In September of 2023, I wrote in these pages about utilizing a Raspberry Pi–primarily based seismometer—a Raspberry Shake—to report earthquakes. However as time glided by, I discovered the outcomes disappointing. Looking back, I notice that my creation was struggling to beat a elementary hurdle.
I stay on the tectonically steady U.S. East Coast, so the one earthquakes I might hope to detect could be ones happening distant. Sadly, the alerts from distant quakes have comparatively low vibrational frequencies, and the compact geophone sensor in a Raspberry Shake is supposed for greater frequencies.
I had initially thought of different types of DIY seismometers, and I used to be postpone by how massive and ungainly they have been. However my disappointment with the Raspberry Shake drove me to assemble a seismometer that represents a great compromise: It’s not so massive (about 60 centimeters throughout), and its resonant frequency (about 0.2 Hertz) is low sufficient to make it higher at sensing distant earthquakes.
My new design is for a horizontal-pendulum seismometer, which accommodates a pendulum that swings horizontally—or nearly so, being inclined only a smidge. Consider a fence gate with its two hinges not fairly aligned vertically. It has a steady place within the center, however when it’s nudged, the restoring pressure may be very weak, so the gate makes sluggish oscillations backwards and forwards.
The spine of my seismometer is a 60-cm-long aluminum extrusion. Or perhaps I ought to name it the keel, as this seismometer additionally has what I might describe as a mast, one other piece of aluminum extrusion about 25 cm lengthy, hooked up to the tip of the keel and sticking straight up. Beneath the mast and hooked up to the underside of the keel is an aluminum cross piece, which prevents the seismometer from toppling over.
The pendulum—let’s name it a growth, to stay with my nautical analogies—is a 60-cm-long bar minimize from 0.375-inch-square aluminum inventory. At one finish, I hooked up a 2-pound lead weight (one meant for a diving belt), utilizing plastic cable ties.
To permit the growth to swing with out undue friction, I drilled a gap within the unweighted finish and inserted the carbide-steel tip of a scribing tool. That sharp tip rests in opposition to a shallow dimple in a small metal plate screwed to the mast. To assist the growth, I used some shifter cable from a bicycle, hooked up by looping it via a few strategically drilled holes after which locking issues down utilizing metal sleeves crimped onto the ends of the cable.
Establishing the response of the seismometer to vibrations is the function of the tip weight [top left] and dampening magnets [top right]. A magnet can be used with a Corridor impact sensor [middle right] that’s learn by a microcontroller [middle left]. Information is saved on a logging board with a real-time clock [bottom]. James Provost
I fabricated a couple of different small bodily bits, together with leveling toes and a U-shaped bracket to stop the growth from swinging too removed from equilibrium. However the principle challenges have been learn how to sense earthquake-induced motions of the growth and learn how to stop it from oscillating indefinitely.
Most DIY seismometers use a magnet and coil to sense movement because the transferring magnet induces a present within the mounted coil. That’s a tough proposition in a long-period seismometer, as a result of the relative movement of the magnet is so sluggish that solely very faint electrical alerts are induced within the coil. One of many more sophisticated designs I noticed on-line referred to as for an LVDT (linear variable differential transformer), however such gadgets appear onerous to come back by. As a substitute, I adopted a technique I hadn’t seen utilized in another homebrewed seismometer: employing a Hall-effect magnetometer to sense position. All I wanted was a small neodymium magnet hooked up to the growth and an inexpensive Hall-effect sensor board positioned beneath it. It labored simply nice.
I figured the immense excursions should mirror some type of gross malfunction!
The ultimate problem was damping. With out that, the pendulum, as soon as excited, would oscillate for too lengthy. My preliminary resolution was to connect to the growth an aluminum vane immersed in a viscous liquid (specifically, oil). That labored, however I might simply see the messy oil spills coming.
So I tacked within the different route and constructed a magnetic damper, which works by having the aluminum vane go via a robust magnetic discipline. This induces eddy currents within the vane that oppose its movement. To the attention, it looks like the metal is caught in a viscous liquid. The problem right here is making a pleasant sturdy magnetic discipline. For that, I collected all of the neodymium magnets I had available, kludged collectively a U-shaped metal body, and hooked up the magnets to the body, mimicking a horseshoe magnet. This labored fairly effectively, though my seismometer continues to be considerably underdamped.
In contrast with the fussy mechanics, the electronics have been a breeze to assemble. I used a US $9 data-logging board that was designed to just accept an Arduino Nano and that features each a real-time clock chip and an SD card socket. This allowed me to report the digital output of the Corridor sensor at 0.1-second intervals and retailer the time-stamped information on a microSD card.
My homebrew seismometer recorded the hint of an earthquake occurring roughly 1,500 kilometers away, starting at roughly 17:27 and ending at 17:37.James Provost
The primary good take a look at got here on 10 November 2024, when a magnitude-6.8 earthquake struck just off the coast of Cuba. Consulting the worldwide repository of shared Raspberry Shake data, I might see that items in Florida and South Carolina picked up that quake simply. However ones situated farther north, together with one near the place I stay in North Carolina, didn’t.
But my horizontal-pendulum seismometer had no hassle registering that 6.8 earthquake. In truth, once I first checked out my information, I figured the immense excursions should mirror some type of gross malfunction! However a comparability with the trace of a research-grade seismometer situated close by revealed that the waves arrived in my storage at the exact same time. I might even make out a precursor 5.9 earthquake about an hour earlier than the large one.
My new seismometer isn’t too huge and awkward, as many long-period devices are. Neither is it too small, which might make it much less delicate to far-off seismic alerts. For my part, this Goldilocks design is good.
