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Project Help and Ideas » My plan - Power Metering

August 12, 2011
by chuckles73
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Diving right in, I've finished the projects in the nerdkits guide.

For my next feat, I'm going to aim a bit higher. A power meter for my house.

Commercial products are rather expensive, and all the other DIY projects that I've found have tended to involve either sticking a resistor on a single line and using a multimeter, or sticking hall effect sensors around the wires in the breaker box.

I want to do it a bit differently.

What I'd like to do is build a set of 6-outlet power strips with onboard MCUs that measure voltage for the strip, and amperage for each of the outlets.

Each MCU will mostly send its data to a central controller that will be in charge of polling each of the power strips regularly and passing that data off to the application that stores/serves the data.

Right now, the plan consists of a few parts:

An ammeter - I'm thinking a ACS756SCA-100B-PFF-T would be the main component here. Each would be attached to an ADC pin on the MCU.

A voltage meter - I'm a bit fuzzy as to what I want to do here. I'm thinking a voltage divider to take it down to <5v, a rectifier bridge to get rid of the reverse polarity, then... shrugs Take readings and do maths, I suppose. I dislike not knowing the science of electricity too well, but hey, learning is sort of what this project is about.

TxRx - ATA5423 looks like a decent fit, though I'll need to do a lot of experimenting to figure out how it works. I'm not sure if I'll want an MCU just to drive this, or if I'll be able to get by using one MCU for all of the metering.

DC power supply - For this one, I'm considering just buying a cheap wall wart and packing it into the enclosure, hooking it directly to the AC input. I doubt I could do it cheaper than a Chinese adapter; though the volt meter probably needs to do much of the same work as this.

Control unit - To cut down on interference if 10 of these things are transmitting in my house on a regular basis, I'm thinking I'll have the control unit use polling instead of the individual units just transmitting their data whenever they have it.

The control unit will also have the job of interfacing out to whatever computer/web server I have serving up the information.

Server - At this point I'm rather fuzzy as to what I'll want this to be. Generally, all I need it to do is get data from the control unit, stick it in a db and possibly serve up reports of various flavours when asked. I'd rather not have a full-time PC dedicated to this (I tend to turn my computers off when I'm not using them), so I was thinking I could just use some of the android hacks on an old phone to get ubuntu running with apache.


So there you have it folks, my master plan.

I have a lot of ideas for the various pieces of this project floating around in my brain right now, but there are also a bunch of things that I just have no idea about.

Any thoughts other than "be careful"? (Not that I'm dismissing safety concerns, I've just seen them in every thread I've read about interfacing with mains power.)

August 12, 2011
by Ralphxyz
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Hi Chuckles73, great project.

[quote] I was thinking I could just use some of the android hacks on an old phone to get ubuntu running with apache. [/quote]

that would mean you would leave a pc running all of the time.

An alternate plan would be to use a external EEPROM, here is a great thread on setting up a I2C EEPROM with the ATmega mcu.

There are even PC USB I2C cards so you could enable a pc to read the EEPROM whenever it is turned on.

That way you could avoid the pc mcu interface (which isn't that hard).

I like that ACS756SCA-100B-PFF-T, I want one.

I assume the output voltage is 0 - 5 volt, with 5 volt Vcc.

It appears it wants a true 5v Vcc for best accuracy, so it might need a separate power supply.

And of course you could really complicate things and hook up each power-strip with it's own mcu and do wireless

communications between them, love it.

Keep us posted on your thoughts and efforts.

Ralph

August 14, 2011
by Ralphxyz
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Those ACS756SCA-100B-PFF-T Hall Effect-Based Linear Current Sensor IC cost $6.00 - $7.00 a piece that would make this a costly project if you monitored each individual outlet.

Now how about making your own curent sensors? Anyone have any idea on how to go about doing that?

Here is a simple current sensor project I wonder if the toroid coil could be replace with just a coil of wire?

Ralph

August 14, 2011
by chuckles73
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I've got a friend who works for digikey. He can usually get the volume prices (or less) on individual items, so they might only cost me $2.80 or so each.

I've looked into a few other methods for measuring current. This seemed to be one of the cheaper and safer methods.

August 14, 2011
by Ralphxyz
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Well yeah if you can get the volume pricing that would make difference.

So any new thoughts on what you would like to do?

Ralph

August 14, 2011
by chuckles73
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Well, I'm going to start by grabbing a few parts and figuring out how to get them working. I'm not going to worry about much more until I can get some prototypes up for the various pieces.

Get it working, then get it working right, then worry about everything else.

I also need to get a decent workbench built up so I can stop messing with electronics on the coffee table...

I'm probably going to get the metering stuff working first because the transceiver I want to use would require SMD soldering. I'm not quite ready for that yet.

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