Description Chip: SIM800L Voltage: 3.7-4.2V Support networks: China Mobile, China Unicom and global quad-band network Module size: 2.5cmx2.3cm TTL serial port for serial port, you can link directly to the microcontroller. No need MAX232 Power module automatically boot, homing network Onboard signal lights all the way . It flashes slowly when there is a signal, it flashes quickly when there is no signal