Mesh Security System (Argon Hub, OLED and MP3 Shields) - Part 2

OLED Display

Figure 5. Argon mounted on Tripler with OLED.

Having demonstrated that we can blink a LED on the Argon, we now want to move onto something a bit more useful. The Argon will form the hub of the Mesh Security System and will connect to an OLED and MP3 shield to indicate system status. In Part 2 we will get the OLED and MP3 shields working.

As shown in Figure 5, connection is simple using the Featherwing Tripler. By mounting the shields horizontally rather than stacking them you can still easily see all the indication LEDs. You will have to solder the headers on the tripler and shields. Do the tripler first. I solder one pin and then check that the header is correctly positioned before soldering the rest. It is a lot easier to rectify an issue with only one pin soldered in place. Once you have completed soldering the headers on the tripler you can use this as a jig to hold the pins in place when soldering them to the shields. This will ensure that the shield pins line up with the headers on the tripler.

Figure 6. OLED Operational

The display board is 128x32 monochrome OLED which has 3 user buttons plus reset. This screen is made of 128x32 individual white OLED pixels and because the display makes its own light, no backlight is required. This reduces the power required to run the OLED and is why the display has such high contrast. The board uses a SSD1306 and connects via I2C (pins D0 and D1), so it is very pin frugal. As I2C is a shared bus you can have other shields which utilise I2C connected at the same time (as long as they have different I2C addresses). The three buttons use:

Button	Pin	Notes
A	D4	No pull-up. Can't be used with Ethernet.
B	D3	100K pull-up. Can't be used with Ethernet.
C	D2	No pull-up.

So all up this shield uses 5 pins (D0 - D4).

The library is available in the Web IDE as oled-wing-adafruit and using the display from the Argon is easy. The library takes care of setting the appropriate input modes and debouncing the buttons for you.

I've reproduced my test code stub below. I always like to get each element of a project working before adding the next. This makes debugging much easier.

MP3 Shield

The MP3 Shield is shown in Figure 5 above. This is before the through hole headers have been soldered onto the shield. The shield version that we are using is the Adafruit Music Maker FeatherWing. This shield uses the the VS1053, an encoding/decoding (codec) chip that can decode a wide variety of audio formats such as MP3, AAC, Ogg Vorbis, WMA, MIDI, FLAC, WAV (PCM and ADPCM). This chip also allows you to adjust bass, treble, and volume digitally.

Figure 7. Argon Block Diagram (showing I/O).

Communication is via a SPI interface which allows audio to be played from an SD card. There's also a special MIDI mode that you can boot the chip into that will read 'classic' 31250 Kbaud MIDI data from the UART TX pin. The hardware SPI pins are needed whenever you are transmitting data from the SD card to the decoder chip. If you are using the wing in the special MIDI mode, they're not used.

D11: SPI MISO - connected to MISO - used by both the SD card and VS1053
D12: SPI MOSI - connected to MOSI - used by both the SD card and VS1053
D13: SPI SCK - connected to SCK - used by both the SD card and VS1053

The Adafruit VS1053 Library does include a constructor to define the SPI pins you want to use, but this doesn't help us because:

1. The hardware SPI pins are already connected by the tripler; and
2. The alternative SPI pins on the Argon are D2, D3 and D4 - which seem to be very popular with shield designers!

Figure 8. Adafruit Music Maker FeatherWing Shield.

Next are the control pins required to play music. From left to right, in Figure 9 below, they are:

MP3_DCS - this is the VS1053 data select pin
DREQ        - this is the VS1053 data request interrupt pin
MP3_CS    - this is the VS1053 chip select pin
SD_CS       - this is the SD Card chip select pin

Figure 9. MP3 Control Pins.

Unfortunately the MP3 control pins connected (via the tripler) to the Argon conflict with the A, B and C buttons connected to D2, D3 and D4 from the OLED shield. Thankfully there is no conflict on pins D0 or D1, so we can still control the OLED with the MP3 shield in place. Obviously the designers of the two shields at Adafruit didn't talk to each other!

Figure 10. MP3 Shield Installed.

To summarise, the Argon pins used to control the MP3 shield are:

SD_CS                = D2;                 // SD Card chip select pin
MP3_CS             = D3;                 // VS1053 chip select pin (output)
DREQ                 = D4;                 // VS1053 Data request, ideally an Interrupt pin
MP3_DCS          = D5;                 // VS1053 Data/command select pin (output)
SPI MISO           = D11;               // used by both the SD card and VS1053
SPI MOSI           = D12;              // used by both the SD card and VS1053
SPI SCK             = D13;               // used by both the SD card and VS1053

Figure 10 shows the MP3 shield in place on the tripler adjacent to the OLED shield. To give myself a bit more room, I removed the OLED shield while soldering the header pins to the MP3 shield. I again inserted the header pins into the tripler before soldering to make sure that everything lined up.

There are two versions of the Adafruit Music Maker, one includes an amplifier and the other just has a 3.5mm connection for headphones or powered speakers. In retrospect I should have got the one with the amplifier built in. Nevertheless I happen to have a Duinotech 2 x 3W amplifier, so I might as well use that. This is the red PCB shown in Figure 10. Before dealing with this, you will want to make sure that the MP3 shield is working.

Thankfully ScruffR has done the hard work of porting the Adafruit VS1053 Arduino library to work with Particle mesh boards. You will need to import this library and the SDFat library in order to get the shield working. This is easy, just search for the libraries in the Web IDE and then add them. Plug in some headphones (assuming you have the same version shield as I do) and you can use the code below to test the operation of your shield. You will obviously need to copy some mp3 files to SD card before you can play them. Make sure that the names of the files are in the 8.3 format or they wont be able to be played.

Duinotech 2 x 3W Amplifier

Rather than use the 3.5mm jack on the MP3 shield, we will connect directly to the Ground, Right and Left pins next to the headphone jack (Figure 11). They are line level, AC coupled outputs which are suitable for connection to an amplifier.

Figure 11. MP3 Shield Audio Out Pins.

The Duinotech 2 x 3W Class D Amplifier (Figure 12) has greater than 90% efficiency and typically delivers 3W into 4 ohm speakers (or 1.5W into 8 ohms). Its operating voltage range is 2.5 to 5.5 VDC.

The amplifier board uses the PAM8403 chip and power output will be determined by a combination of the input voltage supplied and output impedance. As we are using the regulated 3.3V from the Argon and 8 ohm speakers our expected power output from the amplifier is around 0.5W.

Figure 12. Duinotech 2 x 3W Amplifier.

The amplifier pin out description is provided in the table below.

Amplifier Pinout
Module	Function
R+/R-	Right Speaker
L-/L+	Left Speaker
GND	Ground Connection
+5V	Power Supply
5W	Shutdown Control
GND	Ground Connection
LIN	Left Audio In
GND	Ground for Audio
RIN	Right Audio In

Connection between the MP3 shield and amplifier is straight forward.

1. MP3 Shield L and G connect to LIN and Audio GND on the amplifier.
2. MP3 Shield R and G connect to RIN and Audio GND on the amplifier.
3. R+/R- on the amplifier connect to the right speaker.
4. L+/L- on the amplifier connect to the left speaker.
5. +5V and GND on the amplifier connect to the 3.3V and GND pins on the Argon.

In Part 3 we will complete construction of the Argon Hub and 3D print an enclosure for it. We will then move onto configuring the Xenon's.

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