[chbot] Power monitoring

[Andrew Errington]

I built a sensor for a solar hot water backup heater.  It might be at a
lower-level than you are looking for, and you'll have to export the data
yourself.

I decided all that I needed was a simple on/off indication that the load
(heating element) was consuming power.  There are several ways to do it,
but I wanted to avoid opening up the heater and thermostat, or connecting
to the mains power.

Technique number 1:
Use a CMOS logic chip to detect the ac in the supply wire:
http://www.talkingelectronics.com/projects/100%20IC%20Circuits/1-100_IC-Ccts.html#33
(this is one example, but there are many others)

The LED lights when ac is present.  The sense wire is placed next to the
supply wire, so no actual connection to the mains is necessary.
Disadvantage is that it needs a battery.  This will only work if the sensor
is placed *after* the switch/thermostat because it detects that the wire
has become live.  If you put it before the switch then it will always be
on, because the supply is always on.

Technique number 2:
Use a current transformer like this one:
https://www.sparkfun.com/products/11005

The output is an ac waveform coupled with the current in the mains cable.
I half-wave rectified it with a diode and capacitor to drive the input of a
one-wire GPIO DS2406 which I could then hang off a one-wire network and
query from anywhere.  This sensor gives an output when the load is drawing
current, so it can be placed anywhere on the supply line, assuming there is
only one load.  One drawback is that a small load will only induce a small
current, which might not be detectable.  The heater drew 10A, so that was
plenty.  This sensor can be used to give an analog representation of the
current drawn, but I only needed an on/off indication.

Once you have a simple sensor like this working then you can hook it up to
an Arduino or Raspberry Pi and log it.  After that it's just software...

For monitoring the whole house you have three options:
1) current sensor (as technique 2 above) on the main supply line to the
house.  You'd want an analog-to-digital converter to get a number related
to how much power was being used.
2) photocell on your electric meter counting the pulses from the power
consumption LED found on most meter front panels.  The advantage of this is
that it's cheap, non-contact, and entirely digital.
3) data communication with the meter.  Some meters have an IR data port, or
a plug-in data port.  You could establish a connection to the meter and
query it occasionally to get consumption data.

I suppose you could also piggyback onto the smart-meter signals that the
electric company is sending and receiving, but that's a very deep rabbit
hole.

Best wishes,

Andrew



On 12 January 2017 at 06:37, Kiel Hurley <kielhurley at gmail.com> wrote:

> I'm looking for something to measure power usage which can export its
> data. DIY or commercially available (but would prefer it isn't cloud
> connected).
>
> I bought an old fridge off Trade Me and it seems to be on *most* of the
> time, so I'd like to graph it's usage to see what's going on.
>
> Just a single socket for a start. Might add more sockets later, but would
> probably look into monitoring the whole house at that point.
>
> Thanks,
> Kiel
>
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