12V power is applied through a cigarette lighter plug. (A 5V USB cable could have been used, since many newer vehicles have USB. A cigarette lighter to 5V adapter could have also been used) The 12V is filtered and then applied to a switching regulator. This takes the voltage down to 5V. The 5V powers the ATMEGA328 is on a custom PCB. An off the shelf MP3 player with an integrated audio amp was chosen to be a daughter board.
The board used in the assembly is Rev A. The schematic is Rev B. A Rev B PCB layout was not started, since the Rev A of the board was still usable, although with some modification wires.
The MP3 player loads audio files from a microSD card. The main PCB and the supercap perf board were mounted in an old computer speaker. (just one side of the set of computer speakers, there was no need for stereo sound)
The first thing that was apparent was the project could conceivably be replaced by an old cell phone with a custom app. (some apps were found) The app would need to monitor Vin and when the 5V is lost, The audio file to a person about the baby would play. Or, the app could monitor the GPS speed and sound the alert when the vehicle has stopped moving. One downside to this is it might sound at a stoplight or train crossing.
There are a few possible advantages of the stand alone assembly vs. the cell phone.
-No app to configure
-Using supercaps would have a longer lifetime than lithium batteries in a hot vehicle
-Would most likely be louder than most cell phones. At least, the speaker would be larger.
Since hot vehicle interiors tend to cause death more quickly than cold interiors, a thermal trigger point (15C?) was considered. But, cold vehicle interiors can also do harm. So, the announcement/reminder was made temperature independent.