Nuts and Volts - December 2010

Item Quantity 13 21 31 41 51 61 71 81 94 10 1 11 1 12 1 13 1

Reference C1,C2,C4 C3 D1 D2 D3 P1 P2 P3 R1,R2,R3,R4 S1 U1 U2 U3

Description

Capacitor, 0.1uF, 50V ceramic, 0805 Capacitor, 10uF, 16V tantalum, size A LED, Red, 1206 LED, Green, 1206 LED, Blue, 1206 Connector, BS2 programming interface (do not populate) GPS Receiver Module, Parallax #28146 Connector, 9V battery Resistor, 470 ohm, 5%, 0805 Switch, SPST pushbutton, PCB mount, SKHHAQA010 BASIC Stamp II microcontroller, DIP24W 900MHz RF transceiver, Laird Technologies AC4490-200A Linear regulator, L78M05ACDT 5V 500mA, DPAK

Table 1. Bill of Materials.

GOSUB GPS_Get_Lat
GOSUB GPS_Get_Long

Then, the coordinates are transmitted to the base station in a single stream of serial data. The ASCII 'A' and 'B' characters serve as a header and footer, respectively, for the data packet. This makes it easier for the base station receiver to know where the actual start of data is.

contains an on-board gigaAnt chip antenna. Refer to the schematic (Figure 4) and bill of materials (Table 1) for details.

The AC4490 module, U2, is a frequency hopping spread spectrum (FHSS) transceiver that operates in the unlicensed 900 MHz ISM (industrial, scientific, medical) band. It is designed as a drop-in replacement for wired connections and takes in TTL-level serial data, packetizes it as necessary, and broadcasts it wirelessly. Once the module is properly configured, all you need to do is send and receive serial data as you would if two devices were directly connected together — no need to worry about any of the underlying wireless protocols or theory (unless you want to, of course). You can achieve distances upwards of four miles with the right conditions, such as line-of-sight and proper antenna selection. The module provides advanced control via an AT command set, and has optional DES encryption and a choice of wireless network architectures (point-to-point, point-to-multipoint, client-server, or peer-to-peer). D1, D2, and D3 serve as LED indicators. The blue LED, D3, is the power indicator. The green LED, D2, illuminates when the GPS receiver has acquired satellites and is ready to go. D2 will blink if the GPS receiver has not yet acquired the minimum number of satellites. The red LED, D1, illuminates when the GPS coordinates are being transmitted.

A single 9V battery connected through a ubiquitous 5V linear regulator, U3, provides power for the system.

The Firmware

Like the hardware, the code required for the wristband GPS transmitter is straightforward.

Upon power-up, the system initializes itself which consists of configuring the input and output pins, waiting for the GPS receiver to attain a lock on the satellites, and then waiting in a loop until the "distress" button (S1, noted as the "save me" button on the schematic) is pressed.

Once the button press is detected, the system retrieves the current GPS coordinates (using function calls borrowed directly from the stock demonstration code on Parallax's GPS receiver web page).

50 December 2010

SEROUT RF_TX, RF_Baud, ["A", degrees, minutes,
 minutesD.HIGHBYTE, minutesD.LOWBYTE,
 dir, long_degrees, long_minutes,
 long_minutesD.HIGHBYTE, long_minutesD.
 LOWBYTE, long_dir, "B"]

After that, the system sets the "transmitting" LED indicator.

LOW LED_Active
PAUSE 1000

What happens next is a bit confusing and written primarily for ease-of-use during filming. In most real-world cases, once the person presses S1, he would want his coordinates to be transmitted continuously until he was rescued. However, in my case, I wanted to be able to start and stop the coordinate transmission at will using the single button. If S1 is quickly pressed and released, the GPS coordinates will be transmitted over and over again. If S1 is still held down after the coordinates have been sent, the system will reset and the transmission will stop. So, quick push to start, long push to stop.

IF (Alarm_Trigger = 1) THEN
' keep transmitting until button is pressed
' GOTO SendGPSAgain
 ELSE ' if button is held down after
 ' transmitting, reset system
WaitForDepress:
 IF (Alarm_Trigger = 0) THEN GOTO
 WaitForDepress
 ' wait for button to be released
 GOTO Init
  ENDIF

To remove my start/stop feature and change the code to transmit coordinates repeatedly after a single press of S1, you can just call GOTO SendGPSAgain immediately after the PAUSE command and erase the check of Alarm_Trigger.