Mesh Security System using the Particle Argon and Xenon - Part 1

Introduction

Figure 1. Argon Board plus some other bits and pieces.

I wanted to learn about the (relatively) new mesh capable boards from Particle, and decided a good project for this would be a mesh security system for our two garages and carport. These are some distance from the house and so should provide a good test of the mesh network range.

The system design will look something like Figure 2. The three Xenon's will communicate via the RF mesh to each other and to the Argon Hub. The Argon will monitor the state of the Xenon's and indicate the system status using an OLED and MP3 shield. The Argon will also connect to our LAN using WiFi and provide more detailed security status via  a web page. If you were building a for real security system it probably wouldn't be a good idea to publish the details on the internet.

Figure 2. Mesh Security Block Diagram.

For this first article we will focus on getting the Argon up and configured. Subsequent articles will focus on the Xenon's.

Particle Xenon and Argon Boards

The Xenon is a low cost mesh-enabled development kit that can act as either an endpoint or repeater within a Particle Mesh network.

The boards are based on the Nordic nRF52840 SoC (System on a Chip), and communicate using the IEEE 802.15.4-2006 standard to create a PAN (Personal Area Network). Bluetooth and active Near Field Communication (NFC) is also available. They have built-in battery charging circuitry which makes it possible to connect and recharge an appropriately sized Li-Po battery. The Xenon has 20 mixed signal (6 x Analog, 8 x PWM) GPIOs to interface with sensors, actuators, and other electronics. Programming it is very similar to an Arduino. The board is compatible with the Adafruit FeatherWing layout and shields can be connected to the base board using a FeatherWing doubler or tripler.

The Particle Argon is similar but includes Wi-Fi. It can be used as a standalone Wi-Fi device or as a Wi-Fi enabled gateway, repeater, or endpoint for Particle Mesh networks. We will be using it in the second configuration for our network.

The Argon has both the Nordic nRF52840 and the Espressif ESP32 processors on board. As with the Xenon it has battery charging circuitry and 20 mixed signal (6 x Analog, 8 x PWM) GPIOs. Other interfaces include UART, I2C, and SPI.

Programming the boards can be done via an extension to Visual Studio Code or using their online IDE. We will try out both methods.

Connecting the Antenna

The Argon uses two different MCU's for WiFi and BLE/Mesh. The WiFi is done using the ESP32 capability and the BLE/Mesh via the nRF82540. Each communication method uses the following frequencies:

1. WiFi - 2.412 GHz to 2.484 GHz (14 channels)
2. Bluetooth - 2.400 to 2.485 GHz
3. Mesh - 2.4 GHz (uses 6LoWPAN over 802.15.4)
4. NFC - 13.56 MHz

So there is a lot going on around 2.4 GHz if you are using WiFi, BLE and mesh at the same time. This is probably why an external antenna is provided. I assume there is also some smart deconfliction occurring at the hardware or Device OS level.

When talking about the Particle Mesh you may see Thread referenced. Thread is an open mesh networking protocol released by the Thread Group. Particle Mesh uses OpenThread, an open source implementation of Thread released by Nest.

6LoWPAN is an unfortunate acronym that combines the latest version of the Internet Protocol (IPv6) and Low-power Wireless Personal Area Networks (LoWPAN). 6LoWPAN, therefore, allows for the smallest devices with limited processing ability to transmit information wirelessly using an internet protocol. It is a competitor to ZigBee.

Figure 3. Particle 2.4 GHz Antenna

The Argon has 3 antenna connectors (u.FL connector); two on top “BT” (for mesh - nRF52840) and WiFi (for the ESP32), and one on the underside (under the micro-USB connector) for NFC. The Xenon's have 2 antenna connectors; one for “BT” (mesh) and one on the underside for NFC.

The Antenna provided with the Argon is tuned for 2.4GHz so use it for Mesh or WiFi. If you are using NFC, you will need to purchase an antenna tuned for 13.56 MHz. I am going to start out with the external Antenna on WiFi. This is required if you wish to use the WiFi connectivity.

There are two options for the Mesh antenna on the Argon. It comes with an on-board PCB antenna which is selected by default in the device OS and a u.FL connector if you wish to connect an external antenna. If you wish to use the external antenna, you'll need to buy one and issue an appropriate command in the firmware.

Connecting the antenna plug to the u.FL socket on the Argon is easiest done using a pair of long nosed pliers.

First Time Setup

Particle have put together a good video showing how to setup your Argon, so there is no need to reproduce all the steps here. TL;DR - download the iOS or Android app and follow the instructions.

As part of registering your device you will probably have to update the device OS (which abstracts away some of the complexity of programming the Argon). It is all very straight forward and worked well for me. I like the use of the RGB LED to indicate the various states of the device.

Once you've completed the setup you will be able to program your device and send over-the-air (OTA) updates to it.

Flashing the standard blink "hello world" example is a trivial exercise using the Web IDE. I was impressed by how simple this all was. The devs at Microsoft Azure IoT could learn something from this! Next up we will try something a bit more challenging - connecting the OLED and MP3 shields using the FeatherWing Tripler.

Figure 4. FeatherWing Tripler