Monday, 30 October 2017

ESP8266 CurrentCost Wifi Gateway

I have two CurrentCost power monitors - a CC128 EnviR panel in the living room which monitors our entire electricity usage, and an old white first generation panel in the office which monitors the office's electricity usage.

The CurrentCost panels output real time data in XML format every 6 seconds through a TTL serial connection presented as an 8P8C connector on the bottom, and I used to have a WRT54GL wireless access point connected to that, which sent the data to the server.  The WRT54GL was retired some time ago, so I wanted another way of recording the power usage data.  The ESP8266 seemed like a good choice to provide a bridge to my wifi network and I've chosen to use an ESP-12F board.  I could build a small enough device on stripboard to sit in the cavity under the base of the panel and connect to the CurrentCost's TTL serial output.

The 8P8C connector also includes regulated and unregulated power connections, but the 3v PSU is only rated for 80mA which is nowhere near enough to power the ESP8266.  I didn't want to have a second PSU, so opted to use a single USB power supply to power everything.  I've used an LM1117T LDO regulator to drop the supply voltage down to 3.3v, and connected this to both the ESP8266 and the unregulated supply rail of the CurrentCost.  There's a schottky diode between the regulator and the CurrentCost - this isn't strictly necessary, but will protect against someone plugging the CurrentCost PSU in at the same time as the wifi bridge.

GPIO0 and GPIO2 are pulled up by 10KΩ resistors, GPIO15 is pulled down by a 10KΩ resistor - you can probably get away with omitting the resistors and connecting these directly to Vcc / 0v respectively, although using a resistor to pull up GPIO0 allows the ESP8266 to be programmed in circuit by temporarily pulling it down to 0v.

CH_PD and reset are pulled up all the time by tieing them to the Vcc rail.

The connections to the CurrentCost's 8P8C connector are:
  • Pin 1 - Vcc / unregulated power
  • Pin 4 - Ground / 0v
  • Pin 8 - TTL serial data
Component-side view on the left, flipped for a track-side view on the right
Since the ESP-12F has solder pads with a 2mm spacing and the stripboard has 0.1" hole spacing, the ESP has to stand off the stripboard a little.  I've taken the opportunity to use the space between the ESP-12F and the board for the pullup / pulldown resistors.  I've shown the ESP-12F as a translucent block on the stripboard layout above so you can see the components under it.

The power connector is a surface mount mini-USB socket - obviously that's incompatible with stripboard, so I have just tacked on a couple of wires to connect it to the circuit and soldered its mounting lugs down to the track-side.  I find it useful to hot-glue the wires onto the board for mechanical support to avoid breaking off the socket's pins.
Component-side view

Track-side view
Although the CurrentCost outputs XML, I'm not bothering to parse that in the ESP8266.  Helpfully, each XML sentence is terminated with a newline, so it's easy to use Serial.readStringUntil('\n') to grab the whole sentence and post it to the web server.  Data posted over HTTP needs to be URL encoded and there doesn't seem to be a standard library to do this, so I ended up writing a quick and dirty encoder.  A PHP script on the web server parses the XML and feeds the data into RRDTool, and I'm using Cacti to display graphs of the data.

I wanted my graphs to display a total number of kilowatt hours used.  Unfortunately Cacti doesn't have a generic function to do this.  It can do bandwidth summation for network usage (i.e. displaying a "total megabytes" on a "bytes per second" graph), which is almost what I need.  Using that on the "watts" data gives a total number of watt seconds (i.e. joules) used, but no way to divide this by 3600 to produce watt hours.  In the end I added a COMPUTE data source to the RRD file, which records watts divided by 3600, and Cacti can then use that to calculate the total watt hours of electricity consumed.  Since I'm abusing the bandwidth summation functions, this unfortunately adds a "B" (bytes) unit to the total displayed, but I can live with that.

The code, schematic and layout are available from my Subversion repository.  The circuit works for both the CurrentCost EnviR (also known as the CC128) and the first generation CurrentCost, although the baud rate needs to be changed in the code - the EnviR uses a rate of 57600bps whilst the first generation panel uses 2400bps.  I believe there are also second generation panels around that look similar to the first generation ones but run at 9600bps.

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