Welcome to the Tuesday Teardown series, about looking inside the technology around us.
Over two years ago (gosh, time flies), I reported about a low-cost AC metering device called Cost Control:
It seems to be available from several sources, not just Conrad and ELV, under different brand names. Not sure they are identical on the inside, but the interesting bit is that they transmit on 868 MHz and seem to go down to fairly low power levels as well as all the way up to 16A:
So let’s have a look inside, eh? Here’s the back side of the PCB:
No much to see, other than a thick bare copper wire, which probably acts as the shunt resistor.
The rest appears to be built around 3 main chips, two of which are epoxied in, so I can’t see what they are:
Flipping this thing over, we can see the different sections. I had expected a special purpose AC power measuring chip, but it looks like this thing is built around a quad LM2902 op-amp:
Note the discrete diode soldered on the flip side – the topmost solder joint looks pretty bad!
The rest of the analog circuitry and the MPU of some kind running at 4-something MHz is here:
The 24LC02 is a 2 Kbit I2C EEPROM, for the node ID and some calibration constants, I presume.
And here’s the wireless transmitter, running off a 16 MHz crystal:
Being 16 MHz, it’s a bit unlikely that this is a HopeRF RFM12B (or its transmit-only variant), alas. The blob at the center bottom goes to an antenna wire on the other side of the board.
Would love to be able to decode the wireless signal (1 packet every 5s, very nice!). Either that, or find out how they are measuring the power from 1..3600W – the remote actually displays in tenths of a Watt.
PS – See also this forum discussion about decoding.