Archive | January, 2012

What WiFi looks like to your router

13 Jan

Here’s another field visualization from a while back. These folks made a directional antenna for WiFi and mapped each network’s intensity as a function of direction. The blue mist shows a WiFi signal coming through a door near the middle of the image, maybe reflecting off the wall on the left. It’s an image-based technique. It won’t tell you the WiFi strength at the red table farther back in the photo, but it can tell you how to point your antenna when you’re standing at the spot where the “image” was collected.

Thanks to Paul Faget for the link.

60 Hz digital Arduino filter does the job!

10 Jan

60 Hz filter performance measured using function generator

To test the digital filter, I removed the EMF sensing resistor and antenna, and then plugged in a 1V peak-to-peak signal from a sine wave generator at different frequencies, up to the maximum frequency of 250 Hz (half the 500 Hz sampling frequency). It gets a little bumpy at low frequency, probably because we’re averaging over a small number of cycles (less than 3).

For a DC input of 5 volts, the Arduino reports an output of 1023. So the filter is working to extract the 1V amplitude, which is 1/5 *1023, or approximately 200. The signal drops to about half the maximum within +/- 15 Hz of 60 Hz.  Nice!

I made a version of the EMF sensor that reports the 60Hz voltage as a LED output on pin 11, and “everything BUT 60 Hz” on pin 3, where I put a green LED. The red LED lights near active wires and cords, while the green one goes nuts near compact-fluorescent bulbs (CFL). Two colors will help us distinguish more different kinds of events. To wire it up, just add a 3- to 8 Meg resistor between pin A5 and ground, then stick about a 4 inch wire from pin A5 hanging off into space as in the original EMF detector. The Arduino code is after the jump, and there’s also a list of 50 Hz filter coefficients if you live in one of those regions. You’ll have to truncate those and plug them into your code as we did with the 60 Hz, but you won’t need MATLAB to generate them.

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