REVIVING AN ANDROBOT BOB: Part 2
just wasteful. In this case, a 560 ohm
resistor worked out well.
For the most part, I had used 74
series TTL chips in projects and hadn’t
used CMOS chips in new projects. The
Handy Board is all CMOS based for low
power, so I kept that convention when
making the logic for the Sonar MUX
board. One thing that comes up with
CMOS is to never leave any inputs
unconnected! This is something you
must remember when working with
Everything was wired up fine for the
first test but I did it on the bench without
connecting all three selection lines to the
Handy Board. When power was applied,
all five of the relays and the associated
LEDs went crazy! This happened because
an unconnected CMOS input can oscillate wildly and as a result the relays
would energize at random. This wouldn’t
be a problem as long as it was connected to the Handy Board or some other
controller since the input would not float.
To make it work on the bench and
also in circuit, I just added three 10K
pull-down resistors on each data selection line. This ensured that no lines
would be un-connected and would
default each line to a known state. It
took care of the problem nicely. If for
some reason one of the selection lines
goes open, it will always be low at the
sonar MUX board. Otherwise, if a lead
opens up, the relay board could start
acting up and cause problems.
When I went to connect the
Polaroid sonar ranger to the expansion
board, I got quite a surprise. The Handy
Boards expansion had a nine-pin connector. The docs that came with the module
also showed a similar nine-pin connection. Should be easy to connect, right?
(Well if it was, I wouldn’t have had as
much to write about.) As it turns out, the
module I was going to use only had an
eight-pin connector. Since pin 9 is normally used, I had to find the appropriate
pinout for the eight-pin module and then
wire up the connection to the Handy
Board to compensate for the difference.
After some online research of the
Polaroid sonar modules, I found the
details I needed to wire it up. A table
showing the mapping for the nine-pin to
eight-pin modules (at least for the
module I was using) is provided in this
article. Since I didn’t have that odd
nine-pin connector on the Handy Board’s
expansion, I just used a length of ribbon
cable to make the connection.
There are five lines needed to drive
the ranger. I soldered one end of the
cable directly to the expansion board
for GRN, INIT, ECHO, and BINH. A fifth
line for +6V power came from a dedicated voltage regulator on the custom
power board that was specifically there
for the ranger. These all went to an
eight-pin .100 inch straight connector
to plug into the module I was using.
If you do a similar project, you can
avoid all this by using one of the newer
SensComp (replacement for Polaroid)
ranging modules which already has a
matching nine-pin connector for the one
used on the Handy Board. The only
concern is with the power supply for the
sonar if you start altering the Handy Board
power supply like I did on this project. You
need to be sure it gets reliable power.
Table 1 shows pinouts for common
nine-pin and eight-pin Polaroid sonar
modules. NOTE: Before trying out any
sonar modules like this, make sure that
the transducer is connected to the
ranging board or you can damage the
How boards look inside the robot.
but that will never happen again!
WARNING: When the Polaroid
sonar ranging board is in operation,
the voltage applied to the transducer
can reach several hundred volts! Even
though I’ve been aware of this and
worked around these types of sensors
for years, I finally got nailed when
testing this. It only stung for a minute,
After everything was wired up
with the correct pinout for the sonar
module (eight-pin vs. nine-pin), I ran
one of the test routines in the Handy
Board Libraries called sonar_display().
The initial test was done with a single
transducer wired up directly to the
ranger module. It is always best to test
each section and add a little at a time
so it is easier to troubleshoot any issues
that may (okay, will) come up.
When the tests ran, I could hear the
sonar transducer clicking and sending
out the pings like it should but I wasn’t
getting any valid data back on the LCD
screen. It is set up so that if it doesn’t
get a reading or it is out of range, then
it shows as the distance.
Some troubleshooting was in order.
Was the Polaroid sonar board bad? Was
the wiring/pinout correct for the module?
Or was it some other problem? I knew it
must be close or the module wouldn’t
have been firing the sonar at all.
SERVO 03.2008 47