This article was first published in 2009.
Seismometers measure earth vibrations. You’ve probably seen them on TV after an earthquake – the seismographic record shows the magnitude of the ‘quake and, through the use of three or more seismographs, the epicentre of the ‘quake can be established.
The seismometer presented here won’t be finding a place in too many geological laboratories, but it’s cheap and easy to construct, sensitive enough that on a wooden floor it can literally detect a cat walking past. Equipped with a remote probe, it can also be used as a vibration detector of the sort that’s used to assess whether machine bearings are running smoothly.
In short it’s a fun, interesting project that costs nearly nothing to make.
To make the seismometer you’ll need a discarded (but still working) cassette deck that has VU meters. The meters can be either analog or digital. In addition, you’ll also need a couple of speakers – for example, the speakers salvaged from a stereo TV. If you can’t score that lot for under five bucks, you’re not really trying.
If you want to make a fancy new faceplate, you’ll also need your trusty PC, a printer and a scanner.
The seismometer takes advantage of the fact that a cassette deck uses two amplifiers that are designed to work with very small signals. Normally these signals are read off the tape by the heads, but what we do is feed in new signals derived from coils of wire being moved past magnets. Since speakers have very strong magnets, coils with lots of windings and very small internal clearances, they make ideal sensors for the seismometer.
If the speaker basket is firmly attached to the ground and a vibration occurs, the basket and the cone will tend to move at different rates. For example, if there is a sudden movement upwards, the inertia of the cone will mean it gets left behind for a moment. As a result, the magnet will move past the coil, causing a small voltage to be generated in the coil. This voltage is amplified and displayed on the VU meters.
The greater the needle deflection, the greater the amount of vertical vibration that has occurred.
At its simplest, the seismometer will take only a few minutes to make.
Make sure that the power cord is disconnected from mains supply and then take the cover off the cassette deck. Trace the wires (they’ll be shielded) that connect the amplifier printed circuit board to the heads. There will be six conductors in these leads – for each head a common, play and record signal feeds.
Cut these wires and feed them out of the case, so that you can put the lid back on. Replace the lid, power-up the deck, press the ‘play’ button and then connect a speaker across the wires for one channel, trying the various combinations until you find a pair which causes a VU meter to strongly react to any speaker movement.
Then do the same for the other channel.
You may need to extend these cables; in this case we used the RCA cables that came with the deck.
A more complex approach (and what we did here) is to also strip the cassette deck of the surplus parts. For example, the complete tape mechanism was removed. Why? Well, the DC motor, drive belts and springs can find a use in another project, as can the tape counter. It seems a shame to leave them inside the seismometer.
If you take this approach you’ll need to activate the same switches that pressing the ‘play’ button normally does. For example, if when ‘play’ is pressed a single switch is closed, the wires leading to that switch will need to be connected together. On the other hand, you may find that when the cassette mechanism is removed, the PCB is effectively always in ‘play’ mode.
Different speakers will provide different sensitivities. We tried a variety and found that the larger the speaker, the more sensitive the seismometer became.
The speakers shown here (100mm units salvaged from a stereo TV) give the seismometer sufficient sensitivity that in a wood-framed, two-storeys-on-stilts house, anyone walking anywhere within the house gives a noticeable reading. That even includes walking on the concrete pad under the house! As I type this, the seismometer is on my desk; with each normal force keystroke, the display meters are reading just under full-scale!
To make the cassette deck look less like a deck and more like a seismometer, you can make a new faceplate. If the original faceplate is removable (most are), take it off and scan it in to your PC. Then use image manipulation software to construct the new visuals, putting on whatever labels you want. That done, print it out at full size on heavy stock, gloss paper. After that it’s just a case of sticking the paper over the original. We used clear ‘contact’ adhesive film to protect the paper.
If you want, you can also replace the scale behind the VU meters. However, in the case of the seismometer shown here, we elected to keep the original scales.
If the sensitivity of the seismometer is too great, reduce the size of the speaker. Adding weights to the cone also alters response. When exploring the use of different speakers, you need to be aware that a typical house is full of background vibrations. The compressor in the fridge can cause sufficient vibration to swamp other signals, while a PC cooling fan can also cause clearly visible room vibration. So to be able to watch the earth vibration caused by (say) visitors walking up to your front door, you’ll need to remotely mount the speaker sensor away from this house-borne noise – although note that too long a cable will cause a reduction in sensitivity.
Finally, if you want to feed the output signal into a logging system or drive an external display, line-level output voltages are available on the normal RCA outputs of the deck.
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