Vehicle activity tracker

Charger and power management part contains Li-ion charging IC MCP73831. Charging current is configurable and can be set to provide up to 500 mAh. Li-ion battery is connected to DC-DC converter TPS73801. Pressing S1 button enables TPS73801 IC which produces 3.3V at its output pin. When S1 button is released, DC-DC converter remains enabled due to STM32L152 MCU which sets logical one at MCU_PWR_EN pin. PWR_OFF pin is connected to MCU IO pin to detect system power off.

Communication part is implemented using SIMCOMM SIM900 – Quad-band GSM/GPRS module. This module is quite sensitive to power fluctuations and, thus, requires 100 uF tantal capacitor that holds the charge when high current is consumed (peak current for SIM900 can reach 2A). Two transistors GSM_PWR and GSM_RST are driven by main CPU to manage GSM module power on/off and reset.
External antenna can be connected using on-board u.FL connector.
Device supports full-size SIM cards which can be inserted in corresponding SIM slot.

Device is driven by STM32L152RB microcontroller which handles all the communication with GSM and accelerometer/gyroscope modules.
L series was selected as it aims to be used in application which requires low power consumption.
Device can be programmed using on-board SWD slot. UART port is used for debugging purpose.

To determine device rotation with respect to central axis, activity tracker incorporates on-board accelerometer and gyroscope in single case which precisely measures device orientation with 1 degree accuracy.
The orientation threshold is configurable and can be increased to decreased device sensitivity.