www.nutsvolts.com/index.php?/magazine/article/december2010_Grand
mechanism (that held the lifejacket). Zoz modified some
of the control scripts for the MicroPilot system to direct
the airplane to the desired dropzone with pinpoint
accuracy. This system emulates practices by the US
Coast Guard in which rescue equipment, food, or other
payload is manually dispensed from an airplane flying
over the person or vessel in distress.
The two systems were disparate efforts going on in
parallel, but they both relied on a critical piece of
information — a distress signal and location coordinates
from the person in the water. Both systems required the
swimmer to wear a small, battery-powered wristband
transmitter that — when enabled by a button — would
transmit his GPS coordinates to the receiving base
station (Figure 3). Those coordinates — along with
additional data such as wind speed, wind direction, and
distance to the target — would be processed by the
cannon or UAV control system, and a rescue attempt
would commence.
This system received the GPS transmissions sent from
the wristband and processed them accordingly.
This article details the wristband GPS transmitter used
for both systems. While the episode as a whole was
extremely complicated, this piece is very simple. It
requires minimal hardware and firmware, and is something
you'll be able to quickly put together and incorporate into
your own location-tracking projects.
If you want a complete look into the Flying Lifeguard
episode, all technical documentation I created for the
build, including schematics, source code, and
Figure 3. Base station circuitry built on a Parallax Professional
development notes is available on my website at
www.grandideastudio.com/portfolio/pt-flying-lifeguard/.
The Hardware
The wristband GPS transmitter's core hardware
components are a BASIC Stamp II, Parallax GPS receiver
module, and AeroComm (now Laird Technologies)
AC4490-200A 900 MHz RF transceiver module which
Figure 4. Schematic.
VCC1 10
VCC2 11
U2
5V
5V
0.1uF
C1
0.1uF
C2
RXD
TXD
DTR
Programming Interface
1
2
3
4
P1
Header 4
10k
R1
"Save Me" Button
S1 SKHHAQA010
SOUT 1
SIN 2
ATN 3
VSS 4
P0 5
P1 6
P2 7
P3 8
P4 9
P5 10
P6 11
P7 12
U1
BS2
TXD 2
RTS 8
RXD 3
CTS 7
P8 13 P9 14 P10 15 P11 16 P12 17 P13 18 P14 19 P15 20 VDD 21 RES 22 VSS 23 VIN 24
5V
GO1 9
470 R2 D1 Red LED Active
D2 Green LED
Ready
470 R3
GPS Receiver Module
Parallax #28146
5V
GND 5
GND 16
AC4490
1
2
3
4
P2
Header 4
5V
Power Indicator
1
2
P3
Header 2
9V Battery Input
U3 L78M05ACDT
IN 1 OUT 3
5V
470
R4
D3
Blue LED
December 2010 49