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Seismometer

dr pepper

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I'm thinking I'd like to put together a simple seismometer, something to detect road traffic and waves up on the shore.
I dont know if this is practical in the home lab or not.
I have 3 ideas in mind, 1 a loudspeaker with a weight on the cone connected to a filter/amplifier, 2 a pendulum from the lab roof to a weight and a coil to the same amplifier, and 3 I saw something online that uses a Rf oscillator on a center electrode, and 2 antennas each side connected to a diode/cap type Rf detector probe to detect movement of the earth.
Any idea on which is the best, the Rf type claims to have almost no natural frequency of the pendulum rolloff.
The speaker sounds nice & easy.
 

atferrari

Well-Known Member
I'm thinking I'd like to put together a simple seismometer, something to detect road traffic and waves up on the shore.
I dont know if this is practical in the home lab or not.
I have 3 ideas in mind, 1 a loudspeaker with a weight on the cone connected to a filter/amplifier, 2 a pendulum from the lab roof to a weight and a coil to the same amplifier, and 3 I saw something online that uses a Rf oscillator on a center electrode, and 2 antennas each side connected to a diode/cap type Rf detector probe to detect movement of the earth.
Any idea on which is the best, the Rf type claims to have almost no natural frequency of the pendulum rolloff.
The speaker sounds nice & easy.
Firs of all, solve this: where, whatever you implement, would be located (hanging?) and what could affect stability, even air currents and any source of undesired vibrations.
 

gophert

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Nigel Goodwin

Super Moderator
Most Helpful Member
You need a geophone, Sparkfun actually sell one, and there's an Arduino project on their website.



You can buy them MUCH cheaper from AliBaba in China

Here's one ready fitted to a spike and good to go.


We use geophones in one of our projects, which is why I know about them.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The simplest DIY one I've heard of is based on a piezo element taken from a cheap cigarette lighter.

Set up a weight and horizontal arm using a bit of thin steel shim as the hinge, with the piezo supporting the weight.
(The hinged arm prevents lateral movement without adding friction).

Use a very high impedance opamp or FET buffer.

The piezo gives thousands of volts from moderate finger pressure; they apparently give a good output from tiny changes in force from ground vibrations; it sounds reasonable and is easy enough to try!
 

dr pepper

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Thanks Guys,
Of course I should just go to ali baba and just buy one of those, if the country wasnt at a stand still.
So the piezo idea sounds interesting, I have a couple of ford flywheels, 3 piezo's under them to an amp would be a nice & easy setup.
Could I get away with all 3 piezo's into one amp, or should I amplify all 3 & sum them.
 

Nigel Goodwin

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Thanks Guys,
Of course I should just go to ali baba and just buy one of those, if the country wasnt at a stand still.
Deliveries are still happening (although my RS Components one didn't arrive yesterday, and the tracking doesn't say it's out for today yet either), and China seems to be back to 'reasonable' operation.
 

dr pepper

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Most Helpful Member
I'm waiting to order from China, but I think things are going to get way worse so I'm going to delay.
I have some 741's, some 324's and some Lf258's, and a load of passives so I'm going to see what I have piezo wise, I think I have a couple of 2" discs so might lash something up on breadboard.
Thought I might have loads of free time, nope I've been told I'm essential, thats never happened before.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
741's and 324's are pretty well zero spec for opamps, and I've never heard of an LF258? - nor has google.

You really need a high input impedance with piezo's though, so something like the TL07x or TL08x should be OK, or anything with a high input impedance.

BTW, my RS Components parcel arrived this afternoon :D
 

dr pepper

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Sorry Lm258, its a variant of the Lm358.
I dont exactly need an Op07 for sub 10hz signals, but I do need a big old coupling cap.
The input of the '258 is very high z, I beleive similar to the Tlo7 series, it will also run with the inputs at ground with single supply.
My Cpc delivery also came today, I now have padlocks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Sorry Lm258, its a variant of the Lm358.
I dont exactly need an Op07 for sub 10hz signals, but I do need a big old coupling cap.
Or DC couple it - but you shouldn't need a large capacitor at the input anyway, as it's such a high impedance.

The input of the '258 is very high z, I beleive similar to the Tlo7 series, it will also run with the inputs at ground with single supply.
Should be fine then.

BTW, a quick google finds a number of simple examples to play with, here's a couple:


 

Reloadron

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I got curious as to trucks on an overpass once and wondered what it looked like in my backyard. I used a single axis accelerometer which brings us to the type of sensor to use and equally important how sensitive is your sensor. I had a pile of old sensors so I tried a few ro see which gave the most bang for buck or actually pico coulomb/G or mVolt/G.

Accel2.jpg

Hard to see but they are drilled and tapped for a 10-32 screw on the bottoms for mounting, I also had a magnetic base for them and the interfacing cable. Again, any wide range of sensors, including home brew will work, just make sure it is sensitive enough to pick up small shock and vibrations.

Next you want some form of data acquisition to get the data from your sensor and do something with it. You can display it but sometimes we may want to display and record the data. Chart it, write it to a data base, just do something with it. Here is a small example of where I was measuring a floor fan at different speeds.

Vibe Fan 1.png

The results looked like this:
Fan Off To High Speed.png

When I did the seismographic experiments I used a 2 foot long piece of rebar I drove into the ground. I drilled and tapped one end for a 10-32 screw and mounted my accelrometer on the Up end. It was amusing to listen to traffic and note when heavy trucks hit the expansion joints in the bridge. Guessing waves hitting a beach at intervals would be pretty interesting too.

Ron
 

dr pepper

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Most Helpful Member
Interesting thanks guys.
The big cap I was thinking would be for interstage coupling.
I'd put this together using an esp8266 and have a web page for it.
I too would like to see the effect of the road and the railway, not that they'd do any harm they are 1/4 and 1/2 a mile away.
 

Reloadron

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Most Helpful Member
Years ago (been a long time) I made up several circuits using accelerometers like those I posted and used a single AD524 programmable gain op-amp. If it was an AD 524 I remember at the time they were expensive but worked real well.

Ron
 

atferrari

Well-Known Member
When I did the seismographic experiments I used a 2 foot long piece of rebar I drove into the ground. I drilled and tapped one end for a 10-32 screw and mounted my accelrometer on the Up end. It was amusing to listen to traffic and note when heavy trucks hit the expansion joints in the bridge. Guessing waves hitting a beach at intervals would be pretty interesting too.

Ron
Wish I could do something similar in this access to Bs. Aires (from the South). While driving there, the trucks overtake you through the leftmost lane and you really feel it! Sometimes it is even a bit scary.

Acceso Sur.jpg
 

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