How to create a geofence using Arduino

Geofencing is a technology that creates virtual boundaries around a specific physical location using tools like GPS, Wi-Fi, RFID, or cellular networks. It enables control over the movement or use of an object or device within a defined geographic area.

To establish geofencing, virtual parameters are set around a designated area using special software that defines boundaries in any shape or size, such as a simple circular radius around a fixed point. When a device or object crosses the boundary, it triggers predefined actions, such as sending an alert, switching off a device, or initiating tracking.

In this project, we’ll use GPS technology to geofence an Arduino device around a fixed location. When the device moves outside the defined radius, it will send an SMS alert to a registered mobile number, providing the current location of the device.

Components

Arduino Mega x1
SIM900A GSM/GPRS modem x1
Neo-6M GPS module
SIM card
Connecting or jumper wires

Circuit connections

To build this geofencing device, you’ll need to connect a SIM900A GSM/GPRS modem and a Neo-6M GPS module to an Arduino board. We’ve chosen Arduino Mega becuase it includes multiple hardware UART ports, but other Arduino boards can also be used. However, they might require software serial for communication.

To interface the GPS module with Arduino Mega:

Connect the GPS module’s VCC and GND pins to Arduino’s 5V output and GND pins.
Connect the GPS module’s TX and RX pins with Arduino’s RX1 and TX1 pins, respectively.

For the GSM/GPRS modem:

Connect the modem’s GND, TX, and RX pins to Arduino’s GND, RX2, and TX2 pins.
Then power it using an external adapter.

Note, the entire circuit can be powered using a regulated battery supply for portability and reliability.

The sketch

Replace the values of the “inilat” and “inilng” variables with the GPS location’s latitude and longitude coordinates of the desired geofence location. Also, replace the variable “PHONENUMBER” with the number you’ll receive the alerts on.

Then, upload the following code to Arduino.

How it works

A fixed GPS location is hard-coded in the Arduino sketch with its latitude and longitude stored as “initial latitude” and “initial longitude.” Arduino initializes the GSM modem to send an SMS alert to the registered mobile number, which is hard-coded in the sketch.

Arduino then retrieves the current location of the device, calculating the distance between the current location and the initial location using the following formula.

x = Sin((lat1-lat2)/2.0)*Sin((lat1-lat2)/2.0) + cos(lat1)*cos(lat2)*sin((lng1-lng2)/2.0)*sin((lng1-lng2)/2.0)

distance (in meters) = (2*atan2(sqrt(x),sqrt(1.0-x)))* 6371000.0

Remember the latitude and longitude values (i.e., lat1, lng1, lat2, and lng2) are in the radian. If the distance between the two locations is greater than 50 meters, an SMS alert is sent to the registered mobile user with current location.

The code

The sketch begins by using the TinyGPSplus library. This library is required for getting the GPS location from the data received via the GPS module. An object of the TinyGPSplus class is initialized.

Next the variables are declared to store the:

Current latitute in degrees
Current longitude in degrees
Current latitude in radians
Current longitude in radians
Initial latitude in degrees
Initial longitude in degrees
Initial latitude in radians
Initial longitude in radians
Difference between the two latitudes
Difference between the two longitudes
Calculations and distance between the two GPS locations

Variable are also declared to store the registered phone number and to set the status of the alert message. The maximum distance from the initial location or the radius of the geofence is set to 50 meters.

The user-defined function sendATCommand() is used to send the AT commands to the SIM900 GSM modem. In the setup() function, the baud rate for communication for both the GSM and GPS modems is set to 9600 bps. The GSM modem is then initialized for text messages by sending a series of AT commands.

The user-defined function sendAlert() forwards the SMS alert with the current location to the registered mobile number. The SMS is sent by sending a series of AT commands to the GSM modem.

In the loop() function, Arduino checks for any GPS data. If it receives data, it retrieves the current location. The latitude and longitude of current location are calculated with the latitude and longitude of the initial or fixed reference location. The distance between the two locations is calculated according to the equation stated above.

If the calculated distance is greater than the maximum distance (i.e., the radius of the geofence), the alarm variable is set to TRUE and an SMS alert is sent by calling the sendAlert() function. Otherwise, the alarm variable is FALSE.

A delay of one minute is provided upon completion of the loop. Location tracing starts again after this minute is up.

Results

For this project, we configured the fixed reference location in the sketch with the GPS coordinates of our city — approximately 12 kilometers from our current position. Since the distance between the reference location and our current position far exceeded the geofence radius of 50 meters, the device triggered an SMS alert. The alert was sent to the registered mobile number, providing our current location as follows.