Note: The range of DC044 and V_IN pin voltage input is 5-26V, when use 5V as the power, be sure that power supply can provide 2A current. The range of voltage of Lithium battery input power is 3.5-4.2V.

• Three power input interface: DC044 interface and V_IN and a lithium battery interface.
• Start button: When the board is power on, the LED (PWR) will light up. After a long press (about 2 second) on this button, other three LEDs will light. And one of them starts to flash; this suggests SIM808 is beginning to work. When the power supply, GSM and GPS antenna and SIM card are connected to the module correctly, the LED will be flash slowly (3Second de 1second light), that indicates the module is registered to the network, and you can make a call or do something else.
• TTL serial interface: a TTL level interface. Notice: The pin of VMCU is used to control high level of TTL UART, so as to realize to match between 1.25V/3.3V /5V systems. For example, if you want to use the 51 MCU to control this board, the pin of VMCU should be connected the DC5V. And if use STM32 MCU, the pin of VMCU should be connected the DC3.3V. The pins of RXD is the RXD of SIM808 and the pins of TXD is the TXD of SIM808. The pin of V_IN can connect the Power, the function of this pin is the same as DC044.