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Monitoring electricity
consumption using 1-wire
I was fortunate enough to have my electricity meter changed to the new
digital type with a liquid crystal display. One of the interesting features
of this meter is that it has a very bright LED which flashes once every
1/1000 Kwh used. This got me thinking about building an interface to pick up
the flashes via a sensor and feeding this into my one-wire network.
Electricity meter as installed by my
electricity company.
I happened to be in my local Maplin's
(UK electronics store) and found that they sold
Infra-red barrier mini-kits from Velleman (MK120). Info
here
and
here. This consisted of a transmitter and
receiver PCB complete with components. Being under ten pounds, I purchased
this kit to see if it would suit my needs. To my amazement, with a very
minor modification, this was the perfect first interface for the project.
The receiver side of this kit consists of a photo diode which is designed to
pick up infra-red from the transmitter. Looking at the transmitter
circuit diagram (Receiver diagram also
here ), the transmitter LED's were
actually being fed from a astable oscillator. The modulated IR Light was
then being received by the photo diode on the receiver pcb and fed into
various op amp stages to integrate and compare the voltages obtained. This
voltage (when the photo diode received IR light) was compared to a reference
voltage and would switch the final opto to an OFF state. This would then
switch to an ON state when no IR illumination was received. For its intended
application, the buzzer & LED would activate when somebody crossed the beam.
I really needed the output to be the reverse of this so I decided to modify
the receiver pcb by swapping the + and - inputs of the last comparator. This
involved 4 cuts on the PCB and a few bits of wire to transpose the
connections.
I was going to use the final output transistor as the switch for the 1-wire
but thought it would be nice to leave the LED as an indicator. Instead, I
used a spare BC547 transistor & 1K resistor (from the transmitter) as the
switch interface. The buzzer was never installed on the PCB.
The 1-wire interface was the easy bit. One of the common uses for this bus
is for temperature and weather related devices. Several weather instruments
require counters to input the data and this includes rain gauges & lightning
detectors. A company called AAG in Mexico manufacture 1-wire counter boards
(TAI8585) at minimal cost. These consist of a battery backed DS2423 counter
(so no counts are lost) which can be triggered by a reed relay or other
shorting method. This seemed ideal and I purchased one for this project.
Info
here.
These two boards then made up the main interface and the 1-wire output fed
into my Midon Design TEMP08 and then to the Homeseer PC via a serial
interface.
Construction
One of the most important parts of this project was how to mount the photo
diode in front of the pulsed LED without obstructing the meter too much and
raised eyebrows from the meter reader. A 8mm wide length of sheet metal was
bent in to a Z formation with a 5.5mm hole drilled at the end and another
4mm for mounting. This was then sprayed white.
Photo Diode mounted on bracket with
heat shrink to keep the cabling neat.
In the original design, I used telephone cable to solder onto the photo
diode but due to interference from mains spikes being picked up in the
cable, this was later changed to screened cable.
Final installation of photo diode mounted
on wooden back board with screened cable.
The Velleman receiver kit was constructed apart from the photo diode and the
buzzer. R11 was replaced from a 470K to 15K to reduce the sensitivity of the
circuit.
As mentioned before, I needed to swap pins 9 and 10 on the LM324 IC. As the
pads on the PCB could get dislodged, cutting of the board was done after
construction with a sharp knife. A meter set to low resistance was then used
to confirm that the tracks had been completely cut. Wire links as shown were
then added to complete the modification.
Red lines indicate where the pcb tracks
need to be cut.
Wire links required to complete the
modification.
Connection of the photo transistor and 9V battery to the circuit confirmed
correct operation with the on-board LED flashing at the same time as the
meter LED.
I found out by chance that the AAG 1-wire counter could be mounted on top of
the Velleman PCB using the mounting holes designed for the battery holder.
Two 25mm M3 pan head screws were used to mount the AAG board. I needed to
add a small transistor and resistor to allow the one-wire counter to be
triggered from the Velleman board. A 1K resistor with sleeving was soldered
to R12 and pushed through the large hole nearby so that it appeared at the
top of board. The collector and emitter of the BC547 were then soldered
directly to the 1-wire board (across the reed relay) and the other end of
the 1K resistor soldered to the base of the transistor. Finally a
ground/common wire was connected from the Velleman PCB to the 1-wire PCB.
Solder connection of the 1K resistor and
ground connection.
Circuit diagram of transistor interface.
Detailed view of interface and 1-wire board
mounted on Velleman PCB.
Another view of finished interface.
All that was needed was to mount the finished interface within a plastic
box. I added two sockets - one for the photo transistor and the other for
the 1-wire & 12V supply. In addition, I de-soldered the LED and also mounted
this on the side of the box. The 12V supply was obtained from the TEMP08.
Completed interface mounted in plastic box.
Problems
I originally had an issue with the circuit being too sensitive and extra
pulses were detected when switching on the kitchen fluorescent lights. This
was totally eliminated by reducing the sensitivity of the circuit by
replacing R11 on the Velleman PCB from 470K to 15K. In addition, using
screened cable on the photo-transistor also helped. I would recommend
keeping the sensor cable length as short as possible.
One other issue that I did not foresee was that the TEMP08 counter only goes
to 65535 (FFFF) before it resets to zero. As the meter pulses 1000 times
every Kwh, this can add up very quickly. The only way to get raw counter
value from the DS2423 was by using the TEMP08 in debug mode. This was giving
out far too much data and was discounted. I now use Homeseer scripting to
store an incremental counter value in a ini file and adjust the count
automatically even when the counter resets to zero. This is updated every
minute. Two logging files are produced. One is the data recorded for the day
and is created every day automatically. The second is a 2 day log (today &
yesterday). This is used to generate the logging data for the web server
(last hour & last 24 hours).
One of the things this project has highlighted is how much my home costs to
run. Even during the night, I am using over 1KWh to supply computers,
interfaces, fridge & freezer etc and all those other gadgets which glow on
standby!
If you try this yourself, please let me know how you got on.
Data display
Basic monitoring graphs for the last hour and 24 hours can be seen on my
home automation server. This is what you will see:
Real time monitoring of the meter readings
Histograms showing 1 hour data
Histograms showing 24 hour data
UPDATE:
The circuit has now been tried and tested by another 1-wire user in the
Netherlands. Take a look at his site
here.
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