The 'Find me spot' (www.findmespot.com/) device reports the users position via satellite every 5 min.
The problem for using this device is that it quits tracking and reporting the movements of the unit one hour after initiation. We built a small unit to cycle the power and return the device to a tracking mode every half hour. A simple Pic12C629 is connected internally to the battery pack and the On/Off and Help buttons.
Sleep mode keeps the pic from adversely affecting the battery life (3uA draw) and the device starts operating as soon as the batteries are installed.
Fig 1. A view of the outside and top of circuit board
Fig 2. a views of the inside and bottom of circuit board.
Fig 3. Bottom of the board showing pushbutton and power connections and placemeny of the Pic chip
Fig 4. A close up detail of the pushbutton and power connections, via some test points
The unit has operated continually on lithium AA batteries for a week after theses modifications. These batteries were in no way fresh at the beginning of the test as two other tests had the unit running for several days. We estimate a 9 day operational life on fresh batteries.
The Pic software is ridiculously simple. Set the processor up for internal clock, set the watchdog timer for nearly maximum delay (1.15 seconds) and hit the sleep command. When you wake up, increment a counter and go back to sleep. When 30minutes has expired. Power the system off by a 3 sec depression of the on/off switch, power it back on, set it to beacon and start sleeping again.
and the first few lines of the source code:
title "SpotCycle - a simple delay loop for the 12F629"
;
; This Program Controls pins connected to PushButtons
; The chip is connected directly to the battery pins of the
; Revision 1 SPOT device.
; Pin 1 to positive +Vbb can be connected to TP32 on SPOT board
; Pin 8 to negative -Vbb can be connected to TP51 or other ground
; PIN 3 is connected to the On/Off button = TP47
; PIN 2 is the Do button : connect to the .....
; TP3 if you want OK Messages
; TP47 if you want Help Messages
;
; DEVICE OPERATION
; 1) Put batteries in device
; 2) 25 seconds later, the power light will flash.
; 3) Approximately every 30 minutes a position packet will be sent
; 4) Eventually, the batteries will run out (~ 3 weeks ??? )
; - If the batteries freeze up or get jarred, when the batteries
; come back, the system will start up again.
; - If you push buttons, you MAY be able to also send out ok/help/911
; messages, but the power may drop out before they get out.
; - The actual message going out depends on satellite coverage and
; antenna position and random other stuff. You may miss some packets.
; There is no Acknowledgement of reception. this is just how the
; SPOT works.
;