In September of 2023, I wrote in these pages about utilizing a Raspberry Pi–based mostly seismometer—a Raspberry Shake—to file 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 secure U.S. East Coast, so the one earthquakes I may hope to detect can be ones going down distant. Sadly, the alerts from distant quakes have comparatively low vibrational frequencies, and the compact geophone sensor in a Raspberry Shake is supposed for increased frequencies.
I had initially thought of different types of DIY seismometers, and I used to be postpone by how giant and ungainly they have been. However my disappointment with the Raspberry Shake drove me to assemble a seismometer that represents an excellent compromise: It’s not so giant (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 secure place within the center, however when it’s nudged, the restoring pressure could 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 perhaps 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, hooked up to the top 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 increase, 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 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 in opposition to a shallow dimple in a small metal plate screwed to the mast. To help the increase, I used some shifter cable from a bicycle, hooked up by looping it by 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 position of the top 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]. Knowledge is saved on a logging board with a real-time clock [bottom]. James Provost
I fabricated a number of different small bodily bits, together with leveling toes and a U-shaped bracket to forestall the increase from swinging too removed from equilibrium. However the principle challenges have been methods to sense earthquake-induced motions of the increase and methods to stop it from oscillating indefinitely.
Most DIY seismometers use a magnet and coil to sense movement because the shifting 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 gradual 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 exhausting to come back by. As an alternative, I adopted a method I hadn’t seen utilized in every other homebrewed seismometer: employing a Hall-effect magnetometer to sense position. All I wanted was a small neodymium magnet hooked up to the increase and an inexpensive Hall-effect sensor board positioned beneath it. It labored simply nice.
I figured the immense excursions should replicate some form 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 may 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 move by a powerful 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 hooked up the magnets to the body, mimicking a horseshoe magnet. This labored fairly nicely, though my seismometer remains 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 file 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 may see that models 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, after I first checked out my knowledge, I figured the immense excursions should replicate some form 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 may even make out a precursor 5.9 earthquake about an hour earlier than the massive one.
My new seismometer shouldn’t 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 alerts. For my part, this Goldilocks design is excellent.
