Arthur T Knackerbracket has processed the following story:
This basement seismometer is relatively compact yet still sensitive enough to detect the low-frequency vibrations from distant earthquakes.
In September of 2023, I wrote in these pages about using a Raspberry Pi–based seismometer—a Raspberry Shake—to record earthquakes. But as time went by, I found the results disappointing. In retrospect, I realize that my creation was struggling to overcome a fundamental hurdle.
I live on the tectonically stable U.S. East Coast, so the only earthquakes I could hope to detect would be ones taking place far away. Unfortunately, the signals from distant quakes have relatively low vibrational frequencies, and the compact geophone sensor in a Raspberry Shake is meant for higher frequencies.
I had initially considered other sorts of DIY seismometers, and I was put off by how large and ungainly they were. But my disappointment with the Raspberry Shake drove me to construct a seismometer that represents a good compromise: It’s not so large (about 60 centimeters across), and its resonant frequency (about 0.2 Hertz) is low enough to make it better at sensing distant earthquakes.
My new design is for a horizontal-pendulum seismometer, which contains a pendulum that swings horizontally—or almost so, being inclined just a smidge. Think of a fence gate with its two hinges not quite aligned vertically. It has a stable position in the middle, but when it’s nudged, the restoring force is very weak, so the gate makes slow oscillations back and forth.
[...] Most DIY seismometers use a magnet and coil to sense motion as the moving magnet induces a current in the fixed coil. That’s a tricky proposition in a long-period seismometer, because the relative motion of the magnet is so slow that only very faint electrical signals are induced in the coil. One of the more sophisticated designs I saw online called for an LVDT (linear variable differential transformer), but such devices seem hard to come by. Instead, I adopted a strategy I hadn’t seen used in any other homebrewed seismometer: employing a Hall-effect magnetometer to sense position. All I needed was a small neodymium magnet attached to the boom and an inexpensive Hall-effect sensor board positioned beneath it. It worked just great.
[...] The first good test came on 10 November 2024, when a magnitude-6.8 earthquake struck just off the coast of Cuba. Consulting the global repository of shared Raspberry Shake data, I could see that units in Florida and South Carolina picked up that quake easily. But ones located farther north, including one close to where I live in North Carolina, did not.
Yet my horizontal-pendulum seismometer had no trouble registering that 6.8 earthquake. In fact, when I first looked at my data, I figured the immense excursions must reflect some sort of gross malfunction! But a comparison with the trace of a research-grade seismometer located nearby revealed that the waves arrived in my garage at the very same time. I could even make out a precursor 5.9 earthquake about an hour before the big one.
My new seismometer is not too big and awkward, as many long-period instruments are. Nor is it too small, which would make it less sensitive to far-off seismic signals. In my view, this Goldilocks design is just right.
(Score: 2, Interesting) by Anonymous Coward on Sunday January 05, @03:51AM
One involves length- two posts driven into the ground many meters apart, a taught rope between them, and a strain gauge measuring the tension. Of course you have to compensate for temperature.
Another involves a pan of water where you aim a light or laser at an angle and have light sensors on the other side. Minor earth movements cause ripples on the water's surface and will cause changes in the light sensors' outputs.
(Score: 3, Interesting) by VLM on Sunday January 05, @04:35PM
A pretty good rendition of the 1960s Scientific American Amateur Scientist seismograph design. I didn't see credit being given, oh well. Most of modern "maker" culture is re-inventing the past while pretending its new. So, uh, yeah, lets pretend nobody never did exactly the same thing 60 years ago.
Using post 1960s super-magnets is interesting and clean although very expensive. The original used a flapper stuck in oil in a bowl as a dampener.
There was a later design, I'd say in the 70s, which worked around the incredible poor performance of short arms as a sensor for long time period waves by implementing a pretty wild analog op amp solution to filter a simple small capacitor signal which must have been a SNR challenge (I never built it, although I certainly thought about it)
In the modern era I have some ideas I never pursued. One is attach a mirror to "something heavy" then shine a laser at the mirror and set up a web cam recording video a "long distance away" to amplify the signal.
Another is I think if you put a "large weight" on a "large balloon" or air bag, and stuck a low frequency microphone or high resolution barometric sensor (which are cheap now, at least compared to 70s) then you could measure seismograph data as "sound" or pressure fluctuations.
Another fun idea I never pursued is take the 1970s high noise opamp design from Scientific American and replace it with 2020s low noise opamp input stages and a great pile of DSP processing.
His hall effect sensor is interesting but it might be simpler to use something like an optical mouse sensor. I came pretty close to building this. You can prototype it by duct taping an optical mouse to a modest weight and hanging that on a very long pendulum, then you have the painful task of turning mouse signals into movement data (painful for a PC or anything using usb, probably not difficult for a PS/2 attached microcontroller) I was going to go from the roof peak of my garage to the floor, so two stories.
A final idea I had is traditional seismographs go to enormous effort to provide a very low noise environment for a single ultra-expensive instrument. Why not 3-d print and IoT up maybe ten or twenty "merely OK" instruments and toss then around my property and let a computer remove local noise sources?
All of the above are in the class of "in my infinite spare time".