Earlier model Gen 2, SPOT GPS devices, can be purchased second-hand for less than AUD $60. on eBay.
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| Second-hand Spot Gen 2 from eBay |
At the time of writing, this modified SPOT GPS Gen 2, is located approximately 350km east of Sydney, in the Pacific Ocean, and still working well.
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| Tear down |
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| Front of PCB Assembly showing Patch Antenna, Switches and LEDS |
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| Rear of PCB Assembly |
The modifications to the SPOT device consist of:
- Using an Arduino Nano microcontroller to activate the SPOT GPS every four hours.
- Packaging the new assembly in water tight container for deployment on a model boat.
The SPOT GPS is normally operated by pressing buttons to switch it on, and selecting a message type to send, and switch it off.
The microprocessor is used to simulate pressing the buttons and also observes whether the device is currently on or off. This is necessary, because the action to turn on the SPOT GPS is exactly the same action required to turn it off. A human operator can observe the LEDs flashing an know that it is turned on. The microprocessor needs the same insight.
The following is an outline of the minimal steps to be performed by the microprocessor to cause the SPOT GPS to transmit a signal.
Minimal Operating Steps
Step 1
SW5: POWER ON - hold for about 5 seconds .
LED2: POWER LED - Flashes Green every 3 seconds while powered on
Step 2
SW4: Send OK Message. - hold for about 5 seconds
LED5: OK Message LED - Flashes Green every 3 seconds
or
SW6: Send Custom Message. - hold for about 5 seconds
LED8: Custom Message LED - Flashes Green every 3 seconds
Step 3
Once a message has been sent "Message Sent "
(Note: there is no means of acknowledgment of message being received)
LED4: Message Sent - Flashes Green every 3 seconds once a message has been sent.
Note all three green LEDS appear to flash in unison every 3 seconds.
Step 4
SW5: POWER OFF - hold for about 5 seconds .
LED2: POWER LED - ceases flashing every 3 seconds
PCB Notes
The following section covers observations about SPOT GPS Gen 2 PCB with details relating to how a microprocessor would interact with it.
Inner Rings of all switches are ground.
Outer rings can be pull low to ground to activate switch function.
The LEDs are bi-colour Red/Green LEDs with grounded cathodes. The ends marked with an arrow on the PCB are the grounded ends.
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| Front of PCB showing Push Button Switches and LEDs |
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| Rear of PCB showing Test Points with colour coding to indicate their purpose |
Test Points marked RED are linked to the RED LED Anodes
Test Points marked GREEN are linked to the Green LED Anodes.
Test Points marked BLUE are linked to the Button outer rings.
3uA is the measured current flow when the button rings are grounded.
(@3V implies 1MOhm)
The connections between the Arduino Nano and the SPOT GPS Gen 2 are as follows:
Wiring Details
-------------- ------------------------------
| Arduino | | SPOT GPS Gen 2 |
| Nano | | |
| D9 Pin xx |----47kR-->| Power Switch |
| | | |
| D12 Pin xx |<---47kR---| 3Vdc Switched (power sense) |
| | | TP(unmarked) |
| | | |
| D10 Pin xx |----47kR-->| Ok Message Switch |
| | | |
| D11 Pin xx |----47kR-->| Custom Message Switch |
| | | |
| | | |
| | | |
| Gnd |-----------| Gnd |
-------------- ------------------------------
The image below shows the battery connections: BAT1- and BAT1+.
It also shows an unmarked Test Point which provides a switched 3Vdc signal, which provides a 3Vdc signal only while SPOT GPS is on.
This unmarked Test Point is very important because it is the simplest way for the Arduino microprocessor to determine whether the SPOT GPS is on or off.
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| Rear of PCB showing Battery Connections and the Unmarked 3Vdc Switched signal used to indicate that the device is On. |
Power Supply
The SPOT GPS is normally powered using three lithium AAA Cells. These cells have an open circuit voltage of about 1.8Vdc, or 5.4Vdc for three of them.
It was decided to power the SPOT GPS using four AA Alkaline Cells with a combined open circuit voltage of 6Vdc.
I did have a slight concern about operating the SPOT GPS at a slightly higher voltage compared to its unmodified form, but it has been tested continuously in this configuration for over 10 months without any problems.
The Arduino Nano is powered separately with pair of AA Alkaline Cells yielding 3Vdc.
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| The Assembled SPOT GPS with Arduino Nano and Battery Pack ready for packaging. |
Enclosure - Sistema
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| The Complete Assembly prior to the final Silicone Sealing |
The modified SPOT GPS with the Arduino Nano and Battery power supply were packed into 300ml Sistema food container, along with about 100g of Silica Gel desiccant to absorb any moisture.
The Sistema Food containers are a good, low cost housings for electronic projects, with an effective lid closure mechanism. They are not fully water tight, however.
It is necessary to use a sealant to ensure they a fully water tight.
Ordinary silicone sealers purchased from the local hardware store must not be used in confined spaces with electronic equipment. The chemicals released during the curing process is typically acetic acid which is quite corrosive and harmful to small electronic components. The local hardware stores do supply a range of silicone sealer types, including some described as "Neutral Cure". But these are still not suitable for use with electronics.
I use the Silicone Sealant of type Alkoxy. It is safe for use with electronics in a confined space and does not cause corrosion of electronic components. It is only available from specialist suppliers.
Dow Corning Silicone Sealant 798 is an example of Alkoxy type Silicone Sealant.
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| Dow Corning 798 Silicone Sealant |
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