Simple MQTT node

In the previous article I described a basic idea of building my network of interconnected devices. In next few articles I'll describe building of very simple sensor with network interface, which can send data using MQTT protocol.

Node requirements:

• Use well known Arduino hardware
• Software written in plain C
• Cheap components
• Scalable design
• Easy to use

MCU

Because I don't want to create my own board, I have to use some existing one. For that reason I decided to use well known Arduino Nano board with atmega328p MCU. I have lot of these in my shelf. This MCU has 32KBytes of flash storage for code, 1KByte of EEPROM as non-volatile storage and 2KBytes of RAM.

I have plan to use Arduino Pro Mini for final product. It costs about 2.50 USD on ebay.

Another advantage of using Arduino hardware is lot of existing code for it. I can easily connect some components, upload existing sketch and check, if my wiring is correct. After verifying that hardware is connected properly, I could start writing my own software. This safes me lot of work by finding bugs in both hardware and software.

Software

Although I'm using Arduino hardware, I'm not using any Arduino software. I decided to implement all software logic in plain C instead.

I thing that Arduino is really great for learning of programming microcomputers or to make quick proof of concept sketch. But its code is quite messy, not efficient and it limits many hardware possibilities. So for making some serious project it is insufficient.

Use of plain C has many advantages. You have complete control about memory usage and its optimization, interrupts, timers, power consumption and other things. You have to also create Makefile, which brings another control in code configuration, compilation and optimization.

Ethernet controller

There are two common Ethernet controller chips. WIZnet w5100 and Microchip enc28j60. W5100 is chip used in official Arduino Ethernet Shield. It has its own TCP/IP stack implemented in hardware and it is well supported by community. But it is twice as expensive as the enc28j60 chip. Because I want to make many of these devices, price is crucial. I had already some enc28j60 modules in my shelf.

Using enc28j60 chip requires more work in software. It requires to implement own TCP/IP stack in software layer. But it seems that this work is already done by someone else.

Sensors

For development purposes I decided to make a node only with DHT-22 (also known as AM2302) sensor. This sensor can measure relative humidity and temperature in 2s time period. DHT-22 is more expensive variant of DHT-11, which has very limited accuracy and measurement range.

I used this sensor mainly for development. Final software should be capable to use any other sensor or device.

Wiring

Finally, wiring of my prototype looks like this.

Next time I'll describe node software and building MQTT brokers.