{"id":6103,"date":"2011-02-21T11:14:06","date_gmt":"2011-02-21T16:14:06","guid":{"rendered":"http:\/\/scruss.com\/blog\/?p=6103"},"modified":"2013-12-03T20:35:16","modified_gmt":"2013-12-04T01:35:16","slug":"on-the-trail-of-the-elusive-power-cost-monitor-signal","status":"publish","type":"post","link":"https:\/\/scruss.com\/blog\/2011\/02\/21\/on-the-trail-of-the-elusive-power-cost-monitor-signal\/","title":{"rendered":"on the trail of the elusive Power Cost Monitor signal"},"content":{"rendered":"

Catherine<\/a> probably thought I was acting no more strangely than usual last night, when I was holding the Power Cost<\/a> display unit in one hand, frobbing<\/a> the electric stove control with the other, all the while watching a digital clock and cackling gently to myself.<\/p>\n

All this makes me think I’m a bit further on with getting something<\/em> from the Power Cost Monitor. Previous attempts using cheap wireless units<\/a> (and cluelessness on my part \u00e2\u20ac\u201d never forget that) got nothing, so I caved and bought the rather expensive but nice Parallax 433 MHz RF Transceiver<\/a> from Solarbotics<\/a>.<\/p>\n

The Parallax unit adds an analogue output that’s proportional to the instantaneous signal strength. I hooked up the output to the trusty sdfatlib<\/a> analogue logger, set the logger to sample every 50ms (figuring that, every now and again, it’s going to see part of a transmission) and this is what I saw:<\/p>\n

\"\"<\/a>Pretty noisy, I know. But look, there are regular peaks:<\/p>\n

\"\"<\/a>Here I’ve highlighted all the peaks with signal strength \u00e2\u2030\u00a5 200 (in arbitrary units, where 1024 would be full scale). It’s pretty regular; the largest peaks come in a shade<\/em> under every 32 seconds, or a multiple thereof. If you need additional decimal places to validate your worldview, I’m thinking the period’s around 31.86s.<\/p>\n

Observations made during last night’s frobbing and cackling episode seem to confirm that the display updates about every 32s. If you adjust the load mid-cycle, nothing changes until the update. If the display misses an update, it’ll refresh in 64 or 96 seconds.<\/p>\n

I don’t yet know what bitrate the data comes through at, or the protocol. I’ve logged some data recorded at various rates: 433mhz_powercost_data<\/a>. The file names give a hint at the data rate; L1200<\/span>V04 was recorded at 1200bps, for example. I’m guessing that there’s some kind of sync header, then the total value and the current temperature (which was around\/below freezing at the time). I need to work on that.<\/p>\n

Update<\/strong>: I rewrote the logger to use the Arduino’s internal UART, since \u00e2\u20ac\u201d lovely though NewSoftSerial<\/a> may be \u00e2\u20ac\u201d it causes millis() to report wildly inaccurate times at low bit rates. I recorded a bunch more data (powercost-arduino2.zip<\/a>) in a slightly more useful text format. It has three columns:<\/p>\n