RFM69 – Energy Count 3000 / ELV Cost Control

Conrad Energy Count 3000 and ELV Cost Control are wireles 230V electricity consumption measuring plugs Thomas Schulz has cracked the RF protocol of these device back in 2011 on the JeeLabs forum. The signals are FSK encoded and use the 868Mhz band. Thomas was able to receive them with a RFM12B module. Together with a JeeLink V3, it has found popular use with the German home automation software FEHM.

In this post I describe how the RFM69 can take over a task that earlier had to be performed in software.

One of the complications of the protocol is that the preamble of a series of 0x55 is scramble (whitened), and therefore more difficult to recognize. However, after scrambling it is still identical for all packets.

raw data 25 8A F9 59 DA 02 7E 15 ED 67 13 F1 85 D3 AC D2   [payload]   13 E6 84 A8 3B 
descramb 48 BB 55 55 55 55 55 55 55 55 55 55 55 55 55 7E   [payload]   7E 55 55 55 55 
               -------------------------------------- --   ---------   -- -----------
                             preamble                 HDLC  payload  HDLC  postamble

The original Jeelib-based RFM12B programs detection of valid packets was clumsy. (more…)

RFM69 OOK compare antenna’s

The antenna design of a module like the RFM69 is important for the sensitivity to sognal and noise. Also antenna positioning is important. Putting an Nucleo STM32 with RFM69CW very close to a PC will result in a increase of several dBm’s in noise floor.

Gain 7 dB sensitivity with JeeNodes

The basic antenna design commonly used is a quarter wavelength wire antenna with ground plane.
From this informative paper from HopeRF, the manufacturer of the RFM69. Such an antenna has a 1/4 wavelength monopole, and a ground plane that serves to reflect (mirror) the wire so that one gets a half-wavelength dipole effect.

The 86mm wire (at 868MHz) connected to small JeeNode like devices is actually not having much of a ground plane. So this can probably be improved. There are very nice commercial designs for this at 868MHz and 433MHz. A.o from Conrad. So lets putthree 1/4 wavelength antenna’s to the test. The wire is compared to a home build and a commercial 1/4-wavelength with groundplane antenna. (See picture above this article, the blue wire crossing the Nucleo is the wire-antenna). (more…)