Sunday, April 17, 2011

A Wireless Home Energy Monitor to Accompany Nanode

One of the first projects after the Nanode has to be a wireless shield in order to allow Nanode to talk to wireless sensor networks.

In the last couple of years there have been an increasing range of cheap wireless modules available to the hobbyist.

As well as 2.4GHz Xbee and Bluetooth offerings, sub-1GHz is better for in-building propagation. Additionally, if you only want to pass a few bytes between sensing nodes, Xbee and Bluetooth are expensive overkill. Passing a few bytes between nodes is exactly what we want to do with the Nanode, and so a low cost wireless solution working at below 1GHz was sought.

Hope RF produce some nice cheap transceivers that work below 1GHz - and programmable to any of the ISM bands 315MHz, 433MHz, 868MHz and 915MHz. They are available in the UK from Maplin, at just £5.99 for a transceiver.

I had previously experimented with short range wireless, using low cost on-off keyed (OOK) modules. I'd even adapted the SNAP protocol from HiTech Horizons to work across the wireless link, but it was all coded up in PIC assembly code and it was difficult to maintain.

The arrival of the cheap Hope RF modules, and the "Arduino like" JeeNode, meant that progress with with simple wireless networks has been very much accelerated.

JeeNodes are the brainchild idea of Jean-Claude Wippler, a very talented multi-discipline engineer from the Netherlands. J-C had the idea of grafting a RFM12 wireless module on to a BareBones Arduino to make a simple wireless node. The second innovation was to arrange the I/O lines of the AVR into 4 identical ports. Each port consisted of a digital line, an analogue line, power, ground and an interrupt. With this standard interface, any add-on device could be plugged into any port. J-C has also written a comprehensive library to support this port structure.

Glyn Hudson at shared a new design with me for a wireless energy monitor, which he had designed to be compatible with the JeeNode, and with this inspiration I decided to modify it to make it compatible with the Nanode.

Glyn had taken the JeeNode idea of 4 separate ports and an RFM12B wireless transceiver and added the extra hadware needed to do electricity monitoring, temperature measurement and pulse counting - for example from a gas meter.

I decided that the JeeNodes port layout was a good innovation, but if it also had Arduino shield connectors, it could turn a conventional Arduino into a Jeenodes compatible device - as well as all the home energy monitoring interfaces.

So what has been conceived is a bit of a hybrid.

It's a JeeNode wireless shield for an Arduino.
It's a standalone wireless home energy montor.
Its a wireless shield for Nanode
It's a home energy monitoring shield for an Arduino.

Some basic design features:
1. Hope RF RFM12B sub 1GHz transceiver
2. Compatible with Jeenodes firmware
3. Jeenodes "Ports" supported
4. programming via FTDI cable or Vusb port
5. One Wire sensor support - up to 8 DS1802 temperature sensors etc.
6. Connects to one current transformer (CT) and one voltage reference input
7. One interrupt driven pulse counter - for gas meter pulses
8. Supports Nanode local serial bus.
9. Driver for 2 relays or 2 serial slave boards - for central heating/solar hot water control
10 Can be used a basic shield or standalone unit.

With these basic features in mind, I set about laying out a wireless home energy monitoring shield which could also work as a stand-alone wireless node.


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