In September of 2023, I wrote in these pages about utilizing a Raspberry Pi–primarily based seismometer—a Raspberry Shake—to document earthquakes. However as time glided by, I discovered the outcomes disappointing. On reflection, 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 can be ones happening far-off. Sadly, the indicators 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-about different kinds 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 very good 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 comprises 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 drive may be very weak, so the gate makes gradual oscillations forwards and backwards.
The spine of my seismometer is a 60-cm-long aluminum extrusion. Or possibly I ought to name it the keel, as this seismometer additionally has what I’d describe as a mast, one other piece of aluminum extrusion about 25 cm lengthy, connected to the top of the keel and sticking straight up. Beneath the mast and connected 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 increase, to stay with my nautical analogies—is a 60-cm-long bar reduce from 0.375-inch-square aluminum inventory. At one finish, I connected a 2-pound lead weight (one supposed for a diving belt), utilizing plastic cable ties.
To permit the increase 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 towards a shallow dimple in a small metal plate screwed to the mast. To help the increase, I used some shifter cable from a bicycle, connected 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 top weight [top left] and dampening magnets [top right]. A magnet can also be used with a Corridor impact sensor [middle right] that’s learn by a microcontroller [middle left]. Knowledge is saved on a logging board with a real-time clock [bottom]. James Provost
I fabricated just a few different small bodily bits, together with leveling ft and a U-shaped bracket to stop the increase from swinging too removed from equilibrium. However the principle challenges have been the right way to sense earthquake-induced motions of the increase and the right way 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 difficult proposition in a long-period seismometer, as a result of the relative movement of the magnet is so gradual that solely very faint electrical indicators 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 units appear exhausting to come back by. As an alternative, 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 connected to the increase 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 increase an aluminum vane immersed in a viscous liquid (particularly, 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 subject. 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 robust magnetic subject. For that, I collected all of the neodymium magnets I had available, kludged collectively a U-shaped metal body, and connected the magnets to the body, mimicking a horseshoe magnet. This labored fairly nicely, 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 simply accept an Arduino Nano and that features each a real-time clock chip and an SD card socket. This allowed me to document the digital output of the Corridor sensor at 0.1-second intervals and retailer the time-stamped knowledge 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 check 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 positioned farther north, together with one near the place I stay in North Carolina, didn’t.
But my horizontal-pendulum seismometer had no bother registering that 6.8 earthquake. In reality, after I first checked out my knowledge, I figured the immense excursions should mirror some type of gross malfunction! However a comparability with the trace of a research-grade seismometer positioned 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 will not be 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 indicators. In my opinion, this Goldilocks design is good.
