Hardware
Right image shows the inside of the built GPS data logger. The GPS data is stored into the MMC/SD card. The external antenna connector was used by the old GPS receiver but it has been replaced by new one and the antenna connector is no longer used.
Power Supply Circuit
There are some considerations different from generic electronics equipments on designing of power supply circuit of automotive equipment. It is what the power source from the battery is painful to the power supply circuit. The supply voltage at 12/24V system is not that kept to 12/24 volts. Because the battery is continuously charged while the engine is in work, the supply voltage rise to around 14/28 volts, and the voltage dips down to 5 volts on cranking the engine due to high current discharge by the starter. If the equipment uses the ACC power, it is swiched off while cranking the engine but this is not a simply switch off. Because many loads are attached to the ACC line, it should be considerd that the input line is shorted to the ground at ACC switch off. As the result, voltage regurator ICs without immunity of Vi<Vo will easily fail. In this circuit, D2 at the input stage blocks the instant discharge by the voltage dip.
Right image shows an example of the wave form of the load dump test for 12V system. This is called load dump surge that generated by failure of wiring. D1 in this circuit is a zenner diode rated 36V/5W to absorb the load dump surge. The GPS data logger is designed for 12/24V system. For 12V system, D1 can be changed to 18V/5W zenner diode and rated voltage of C1 can be changed to 16V.
For easy installation, the GPS data logger is powered by only ACC line. No power switch is supported and start/stop the logging by ACC on/off. The energy for file close process after black out is held in C1. Of course it should be not only proof against painful input but also avoid incorrect operation due to unstable supply voltage. This requires also some considerations on software design.
Power Control of MMC/SDC
When a short time dip of input voltage occured, Vcc may rise again before it reaches to 0 volt. In this situation, the power sequence may not meet the requirement of the MMC/SDC. As the result, the reset sequence of MMC/SDC is not work and some MMC/SDC cards get wrong state and no command is accepted. Once the card gets the wrong state, no way to exit this state except applying a correct power sequence to the card. To support this procedure to always reinitialize the MMC/SDC, a power control circuit to the card socket is required.
