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I was wondering if people had ideas to help move the following forward...

If you can mount a massive pendulum from a "rock solid" support (the wall in a poured concrete basement, for instance), you have yourself a seismometer... If the ground moves, the pendulum will appear to, as it will stay still. Inertia, and all that.

If you mount an LED on the bottom of the pendulum, and put some kind of "webcam" under that, looking up, and record what the webcam sees, you will have a record of seismic activity.

I put "webcam" in quotes, because I doubt you'd want, literally, a webcam. But some device with a silicon "eye". Ask the eye which pixel is brightest from time to time, and that's all you need to record... you don't record the whole image again and again!

You'll want to read and re-read the sensor frequently. And record what is seen frequently... although you can probably put something in the software to say "if the LED hasn't moved, don't record the position AGAIN".

Ideas... especially in respect of the sensor? One of Sparkfuns little CCTV units?
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I don't think you connected the grounds, Dave.
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How about an optical mouse?
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What about bouncing a laser off of it so that the edge of the returned circle of light falls on the center of a photo sensitive component - so that the voltage passed by the component varies with how much of the bounced light covers the component.  Like the 'laser listening device' that can be found on the internet?
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Why not just use a Geophone?
'cos there's no slot for a SIM card?
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The idea should work - its a variation on Foucaults pendulum. Formulas you might need - http://en.wikipedia.org/wiki/Pendulum

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You'll want to read and re-read the sensor frequently. And record what is seen frequently... although you can probably put something in the software to say "if the LED hasn't moved, don't record the position AGAIN".
That is known as runlength compression, in fact what you write is {time, value, count} tuples - Time is optional but easy for processing later

Q: what do you want to measure?
- the fact that there is movement (true - false)
- the amplitude of the movement (0...n)

Do you live in an EQ zone?

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Thanks for the interesting comments... keep 'em coming!

Want to measure: As much as possible... i.e. the direction and amplitude of the pendulum's movement relative to basement.

Live in earthquake zone: They are rare and small, but do occur. Mostly a "can I do it?" exercise.

Geophone: I suspect the pendulum would be more sensitive, and less prone to pick up the effect of footsteps or big vehicles going by the house. Also: Doesn't give information on direction of movement.

Laser/ Amplitude... I guess I'm just a digital junkie.... No problems of compensation for accumulation of dust, etc. Sounds like a good sensing idea to implement alongside the digital "answer", to see how they corespond.

Optical mouse.... interesting idea! Might be hard to position mouse close enough to reliably "read" target on bottom of pendulum... but probably not impossible. Avoids need to feed power across the pendulum's suspension. I wonder what the resolution/ response time trade offs would look like. Main dis-incentive: I don't wan't to deal with talking across the mouse's USB interface.

====
Further new idea, prompted by the laser suggestion....

If two small mirrors were mounted at 90 deg to each other on outside of pendulum, at an to vertical, and each had a vertical beam of light bounce off it, the "spot" thrown off would move up and down the wall it hit. Two simple linear arrays of light detectors (probably something like the sensor in a scanner?) could determine the pendulum's position. Anyone know of the sort of sensor this would entail? One of the beauties of this solution is that by varying the angle the mirror was mounted at, you could change the sensitivity and range of the instrument.
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idea:
you could place a horizontal line laser near the pendulum so a shadow is cast on the wall with sensitive detectors. If the laser is placed near the pendulum there is an angular magnification (I don't know how to call it otherwise).  A slight movement of the pendulum gives a larger displacement on the wall. It is easy if the pendulum is a cilinder to detect X and Y movements. Then one can calculate in which direction the cilinder moves first, and this might be indicative for the direction of the epicentre.

idea ?
I can imagine a sphere floating in a magnetic field as pendulum. Then 3 dimensions might be possible?
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idea:
you could place a horizontal line laser near the pendulum so a shadow is cast on the wall with sensitive detectors. If the laser is placed near the pendulum there is an angular magnification (I don't know how to call it otherwise).  A slight movement of the pendulum gives a larger displacement on the wall. It is easy if the pendulum is a cilinder to detect X and Y movements. Then one can calculate in which direction the cilinder moves first, and this might be indicative for the direction of the epicentre.

idea ?
I can imagine a sphere floating in a magnetic field as pendulum. Then 3 dimensions might be possible?

Rob:

I think that idea might go to he11 in a hand basket if there is too much motion in three dimensions. You can't even be sure that the wall be there to hold the sensors in a large quake...  smiley-evil

Imaginative though...

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You can't even be sure that the wall be there to hold the sensors in a large quake... 
If the quake is that large you don't wait for the Arduino to tell you there's a quake ...

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You can't even be sure that the wall be there to hold the sensors in a large quake... 
If the quake is that large you don't wait for the Arduino to tell you there's a quake ...




Good point! INCOMING!!!!
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I can imagine a sphere floating in a magnetic field as pendulum. Then 3 dimensions might be possible?
Didn't I see that in a sci-fi movie?  Or perhaps on TV ("Alias" or "24"?)

Definitely from sci-fi
a quick google "gadget floating sphere magnetic" gave things like - http://www.wired.com/gadgetlab/2010/11/hacker-builds-floating-jedi-training-remote-droid/

such a thing could become a nice EQ sensor smiley
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Serendipity?

Over at the "Linear CCD" discussion in this forum, they are talking about the sort of linear sensor arrays that my "light beam" approach would need... $16 for something that might work...

http://www.taosinc.com/productdetails.aspx?ID=10

(I just mention it, in case others found the idea of the sensor array attractive, wondered where to get one.)

You can of course get fancier sensor arrays up to your willingness to $pend.
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Mueller Device .... But back to a practical level, note that keeping something floating in mid-air involves positional feedback and modulating magnetic fields that render the object relatively insensitive to things like seismic movement.

Can't you create a magnetic bowl in which the ball floats with fixed magnets?

ThinkingOutLoud:
- Put 6 or more magnets with North up in a circle - maybe with a small angle
- cover the ball with coinlike magnets with North to the outside. ..
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Nice device to detect a movement in the Z direction ==> the Shear waves

Just read an interesting story in the Scientific American April 2011 edition Page 54 - 59
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You could try using a 2-axis accelerometer mounted to a pendulum, using very light guage wire to connect it to an arduino so it don't interfere with the movement of the pendulum.
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