The SIM800 Cellular Module and Arduino: A Powerful IoT Combo

The SIM800 Cellular Module and Arduino: A Powerful IoT Combo

The SIM800 from SimCom and the Arduino UNO are two of the most popular development modules to get you started with IoT.  This article discusses how to connect physical objects and the digital world in new and creative ways.

The Internet of Things (IoT) has been a trending field in the world of technology. From consumer electronics to business and industrial processes, IoT has changed the way we work.

The SIM800 from SimCom and the Arduino UNO are very popular due to their support from the hobbyist and developer community.

The SIM800 is a cellular communication module that can make calls, send email and SMS texts, and even connect to the internet.

The module is intended to operate like a mobile phone, but it needs external peripherals to function properly. The SIM800 can do a lot of things but in this article we will focus on the internet capabilities of the module.

Adding the SIM800 module with an Arduino will enable you to develop countless innovative projects. Your imagination is the limit.

The SIM800 module is not only great for maker projects, but it can also be an affordable and viable option for use as a cellular communication module in a production product.

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Considerations

Let’s start by reviewing the pros and cons of the SIM800 so you can decide if this module is the right solution for your project.

Pros

• Easy to develop and a popular choice among both hobbyists and product developers
• Extensive features including voice calls, SMS, and internet access
• Small size is ideal for portable products and wearable tech
• Very affordable module from a Chinese manufacturer
• The manufacturer of the SIM800, SimCom, also offers numerous other communication modules including the SIM868 which includes built-in GPS functionality.

Cons

• Older 2G technology with limited data speeds (GSM/GPRS). GSM is also being phased out in most regions.
• Not necessarily the smallest, lowest power or highest performance cellular module available. Nonetheless the SIM800 is a great general purpose cellular module.

WARNING: The SIM800 is an older GSM cellular module, and unfortunately most regions of the world are actively phasing out GSM cellular networks, so it is not recommended for new designs.

Programming & Prototyping

Basic Connections

The connections for a basic serial communication setup between an Arduino and the SIM800 are quite simple. All you need is three lines between the SIM800 and the Arduino. This includes ground plus UART send and receive lines.

You need to have the following hardware components and installed software on your PC as listed below.

Figure 1: SIM800 and Arduino UNO basic connections.

Components

• SIM800L breakout board
• Arduino UNO board or similar
• MicroSIM Card
• USB cable (Arduino to computer)
• 3.7V Lithium Polymer Battery (Supply for SIM800L)

Software

• Arduino IDE (Installed to your PC)

References

• Arduino Reference
• SIM800 AT Commands reference

Pin connections:

Now, insert the microSIM card into the back of the SIM800 module. Then, connect the SIM800 TX and RX to the Arduino D8 and D7 respectively. These are the serial UART connections which allow communication between the two modules.

Keep in mind that the send pin on the SIM800 connects to the receive pin on the Arduino, and the receive pin on the SIM800 connects to the send pin on the Arduino.

Note that the Arduino Uno uses 5V GPIO whereas the SIM800 uses 3.3V level logic and is not 5V tolerant. This means the TX signal (D7) coming from the Arduino Uno must be stepped down to 3.3V so as not to damage the SIM800. There are several ways to do this but the easiest is to use a simple resistor divider (1.69k for the top resistor, and 3.3k for the bottom resistor is a good choice).

Next, connect the antenna to the mini socket located in the upper left area of the module.

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Power Supply for SIM800

The SIM800 needs to be powered by 3.7V which is a common standard among most cellular modules. One might think of using the Arduino’s 5V and 3.3V supply from the Arduino but this is not advisable.

The SIM800 is specified to use a power supply in the range of 3.4V to 4.3V. So, using 5V could damage the SIM800 module and 3.3V is not enough to reliably power it.

Therefore, an external 3.7V Li-ion Polymer battery is used as the power source for the module.

Ideally, a power decoupling circuit is added to the input supply. We will discuss this circuit in more detail in the next sections.

Serial Monitor

The serial monitor is your interface with the Arduino and it communicates with your SIM800 as well. You can use the serial monitor to check the current status of the device. Also, you can send commands through the serial monitor to communicate with the Arduino.

Simple Serial Communication

The code below will enable the Arduino to communicate with the SIM800. Initially, we set up the pins and initialize a serial communication link between the two devices. Then, we set the baud rate to 9600.

In the looping part of the code, the SIM800 will continuously wait for any inputs from the serial monitor and send it to the SIM800 through the D7 pin (RX for SIM800). It will also continuously read the D8 pin (TX for SIM800) if the SIM800 has any responses.

Once your setup is complete, and your Arduino is connected to your PC, compile the code above and upload it to the Arduino.

Now that we have established a basic connection let’s try to communicate with the SIM800 module by sending AT commands through the serial monitor.

AT commands are short command keys used to execute the desired functions on the SIM800. We send these codes through the Arduino and it will relay the commands to the SIM800 for execution.

The most basic AT command you can use is “AT”. When you type or request this command in the serial monitor the response will be “AT” as well. This response verifies that the Arduino is communicating properly with the SIM800.

Take note that other commands will have different responses based on your requests.

Downloading a Simple Web Page from the Internet

It is fascinating to see how the SIM800 module can download a web page. You can do this by executing a series of AT commands through the serial monitor.

One thing to do first is to check for signal quality. There are times that we may not be able to connect to a network internet. What we need to do is to check for signal quality. Use the “AT+CSQ” command to check the cellular signal level.

This is not a prerequisite for establishing a connection to the internet but it will be helpful when monitoring the signal level during debugging.

In this walk through, we will request a web page that contains weather forecast information in JSON format from the Weather Underground website.

Let’s first check the signal quality by typing “AT + CSQ” to verify.

Next, let’s set the device to full functionality mode. This is already the default mode but just to make sure, we will explicitly define it.

Next, we connect to a data network.

The next commands are important settings for connecting to your network properly. Verify the correct APN for your data network. I am using SMART as my network and ‘internet’ is its APN.

Next, let’s establish a web session by setting up a browser-like environment for the SIM800. First we initialize and set the HTTP settings.

Then input the website URL you wish to download.

The next command tells the SIM800 to start downloading the web page.

Once the page has completely downloaded, you may view the whole content of the web page through the serial monitor.

Below is the list of the most common AT commands for the SIM800. There are even more AT commands available, but this list will get you started with defining settings and connecting to a network internet.

What else can you do with the SIM800?

The SIM800 can be embedded into hand-held or miniature products such as wearables. Also, you can build a simple home automation system, a remote wireless sensor or a GPS tracker using the SIM800 and an Arduino.

NOTE: This is a long, very detailed article so here's a free PDF version of it for easy reading and future reference.

Sample Project: Low-cost Weather Forecast for Farmers

One of the projects I’ve created is a low-cost, weather forecasting device using the SIM800 and a Gizduino (an Arduino clone). The project provides a 10-day weather forecast represented through colors.

The device locates the user and downloads weather data from Weather Underground, a popular weather forecasting service. Here’s the link to the full case-study.

One of the SIM800 features used in this project is the ability to identify the location of the device. A rough estimate of location was sufficient for this project, even though it is not nearly as accurate as GPS.

The acquired location is used as the reference location for the weather data being requested. For such an application there really is no need for the accuracy provided by GPS, and cellular triangulation provides more than sufficient accuracy. The series of commands are listed below:

SIM800 Schematic and PCB Layout Considerations

If you wish to embed the SIM800 module into your product, you should keep the following considerations in mind.

The SIM800 datasheet suggests adding a power decoupling circuit to the input supply of the SIM800 module. This will provide a stable power supply and prevent surges that may damage the SIM800 module.

Three capacitors (100uF, 33pF, and 10pF) and a Zener diode connected in parallel with the supply make up the decoupling circuit.

Figure 2: Power decoupling circuit for SIM800 3.7V supply

GSM Antenna

There are two ways you can add an antenna to your product or device when using the SIM800. You can use a mini RF coax connector (recommended) or an SMA connector.

The RF interface has an impedance of 50 ohms. The trace impedance with the antenna should also match this impedance with 50 ohms as well.

Designing RF traces for the SIM800 antenna is critical and complex. You may want to ask for guidance from an RF expert since antenna traces are one of the most common mistakes made on PCB layouts.

To avoid any unnecessary FCC certification complications you should closely follow the manufacturer’s layout guidelines.

Summary

The SIM800 is an excellent choice for providing 2G cellular functionality to both maker projects and production products. It’s small size, low-cost, and ability to communicate via a simple UART interface using AT commands makes it an excellent choice for many applications.

The SIM800 is also a very popular module with a large support community of makers and product developers than can help you work through any technical issues. They are broadly available from numerous suppliers and a variety of development kits based on the SIM800 exist to help you get started.

This article was contributed by Joseph Ricafort. Joseph is a Test Engineer who builds electronics projects in his spare time. He wants to build small projects that provide big impacts to those in need. His weather forecasting project for farmers was selected as the winning project for a recent Predictable Designs contest.

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