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Internet of thing (IoT) systems involves multiple devices that are connected to the cloud. These devices are not always near a power station, so their battery need to last for long periods of time. Microcontrollers are adapted for simple tasks like taking a measurement or activating a signal and since they have simple architectures, they need low power to run.
Arduino is a big player in the IoT field, their open source platform allows writing software for microcontrollers. These devices can communicate using standard protocols like WiFi, Bluetooth or Low Power Wide Area (LPWA) networks. LPWA protocol is adapted for applications that need to communicate in remote places where there is no WiFi available. Sigfox is a great player in this field.
Sigfox partnered with Arduino to produce the Arduino MKR FOX 1200, which can connect easily with Sigfox network. The Arduino MKR FOX 1200 is a designed to run Arduino code, use Sigfox network to communicate and have a battery which run for long periods of time.
In this article I will show how to set up the device, send messages to the Sigfox network and create a serverless architecture to display the collected messages.
Arduino MKRFOX1200 has been designed to offer a practical and cost effective solution for makers seeking to add SigFox connectivity to their projects with minimal previous experience in networking.
Its USB port can be used to supply power (5V) to the board. It has a screw connector where to attach a 3V battery pack. The board consumes so little that it runs on two 1,5V batteries type AA or AAA for a really long time.
It has 8 I/O pins allows for different applications. For more information about the board, you can see Arduino’s page.
The board can be programmed using C or C++. Arduino provide it’s own IDE to write the software, compile it and send it to the device.
For command line lovers there are other options like platformio.io or ino package, but it involves dealing with complex Makefiles, so I still use Arduino IDE for matter of simplicity.
The first part is to get the device ID and PAC number. Sigfox provide an Arduino sketch file, which connects to the board and shows the ID and PAC number in the serial console.
This information is need in the sigfox backend to configure the device and receive the messages. Sigfox has a very well documentation to register their devices.
For development environment, I connected the device from the micro usb input to my laptop usb port. This allows me to power the device and use the serial communication at the same time.
For production environment, I power the device using the battery connection with 2 AA batteries. The constructor announces at least 6 months of power, which I think might be possible using the low power mode between sending messages.
The following image shows a Fritzing schema, which shows clearly how to connect each wire. This is very useful when you want to recreate the project.
I used the sample code from Antoine de Chassey to get information from the DHT22 and send it to sigfox cloud.
Sigfox allows to send up to 140 messages per day, each message can have a maximum length of 12 bytes. This is enough for cases when you need to send only small amount of information, like data from measurements.
As a reminder, 1 byte == 8 bits. So if we want to sent 1 unsigned byte, it can be from 0 to 255. If we need to add a sign, it can be form 0 to 127. For 2 bytes: the range is 0-65535 for unsigned and the half for signed values.
In Antoine de Chassey’s code, this message is passed using the following C++ structure:
typedef struct __attribute__ ((packed)) sigfox_message {
int16_t moduleTemperature;
int16_t dhtTemperature;
uint16_t dhtHumidity;
uint8_t lastMessageStatus;
} SigfoxMessage;
This message contains the following elements:
Finally, we only used 7 bytes in each message. Smaller message reduces transmission times, and increases battery.
Sigfox API allows to obtain messages from all of the registered devices within
a maximum date history of 3 days and a maximum of 100 messages. The endpoint
correspond to the route /devices/${DEVICE_ID}/messages, where DEVICE_ID is
the id of the device. More information about can be found at the Sigfox endpoint
documentation.
I used the library Axios in Javascript to do a get request to the API:
const params = {}
if (limit) {
params.limit = limit
}
if (start) {
params.since = start
}
if (end) {
params.before = end
}
const myurl = `https://api.sigfox.com/v2/devices/${DEVICE_ID}/messages`;
const resulting = await axios.get(myurl, {
params: params,
auth: {
username: UNAME,
password: UPASS
}
});
The variables UNAME and UPASS correspond to authentication credentials, which can be obtained in your Sigfox account page as you can see in the Sigfox API documentation. The parameters limit, since and before allows to control the time range of the response.
The result of from the API seems like 7707d713649c000078c40000, which corresponds to the message in bytes encoded in a hexadecimal string. I used the npm sigfox-parser package to parse this output.
var parser = require('sigfox-parser')
function convertPayload(payload) {
var parsed = parser(payload, format);
var moduleTemp = (parsed.moduleTemp / INT16_t_MAX * 120).toFixed(2);
var dhtTemp = (parsed.dhtTemp / INT16_t_MAX * 120).toFixed(2);
var dhtHum = (parsed.dhtHum / UINT16_t_MAX * 110).toFixed(2);
var heatIndex = computeHeatIndex(dhtTemp, dhtHum, false).toFixed(2);
var result = {
moduleTemp: moduleTemp,
dhtTemp: dhtTemp,
dhtHum: dhtHum,
heatIndex: heatIndex
}
return result
}
The parser function takes two inputs:
var format = 'moduleTemp::int:16:little-endian dhtTemp::int:16:little-endian dhtHum::uint:16:little-endian lastMsg::uint:8';
Once the result has been parsed, it can be used as a Javascript object. I used ChartJS library to plot the result as you can see in the demo page powered by a free heroku instance. The server keep the token save from the web client.
One of the biggest advantage of this serverless architecture is the fact that it doesn’t involve a database, which reduces the complexity of the web application. The code source of the application can be found on this github page.
The Arduino MKR FOX 1200 allows to easily connect IoT with Sigfox network and that way to the cloud. The device consumes so little that it runs on two 1,5V batteries type AA or AAA for a really long time. The constructor announced at least 6 months, which I’m currently testing right now. This kind of device is completely adapted for outdoor applications that don’t have WiFi connectivity or electrical power.
Sigfox proposes an API which allows to get the device messages and simplifies a dashboard architecture.
I would like to thank Sigfox’s team for organizing the Hackster.io Sigfox Universities Challenge 2019, where I got the top 10 price and obtained an Arduino MKR FOX 1200.
I tried the device on prudction environment and having the debug mode on (which means that the led light is turned every time that there is a message transmission) and the battery lasted 3 months. I guess that the led lights reduce the expected life of 6 months.
Having the debug mode off (no led light) the device battery should last more than 4 months, I didn’t get to know how much more it lasted because my Sigfox subscription year ended, so I could use the network any more.
When debug mode was off, I had some issues to wake up the device after entering to the power saving issue. Hopefully, I found this useful post in the Arduino forum which solved the problem.
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