Updates in the version 3:

1. Power circuit update: previously a 3.7v battery is needed as the A9G power supply. 

in the new version 3, the module can be powered either by battery , or directly by USB.

2. Battery voltage Monitor, to check the battery voltage

3. USB voltage monitor, to check if there a USB power connected;

4. 3.3V DC-DC module updated to 1A Version

5. SIM808 DTR connected to MCU, to make SIM808 enter sleep mode