IntroductionNow that the hardware is complete, we come to the exciting part: running the software that retrieves the data from our receiver XBee and saves it to our computer or uploads it to a database or updates our twitter feed or.whatever you'd like! Here is how it works, the XBee inside the Kill-a-Watt is hooked up to two analog signals. One is the voltage signal which indicates the AC voltage read. In general this is a sine wave that is 120VAC. One tricky thing to remember is that 120V is the 'RMS' voltage, and the 'true voltage' is +-170VDC. ( basically it's a way to indicate how much 'average' voltage there is.) The second reading is the AC current read. This is how much current is being drawn through the Kill-a-Watt.
If you multiply the current by the voltage, you'll get the power (in Watts) used! The XBee's Analog/Digital converter is set up to take a 'snapshot' of one sine-cycle at a time.
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Each double-sample (voltage and current) is taken 1ms apart and it takes 17 of them. That translates to a 17ms long train of samples. One cycle of power-usage is 1/60Hz long which is 16.6ms. So it works pretty well! Lets look at some examples of voltage and current waveforms as the XBee sees them. For example this first graph is of a laptop plugged in.
You'll see that it's a switching supply, and only pulls power during the peak of the voltage curve. Light bulb on dimmer switch The XBee sends the raw data to the computer which, in a python script, figures out what the (calibrated) voltage and amperage is at each sample and multiplies each point together to get the Watts used in that cycle. Since there's almost no device that changes the power-usages from cycle-to-cycle, the snapshot is a good indicator of the overall power usage that second. Then once every 2 seconds, a single snapshot is sent to the receiver XBee Install python & friends The software that talks to the XBee is written in python. I used python because it's quick to develop in, has multi-OS support and is pretty popular with software and hardware hackers. The XBees talk over the serial port so literally any programming language can/could be used here. If you're a software geek and want to use perl, C, C#, tcl/tk, processing, java, etc.
You'll have to read the serial data and parse out the packet but it's not particularly hard. However, most people just want to get on with it and so for you we'll go through the process of installing python and the libraries we need. Download and install python 2.5 from I suggest 2.5 because that seems to be stable and well supported at this time.
If you use another version there may be issues. (this will let us talk to the XBee thru the serial port).
If you're running windows (this will add file support). (this is how the twitter api likes to be spoken to) you'll need to uncompress the tar.gz file and then run the command 'python setup.py install' to install.
Now you can finally we will demonstrate here! We're going to download it into the C: wattcher directory, for other OS's you can of course change this directory. Basic configure We'll have to do a little bit of setup to start, open up the wattcher.py script with a text editor and find the line: SERIALPORT = 'COM4' # the com/serial port the XBee is connected to. Change COM4 into whatever the serial port you will be connecting to the XBee with is called. Under windows it's some COM x port, under linux and mac it's something like /dev/cu.usbserial- xxxx check the /dev/ directory and/or dmesg.
You should get a steady print out of data. The first number is the XBee address from which it received data, following is the estimated current draw, wattage used and the Watt-hours consumed since the last data came in. We have wireless data! Calibrating Now that we have good data being received, it's time to tweak it. For example, it's very likely that even without an appliance or light plugged into the Kill-a-Watt, the script thinks that there is power being used.
We need to calibrate the sensor so that we know where 'zero' is. In the Kill-a-Watt there is an autocalibration system but unfortunately the XBee is not smart enough to do it on its own. So, we do it in the python script. Quit the script by typing in Control-C and run it again this time as C: python25 python.exe wattcher.py -d note the -d which tells the script to print out debugging information. Now you'll notice that the numbers are pretty much all the same. That's because there's nothing plugged into the tweetawatt and so each 1/60 Hz cycle has a flat line at 'zero'. The A/D in the XBee is 10 bits, and will return values between 0 and 1023.
So, in theory, if the system is perfect the value at 'zero' should be 512. However, there are a bunch of little things that make the system imperfect and so zero is only close to 512. In this case the 'zero' calibration point is really 498.
When it's off there is a 'DC offset' to the Amp readings, as this graph shows. See how the Amps line is now at 0 steady, there is no DC offset. Logging data It's nice to have this data but it would be even nicer if we could store it for use. Well, that's automatically done for you! You can set the name of the log file in the wattcher.py script.
By default it's powerdatalog.csv. The script collects data and every 5 minutes writes a single line in the format Year Month Day, Time, Sensor#, Watts for each sensor. As you can see, this is an example of a 40W incandescent lightbulb plugged in for a few hours. Because of the low sample rate, you'll see some minor variations in the Watts recorded.
This data can be easily imported directly into any spreadsheet program. You have been successfully subscribed to the Notification List for this product and will therefore receive an e-mail from us when it is back in stock! For security reasons, an e-mail has been sent to you acknowledging your subscription. Please remember that this subscription will not result in you receiving any e-mail from us about anything other than the restocking of this item. If, for any reason, you would like to unsubscribe from the Notification List for this product you will find details of how to do so in the e-mail that has just been sent to you!
Summary: This story is about a girl named Cassandra who seems to be troubled and experiencing the sense of hopelessness and lost. Doesn't care about anything, gets in a lot of trouble. She skips school and smokes all day. Has a best friend named Rhonda who sings and is trying to sign a contract, which excites the both of them very much. Cassandra gets asked by these two girls, Melanie and Gladys if she is able to take them to her fathers house to deliver a package to him, as a result of being paid.
Have a little arguments on there way there and back. As a result of the story, Cassandra finds out that Rhonda has been shot by her husband and she cant accept the fact that it has happened. Point of View: The Story is written in third person.
'Cassandra had to get Melanie.' There is a distance between the narrator and the events. The narrator has complete information (thoughts and feelings) from Cassandra and limited information from the other characters. “She could not hear what they were saying but she could see Melanie shaking” Even though the story is told in third person it is mostly based on Cassandra´s perspective '.if I take off and leave them here.Cassandra thought' The narrator´s proximity to Cassandra makes the reader sympathize more with Rhonda´s death, because of the friendship between them. Tone: Informal Sarcastic dialogues “It never crossed my mind to do that”(50) The informal tone between the characters confirms their age and relationship Cassandra is characterized by her offensive and rude tone. “I don´t give three fucks who he is!”(48) The narrator does not describe feelings. The dialogues transmit feelings and emotions.
“This is Rhonda´s best friend, Mama” Anita said.” (53) Dialogue: Reveals a lot about the characters and expresses the themes: “You don’t know what you’re talkin about, girl’ Cassandra said. ‘Rhonda’s gonna stay the same. You don’t even know her all that well’” “They all change, and Rhonda’s gonna change the most.” “Oh, fuck you, Melanie!” (40) “If we get lost, it ain’t my damn fault” (45) “Thas why nothin ain’t right no more,’ Cassandra continued, “cause some dumb bitch like you think this dick-head and that dick-head is so cute! Get some brains, girl! I get so sick and tired of you!” (49) Main Characters: Cassandra: Main character of the story, who is a high school student who seems to be going through a lot of hate and anger towards the world with the exception of her best friend Rhonda and her parents. Both her parents died in a car accident, which may be a reason for her to rebel against anyone. She has an older sister and niece, who she stays with from time to time.
She's very sarcastic and loves to be in trouble. Conflict: Cassandra has lost her parents and has now lost one of her closest friends.
She has been left with feelings of displacement and this issue is never fully resolved, only soothed for a moment when Anita sang her to sleep with the song “Will You Love Me Tomorrow.