Computing stuff tied to the physical world

The AC current sensor node lives!

In Hardware, Software on Oct 28, 2011 at 00:01

At last, everything is falling into place and working more or less as intended:

    OK 17 172 31 173 31   <- 25 W incandescent
    OK 17 169 31 177 31
    OK 17 40 0 41 0       <- open
    OK 17 245 95 40 0     <- 75 W incandescent
    OK 17 140 95 245 95
    OK 17 43 0 140 95     <- open
    OK 17 39 0 43 0
    OK 17 118 2 42 0      <- 2W LED-light
    OK 17 211 2 97 2
    OK 17 219 2 102 2
    OK 17 107 2 219 2
    OK 17 89 2 107 2
    OK 17 40 0 82 2       <- open
    OK 17 39 0 40 0
    OK 17 38 0 39 0
    OK 17 219 53 38 0     <- 40 W incandescent
    OK 17 234 53 219 53
    OK 17 43 0 234 53     <- open
    OK 17 149 75 43 0     <- 60 W incandescent
    OK 17 23 0 149 75     <- open
    OK 17 42 0 23 0

That’s the log of packets coming in from the AC current node, as I inserted and removed various light bulbs. Still through the isolation transformer for now.

As you can see, every change is very clearly detected, down to a 2W LED light. These are all the bulbs with an E27 fitting I happened to have lying around, since the test setup was fitted with one of those. Some other time I’ll try out light inductive loads, chargers, and eventually also a 700 W heater as load.

I’m not interested that much in the actual readings, although there is a fairly direct relationship between these wattages and the 2-byte little-endian int readouts. The fluctuations across readings with unchanged load are small, and the no-load readout is lower than in my previous tests, perhaps the shorter and more direct wiring of this setup is helping avoid a bit of noise.

One problem I see is that some packets are lost. Maybe the way the wire antenna is laid out (quite close to the PCB’s ground plane) prevents it from working optimally.

The other problem seems to be that this node stops transmitting after a while. I suspect that the current draw is still too large, either on the ADC side (500+ samples, back to back) or due to the RFM12B packet sends (unlikely, IMO, after the previous test results). At some point the voltage over the supercap was 3.1V – I’m not sure what’s going on, since after a new power-up the unit started transmitting again.

Hm… or perhaps it’s that plus an antenna problem: when I rearranged it, things also started working again (I’m always cutting power before messing with the test circuit, of course).

But all in all I’m delighted, because this unit has a really low component count!

  1. That is really impressively sensitive!

    630-731 for 2 Watts (8mA) 38-43 for unloaded.

    If you want to test with lower loads I think you’ll need to start using resistors.

    Re: Stopping working, you could try sleeping it a little longer to see if that does the trick. If so, then it looks like you might need to up the charge current by reducing the resistance a little. 180K each might be all it takes.

  2. Very impressive, stable results. But your are still measuring current, not power, are you? The small fluctuations you get could also be due to small changes in grid voltage?

  3. Quote: ‘Re: Stopping working, you could try sleeping it a little longer….’

    Misunderstood this reading it first time. I am not sure that it helps if JC would sleep a little longer after work…. :-)

  4. Not sure I understand the reference to a ‘2W LED’. Is that a multi-emitter device, perhaps 2W per element, series connected to support 230v input? Or a single 2W element with some tiny supply (switching or dropper) to match to the mains?

  5. The 2W LED is some sort of night-light built into a bulb. I suspect it has some switching stuff in there as you say, which shows as variation in the readings.

Comments are closed.