{"id":61669,"date":"2025-06-10T15:28:00","date_gmt":"2025-06-10T19:28:00","guid":{"rendered":"https:\/\/www.engineersgarage.com\/?p=61669"},"modified":"2025-06-11T12:15:48","modified_gmt":"2025-06-11T16:15:48","slug":"articles-raspberry-pi-sim900a-gsm-gprs-modem-voice-call-sms","status":"publish","type":"post","link":"https:\/\/www.engineersgarage.com\/articles-raspberry-pi-sim900a-gsm-gprs-modem-voice-call-sms\/","title":{"rendered":"RPi Python Programming 22: Calls & SMS using a SIM900A GSM-GPRS modem"},"content":{"rendered":"

In the previous tutorial<\/strong><\/a>, we covered the AT commands that are supported by a SIM900 GSM-GPRS modem. We\u2019ve already discussed how to interface a SIM900A GSM-GPRS modem with Raspberry Pi, Arduino, as well as other microcontrollers and desktop computers. Now, it’s time to use this modem to make and receive calls, and send and receive SMS.\u00a0<\/span><\/p>\n

In this tutorial, we\u2019ll demonstrate voice call and SMS messaging using a SIM900A with Raspberry Pi (RPi) and a desktop computer.\u00a0<\/span><\/p>\n

Communicating with a GSM-GPRS modem using Python<\/span><\/strong>
\n<\/span>We\u2019ve previously learned about serial universal asynchronous receiver\/transmitter (UART) communication using RPi\u2019s serial TTL port.\u00a0<\/span><\/p>\n

Simply interface the GSM modem with RPi\u2019s serial TTL port and open a serial communication port with the modem using the serial.Serial() method. Next, you can send the AT commands to the modem using the serial.write() method and receive a response from the AT commands using the serial.read() method.\u00a0<\/span><\/p>\n

The response from the modem can be stored in a variable and printed on the console the using print() method. It\u2019s also possible to design a\u00a0<\/span>GUI application using the TTK library<\/a><\/strong>\u00a0and a text editor so the data is sent and received over a serial port \u2014 and shown within a Python application.\u00a0<\/span><\/p>\n

In fact, Python is platform-independent and can be used to set up serial communication on any device and any other platform (i.e. not just RPi\u2019s Linux platform but also others such as Win32 and macOS).<\/span><\/p>\n

On a desktop system, the modem can be interfaced using a USB-serial board. To do so, first figure out the port name where the GSM modem is interfaced with the desktop, via the USB interface. We\u2019ve covered how to\u00a0<\/span>find a serial port name on a desktop here<\/strong><\/span><\/a>.\u00a0<\/span><\/p>\n

Typically, the modem communicates at 9600 bps baud rate and 8N1 UART data encoding. The modems are designed to auto-detect the baud rate. While the data encoding scheme remains 8N1, the maximum 115200 bps baud rate can be used to communicate with the modem.\u00a0<\/span><\/p>\n

Booting the SIM900A GSM-GPRS modem<\/span><\/strong>
\n<\/span>To begin, we must prepare the GSM modem for cellular communication. Insert a SIM card into the SIM cardholder of the SIM900A modem. The modem is similar to a mobile phone and requires a SIM card to connect with a cellular network.\u00a0<\/span><\/p>\n

Most of the modems have SIM cardholders for Mini SIM (2FF form factor). However, Micro SIM (3FF form factor) and Nano SIM (4FF form factor) can also be plugged in so long as the SIM contacts touch the cardholder pins properly.\u00a0<\/span><\/p>\n

Next, turn on the GSM modem by connecting the modem to a power adaptor (5V-12V, 2A). The modem\u2019s status LED or Network LED will start blinking. It typically blinks at a faster rate while the modem connects to the mobile network. Once the modem is connected to the subscribed communication service provider (CSP), it blinks every three seconds. It may take up to a minute for the modem to boot (just like a mobile phone boot) and connect with a subscribed mobile network.\u00a0<\/span><\/p>\n

If you ring the plugged-in SIM, the modem will start ringing via the buzzer and flash the ring indicator LED on it. This confirms the modem is working. In fact, when you call the SIM number, the call is automatically picked up by the modem after a few rings. And if you connect a speaker and microphone to the modem, you can speak to and receive calls on the plugged-in SIM. So, the microphone and speaker input are actually on the modem.\u00a0\u00a0<\/span><\/p>\n

Interfacing SIM900A with RPi<\/span><\/strong>
\n<\/span>The SIM900A modem is a 5V device and Raspberry Pi is s 3.3V single-board computer. We cannot directly interface the TTL serial port of the SIM900 modem with RPi\u2019s serial TTL port.\u00a0<\/span><\/p>\n

If you directly connect the serial ports of modem and RPi, it will transmit the serial data to the modem because the 5V modem can tolerate 3.3V UART voltages. But when you try to receive data from the modem on RPi, the 5V UART signals from modem can damage or destroy RPi\u2019s serial receiver pin.\u00a0<\/span><\/p>\n

To be safe, let\u2019s use a 5V to 3.3V TTL Logic Shifter. A simple 5V-to-3V3 logic shifter can be designed using 1N4148 diode as shown in this circuit diagram:<\/span><\/p>\n

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Alternatively, you can use a 2, 4, or 8-channel level switching I\/O module. This is an 8-channel level switching bidirectional I\/O module:<\/p>\n

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Next, connect the modem’s Rxd, Txd, and ground pins with those of RPi via the level-switching circuit or I\/O module. Now, you are ready to communicate with the modem over the serial port.<\/p>\n

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Making a call from Raspberry Pi<\/span><\/strong>
\n<\/span>To enable serial communication with the modem on RPi, you must import the serial, RPi.GPIO, os, and time libraries. The os and time library are necessary to use the sleep() function, which ensures RPi awaits responses from the modem.\u00a0<\/span><\/p>\n

Begin by setting the pin numbering system to the board numbers and access RPi\u2019s serial TTL port using the serial.Serial() method. The port name on Raspberry Pi is \/dev\/serial0. Alternatively, the port name can be \/dev\/ttyS0 or \/dev\/ttyAMA0 \u2014 use whichever one is the primary UART on the\u00a0<\/span>respective Raspberry Pi<\/a><\/strong>.\u00a0\u00a0<\/span><\/p>\n

Send the AT command “AT” to the modem using the serial.write() method to check communication with it. Read the response from the modem using the serial.read() method and print it to the console using the print() method or to a GUI interface. The modem will respond “OK” if it\u2019s properly working and connected to Raspberry Pi.\u00a0<\/span><\/p>\n