Figure 5. Prototype circuitry mounted in a waterproof
Tupperware bowl and held together with tape.
The great thing about prototypes is that they don't
have to be pretty to get the job done!
prototype using a BASIC Stamp Board-of-Education with
the GPS receiver and AC4490 connected to it with wire
harnesses. We'd be using this version for some real-world
testing, so it had to be waterproof. I crammed it into a
Tupperware bowl and held it all together with some
Gorilla Tape and hot glue (Figure 5). Instead of using a
pushbutton switch that would require me to physically
activate it within the waterproof enclosure, I used a
reed switch mounted inside the lid and enabled the
system with a magnet.
In the small cove outside our warehouse in San
Francisco, I swam out approximately 20 meters with the
bowl and circuitry strapped to my arm. After verifying that
Zoz could receive my GPS coordinates, I increased my
distance from shore little by little until I was almost at the
other end of the water. Our tests were enough to prove
that the concept worked and I proceeded to design the
custom circuit board for our "production" model.
Since there are very few components in the circuit,
designing and routing the PCB was relatively painless. The
final board was two layers and measured 2. 9 inches long
by 2.1 inches wide (Figure 6).
Figure 7 shows the final assembled unit. The AC4490
module connects to my board via a double-row header on
the back side. The front side of the board contains all the
other components. While I could have spent time
dramatically shrinking the design by replacing the larger
off-the-shelf modules with discrete GPS and RF
functionality and going with a smaller microcontroller and
battery, it wasn't necessary. This design worked just fine
for our purposes. The 9V battery (not shown) sits to the
side of the board.
As the finishing touch, Zoz used Rhino 3D to design a
two-piece, arm-mountable enclosure (Figure 8) complete
with red "distress" button (which made physical contact
with the pushbutton switch on the PCB) and Prototype
This logo. The circuitry and battery sit snugly inside. The
enclosure was printed out on a Z-Corp Z450 multicolor
3D printer — a rapid prototyping tool that builds solid
models out of layers of powered plaster glued together
one after another. To make the enclosure waterproof, the
two halves were infiltrated with epoxy and sealed together
Figure 6A-B. PCB layout: Top layer with silkscreen (left); bottom layer (right).
December 2010 51