Watt Meter build walks you through Power Measurement basics

You almost never hear of a DC Watt Meter – one just does some mental math with Volts and Amps at the back of one’s head. An AC Watt Meter, on the other hand, can by pretty useful on any workbench. This handy DIY Digital AC Watt Meter not only has an impressive 30A current range, but is designed in a hand-held form factor, making it easy to carry around.

The design from Electro-Labs provides build instructions for the hardware, as well as the software for the PIC micro-controller at its heart. A detailed description walks you through the schematic’s various blocks, and there’s also some basics of AC power measurement thrown in for good measure. The schematic and board layout are done using SolaPCB – a Windows only free EDA tool which we haven’t heard about until now. A full BoM and the PIC code round off the build. On the hardware side, the unit uses MCP3202 12 bit ADC converters with SPI interface, making it easy to hook them up to the micro-controller. A simple resistive divider for voltage and an ACS-712  Hall Effect-Based Linear Current Sensor IC are the main sense elements. Phase calculations are done by the micro-controller. The importance of isolation is not overlooked, using opto-isolators to keep the digital section away from the analog. The board outline looks like it has been designed to fit some off-the-shelf hand-held plastic enclosure (if you can’t find one, whip one up from a 3D printer).

Although the design is for 230V~250V range, it can easily be modified for 110V use by changing a few parts. Swap the transformer, change the Resistive voltage divider values, maybe some DC level shifting, and you’re good to go. The one feature that would be a nice upgrade to this meter would be Energy measurements, besides just Power. For an inside look at how traditional energy meters work, head over to this video where [Ben Krasnow] explains KiloWatt Hour Meters


Go On a Power Trip with Powerduino

powerduinoThings don’t always run the way we want them to or operate at the ideal temperature out of the box. Instead of spending extra for power controls that may or may not meet your needs, wouldn’t it make more sense to dial in the ideal level from the source? That’s what [dekuNukem] had in mind when he decided to make Powerduino, an arduino-compatible programmable power strip.

With Powerduino, [dekuNukem] can control the electrical consumption of all kinds of things without ever worrying about the irreversible deadliness of mains voltage. It actually uses a Teensy 3.1 which can be programmed with the Arduino IDE through the micro USB connector. He’s really tricked it out to the point of putting Kill A Watt meters to shame. A wi-fi module lets him control any of the outlets from anywhere, and the RTC module lets him make customized schedules for them. Powerduino has an SD card slot for logging energy consumption, and a 20 x 4 LCD screen makes it easy to directly interface with the power strip.

The Powerduino code is up on GitHub, and [dekuNukem]’s walkthrough video is after the jump.

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The effect of code on power consumption

Of course putting a microcontroller into sleep mode or changing the clock rate has an effect on the power consumption of the chip, but what about different bits of code? Is multiplying two numbers more efficient than adding them, and does ORing two values consume more power than NOPping? [jcw] wanted to compare the power draw of a microcontroller running different loops, so he threw some code on a JeeNode and hooked it up to an oscilloscope.

For his test, [jcw] tested two instructions: multiply and shift left. These loops run 50,000 and 5,000 times, respectively (bit shifting is really slow on ATMegas, apparently) and looked at the oscilloscope as the JeeNode was doing its work.

Surprisingly, there is a difference in power consumption between the multiply and shift loops. The shift loop draws 8.4 mA, while the multiply loop draws 8.8 mA. Not much, but clearly visible and measurable. While you’re probably not going to optimize the power draw of a project by only using low-power instructions, it’s still very interesting to watch a microcontroller do its thing.

Open Source Wireless Mesh Networking Energy Meter

[Jay Kickliter] writes in to tell us about his open source energy/power meter. With his buddy [Frank Lynam] they designed a small device that crams into existing power boxes and uses and 8 core propeller (P8X32A) microcontroller to perform true RMS voltage and current measurements using a current transformer. [Frank] and [Jay] don’t stop there. The meter also features an xbee pro 900 MHZ to provide wireless (and even mesh networking) capabilities to the whole ordeal.

[Jay and Frank] estimate a total unit cost of around $80 (US) per prototype. With volume the price goes down by about half. With a larger number of units, and the magic of mesh networking, we could see cheaper xbee’s driving the cost down some. Check out the Google code page for details or the schematic (pdf) if you are interested.

So far the project is in the beta stages, and only features a single module sending data to a PC running an OS X Cocoa application. [Jay] is about to be otherwise occupied by the Merchant Marines and [Frank] the Navy, so they figured we could have a go at it for awhile.

We have seen other hardware used to monitor power consumption, but cramming this circuit into each power box is a neat idea.

[Frank] explains the whole project in the video after the jump.

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Clamp sensor power monitoring

[Bill Porter] has joined in the pursuit of an inexpensive yet effective way to monitor his power usage. He calls his project the Not So Tiny Power Meter, and shared both his successes and follies involved in seeing it through to implementation. There are problems; sizing issues with enclosures and his PCB, issues with noise when measuring low-current signals with the clamps, and even some wireless communications issues. But it looks like he’s got the system running despite these hurdles. Right now it streams data wirelessly so that he can display the current energy consumption, but he plans to add Google Power Meter functionality next.

We’ve seen commercial units using the same sensing principles that have been hacked to show data online.