Rather than hacking into a cell phone, it became clear that we could buy off-the-shelf M2M cell phone boards that would be easier to program and more reliable.

Several problems identified during Phase One were addressed in the Phase Two design:

• The operation of the cell phone, having been designed with a human interface, was often unpredictable. Occasionally, messages would pop up prompting a human reply (like a notification that the phone was charging). This made programming the device difficult because all of the possible messages and the timing of their appearance were not predictable. For example, at one point the devices stopped working because the cellular service provider required all of its users to upload new operating software. An appropriate user response was not possible in field-installed, pre-programmed units. Because of this problem, we adopted industry-standard M2M (Machine to Machine) methods of sending data over a cellular network. The Phase Two communications assembly, mounted externally to the nest in a 3" diameter PVC pipe anchored in a bucket of concrete, contains another custom circuit board. This "communications board" has a MSP430FR microprocessor, an RS485 transceiver, a 3.3V power supply for the microprocessor and the sensor, a switchable power supply for the phone board, and headers to plug in the M2M cell phone board.
• Text messages and disposable cell phones were not a cost-effective way to send large amounts of data. Phase Two uses FTP protocols with devices and data plans that have much less expensive data charges.
• Hacked cell phones would be difficult to produce in the quantities needed by wildlife managers, so the new design uses off-the-shelf, plug-in cell phone boards and custom circuitry that can be mass produced.
• Phase One used single-use D cell alkaline batteries. Phase Two uses rechargeable NiMH AA batteries and achieves longer battery life, as the package is carefully designed for low energy needs.
• The silicone filling the Ping-Pong balls that housed the sensors in Phase One never cured. For Phase Two, we made a casting of a Ping-Pong ball and used it to cast solid polyurethane "eggs" around the sensor circuit boards. The polyurethane is fully cured in less than a day, does not out-gas, and is very hard and durable.
• The sensors had reliability problems communicating with the cell phones because of the long distance between the sensor and the microprocessor in the Communications Unit. To correct this, a custom 1" x 1" "Smart Sensor" board was designed to include, besides the sensors, a microprocessor and an RS485 transceiver that allows reliable communication over very long cable lengths. The board and components are encapsulated in epoxy and then sealed in a polyurethane ball for environmental protection. Most of the turtle-specific calculations are now done in the Smart Sensor.