. In this configuration, any potentiometer — regardless of
its value — will do the same exact same thing.
The next question to answer is why use a 4.5K Ω
potentiometer? First, from Ohm’s law, the current that will
go through the resistor will be limited to 1.1 mA (V/R). Since
this is a battery-operated robot, you will want to minimize
the amount of current that is being wasted going through
Circle #110 on the Reader Service Card.
SERVO 07.2004 47