The serial port is a TTL serial port and can be directly linked to the microcontroller. No need for MAX232, the module automatically turns on when powered on and automatically searches for networks. Onboard one signal indicator light (slow flashing when signal is present, fast flashing when no signal is present) Small size GSM module Full machine patch processing, reflow soldering, stable performance The power supply can be directly powered by a potassium battery or 5V. Chip: Working Voltage: 3.7~4.2V Quad-band 850/900/1800/1900MHz Connect onto any global GSM network with any 2G SIM Make and receive voice calls using a headset or an external 8 speaker and electret microphone. Send and receive SMS messages. Send and receive GPRS data (IP, https, etc.) Scan and receive FM radio broadcasts. Lead out buzzer and vibrational motor control port. Material: plastic+metal colour: As Shown Package Contents: 1 x GSM Module 1 x Antenna Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.