that $30 VEX add-on kit into something that can
actually drive servos and digital outputs. One last
note before we move on. As the timing is controlled
by the VEX transmitter, we can actually run this circuit
using the internal 4 MHz clock source. If you do this,
you can leave R3, the OSC socket, and the resonator
off the board. I put them onto mine so I have options
— you can see in the photo of the completed board
(Figure 3) that R3 and the socket are installed, but
the resonator is not.
DOUBLE IT UP
Having such a svelte circuit leaves us with a bit
of a dilemma when using ExpressPCB’s mini-board
service: There’s a ton of unused board space. Should
we let this go to waste? Absolutely not! Let’s double it
up. When I started laying out the board, I found that
the circuit would comfortably fit in half the space of a
standard mini-board. Excellent — let’s just copy-and-paste
and get two boards for the price of one.
Not so fast, there, chief. Before we double-up any
of your boards, we need to do a thorough check of the
layout using a link to the schematic. This will save us a lot
of trouble later; not all (as I found out), but most. Save the
single board file separately so you can come back and
update it if necessary.
I did, and here’s why. While having lunch with my
“networking” pal, Peter, he talked about making generic
boards as generic as possible, and this really is the case
with this board. It dawned on me — especially having just
written a servo animation driver for the Prop-SX — that I
could add another connector and make this board a
standard servo controller.
If you look closely at the
layout, you’ll see that the RJ- 11
sits on top of a three-pin header;
this allows me to stuff the board
two different ways based on what
I want. The RJ- 11 allows me to
make the standard VEX decoder,
or use phone cable for my input.
If I want to create a standard
servo controller for a BASIC
Stamp or SX project, I’ll replace
the RJ- 11 with a three-pin servo
Figure 4 shows a screenshot
from ExpressPCB with the
completed layout for one board.
After this file is saved, it’s a simple
matter of copy, paste, and then
adjust position (while everything
is still highlighted) of the duplicate
parts. Save the double board as a
separate file. And note that once
we’ve doubled things, using the
“Highlight Net Connections” tool
■ FIGURE 3. VEX Decoder Ready For Testing.
is no longer functional as we have duplicated part
Since we did a “background” servo driver last May
I won’t go into that, but what I will show you is how
I created the servo animation driver I mentioned earlier.
Many artists use a program called VSA (Visual Show
Automation) for running props and servo-based
animatronic displays. VSA allows one to integrate servo
movement and sound very easily, and has become a
favorite, especially with its low price (about $50).
VSA uses the SEETRON (Scott Edwards) MiniSSC
protocol as its default. Being a very clever guy, Scott made
■ FIGURE 4. Single Board Layout.
May 2008 19