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I've been messing around with the design of a geophone. This is a sensor that is coupled to the ground (sits on the ground) and records vertical movement.

I will use a phototransistor to detect movement of a suspended weight. The weight could be a permanent magnet so a coil could act on it. I'd like to build a circuit that would energize a coil + or - to oppose any movement of the weight. So I don't have to be concerned about an occilation period of the suspended weight. I just need to record the current fed to the coil. I have an lm324 quad op amp I'm planning to use. Thanks Peter

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    \$\begingroup\$ This question is overly broad and difficult to answer in its current form. \$\endgroup\$ – Olin Lathrop Dec 4 '11 at 21:31
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    \$\begingroup\$ I can't even work out what the question is asking... \$\endgroup\$ – Majenko Dec 4 '11 at 22:37
  • \$\begingroup\$ Proper geophones aren't very expensive. \$\endgroup\$ – Leon Heller Dec 4 '11 at 23:29
  • \$\begingroup\$ tRY TO FIND THE wIRELESS wORLD FORCE BALANCE UNIT. probably USED TERMS FLUX GATE AND SPEAKER \$\endgroup\$ – Russell McMahon Dec 5 '11 at 12:52
  • \$\begingroup\$ It's unfortunate that the question was closed by those who don't understand it. What is being requested is a circuit for magnetically driving a mass to maintain position in response to optical error feedback, so that either the optical feedback or the drive signal can be tapped out as a data source. \$\endgroup\$ – Chris Stratton Dec 5 '11 at 20:51
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Long ago, Wireless world published a flux gate force balance circuit using a loudspeaker as the position driver. Gargoyle probably knows - I don't have time just now to dig. I MAY have the magazine if someone finds the reference.

This amazingly simple but capable fluxgate magnetometer paper should be of interest. A cleaver sensor. I've seen similar suggested elsewhere - some superb results are claimed. Brown University. They say -

  • A simple fluxgate magnetometer can be constructed out available equipment in the lab. It can easily measure the magnetic field of the earth .You will need the following equipment: (1) A function generator,( 2) an oscilloscope, (3) Pasco waveform analyzer, (4) a Thorton amplifier ,(5) a spool of magnet wire, and (6) 1 meter of 18 gauge stovepipe wire( iron wire that becomes magnetically saturated at low magnetic fields)

DIY fluxgate sensor and some good references.

Example only magnetometer circuit from the above page.

enter image description here


Brooke Clark's super sensor links page should be useful.


These people who patented the circuit below think it's a force balance magnetometer based on a geophone:

  • The invention concerns methods and apparatus for improving the performance of a force-balance accelerometer based upon a conventional, single-coil, velocity geophone. Specifically, the operating temperature range is increased through the use of both a temperature-compensating reference impedance, and a new electronic circuit architecture. Two specific types of temperature-compensating reference impedances are disclosed. One is a pure DC-resistance, with the temperature coefficient of the DC resistance matching that of a single coil of a conventional geophone. A second reference impedance adds a series reactance which closely matches the ratio of total impedance to DC-resistance, and the temperature coefficient of this ratio, with that of the geophone coil.

    A method is also described which provides for decreased magnitude of a reference impedance required in an accelerometer. This allows for a significant reduction in the physical size of the reference impedance. The reduction in size reduces the physical size of the impedance and the sensitivity of the accelerometer to external time-varying magnetic fields.

enter image description here


The "missing link" in the above which I could (easily :-( ) have made clearer is that a traditional geophone is a simple open loop device where a coil is moved near a magnet by a vibration source and induces a voltage which is analysed. Such instruments are affected by self resonance.

What Peter referred to and what the Wireless World article I mention addresses, was the use of feedback to null the position of the moving element against the perturbing forces and produce a feedback error signal as the output. Such a system is less affected by the mechanical characteristics of the sensor device.

The WW circuit implemented a fluxgate detector to provide feedback. My mention of the magnetometer is in that context but I agree, taken in isolation, it is not clear how it relates.

Finding the WW article would be nice. I'll try in due course. Anyone?

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  • \$\begingroup\$ Nothing like a geophone \$\endgroup\$ – Leon Heller Dec 4 '11 at 23:27
  • \$\begingroup\$ @LeonHeller - Perhaps "not exactly like all other geophones" :-). The original Wireless World article were essentially used as precision scales. A Fluxgate allows VERY sensitive measurement of signal change. The loud speaker acts as a "nice" mechanical to electric transducer that uses feedback to null mechanical forces electrically. Seems to me eminently usable a a geophone (but, I may be wrong). There are probably better methods that more thought would turn up. But what I mentioned seems to match what he described. Maybe not. || \$\endgroup\$ – Russell McMahon Dec 5 '11 at 3:25
  • \$\begingroup\$ @LeonHeller Presumably if you look at my overall suggestions and find you agree with them you'll remove the -1 vote against yourself? \$\endgroup\$ – Russell McMahon Dec 5 '11 at 3:28
  • \$\begingroup\$ Nothing like this ? freepatentsonline.com/6918299-0-large.jpg \$\endgroup\$ – Russell McMahon Dec 5 '11 at 3:31
  • \$\begingroup\$ The "missing link" in the above which I could have made clearer is that a traditional geophone is a simple open loop device where a coil is moved near a magnet by a vibration source and induces a voltage which is analysed. Such instruments are affected by self resonance. | What Peter referred to and what the Wireless World article I mentioned addressed, was the use of feedback to null the position of the moving element against the perturbing forces and produce a feedback error signal as the output. Such a system is less affected by the mechanical characteristics of the sensor device. \$\endgroup\$ – Russell McMahon Dec 5 '11 at 4:48
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There is huge gap between having a LM324 and coil, and a working system.

First you will need something that can detect small movements and indicate deviation from the desired position. The signal from this sensor is what the servo system will try to null out. Then you will need a means of driving the coils from the relatively weak signal out of the LM324. But the most difficult part will be tuning this properly, especially if you want to do it all in analog as it seems you do.

Nobody is going to hand you a complete circuit. They can't even if they wanted to because we know little about your particular details. Try asking more specific questions.

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