Here is the preamp.
I was going to place it right at the geophone.
The two exact coils will change in step
with temperature to keep a stable DC
baseline. Minimal drift.

http://gmvoeth.home.mchsi.com/AMP001.jpg

Any reason this should not be right ?

Will Larry sell only the coils ?
I might get two to try my ideas.

I looked into getting ten wound but the company
was outragious in its (retooling fees)
It seems they cant simply do it even tho they
wind the things for a living.

Thanks for your response.

Best Regards,
geoff

PS: if PSN had a binary news server.
      we could post any kind of file
      for people to have/look at ?
      Could someone create a [alt.binaries.seismic.psn]
      news group, I understand there is a complex process
      but the creation is free and almost all ISP have
      one as part of their service. I have tried to understand
      the process but like Linux OS I cant understand it.
      I have tried Linux several times but cant make it functional.
      Fedora 10 installs OK but I just cant get it to be functional.

----- Original Message ----- 
From: "Brett Nordgren" 
To: 
Sent: Sunday, July 18, 2010 10:57 AM
Subject: Re: Velocity Sensor Question ?


> Hi Geoff,
> 
> At 07:30 AM 7/18/2010, you wrote:
>>Hello Mr. Nordgren;
>>
>>The question I have is thus:
>>
>>Is that coil right for the magnets or,
>>Should it be rectangular in shape with the
>>two vertical sides of the rectangle outside the magnetic flux ?
> 
> Ideally you want your sensor to be linear.  If you move the coil at a 
> constant speed over its mechanical range you would like to see a 
> relatively constant voltage out.  You can achieve that in two 
> ways.  Have the magnet pole faces large enough that all the coil 
> wires stay well within the magnetic region, or alternatively, make 
> them small enough that all the magnetic lines stay within the region 
> filled with coil wires.  When you have the edges of the magnetic 
> field moving across the outside or inside edges of the coil, the 
> linearity suffers, though perhaps not enough to worry about too much.
> 
>>I figure I may need like 1206 feet of 36 AWG(B&S) copper enameled wire
>>for a 2100 turns 500 ohm coil ?
>>
>>The 2100 turns are of enamled wire without a heavy coat of enamel.
>>
>>Is 2100 turns enough ?
> 
> I guess the question would have to be, enough for what?  It all 
> depends on how sensitive you plan to make your signal detection 
> circuit--that is, how much amplifier gain do you plan to have and if 
> you are connecting to an A/D device, what is its sensitivity?  In 
> general, I would try to start with the smallest wire and the largest 
> number of turns I could easily manage.  Coil resistance of several K 
> Ohms wouldn't be unreasonable.  The coil Larry sells I believe has 
> 10,000 turns and is 9,000 Ohms.
> 
>>Would it be better if the coil had a copper or iron core ?
> 
> Definitely no.  Iron would "suck in" the magnetic lines, away from 
> the wires where you want them to be.  Copper, unless it made a 
> complete loop, wouldn't do much since it wouldn't be in the magnetic 
> field, certainly nothing particularly helpful.
> 
>>This would mean a custom coil rectangular instead of circular.
> 
> Possibly you'd get slightly better linearity with a rectangular coil 
> and rectangular magnet pole pieces, but either shape should work 
> reasonably well for what you are trying to do.
> 
>>I think it may be possible to have two identical
>>coils center tapped in the middle with a single
>>rare earth magnet in the middle between the two
>>coils then you have the right setup for
>>a proper op amp differential circuit.
> 
> I may be wrong, but I am suspecting that you are wanting to connect 
> one output to each of the two inputs of an op amp.  If that is what 
> you are thinking about, the problem will be too much gain.  Typical 
> op amps have voltage gains of 100's of thousands, or more often, 
> millions.  Generally for approximate analyses designers assume that 
> their gain is infinite.  That means that extremely tiny input signals 
> (noise) would have the op amp output bouncing between its voltage 
> limits--not very useful.
> 
> Usually op amps are connected up as single ended amplifiers using a 
> two-resistor feedback circuit, which makes a very nice voltage amplifier.
> 
> See: 
> http://www.analog.com/static/imported-files/tutorials/MT-032.pdf   Figure 3.
> 
> If you do that, one coil will work fine, and by changing the resistor 
> values you can choose the amplifier gain to complement your coil 
> sensitivity.  The only advantage I can think of for some kind of 
> differential coil setup is that it might not be as sensitive to 60 Hz 
> hum.  Though it probably wouldn't be that hard to connect a 
> differential coil to a single-ended amplifier, sort of like a guitar 
> "hum-bucking" pickup.
> 
> An instrumentation amplifier *would* allow for independent 
> connections to a pair of coils, but they tend to be a lot more 
> expensive and their gain usually can't be adjusted as precisely.
> 
> See: 
> http://www.analog.com/static/imported-files/tutorials/MT-032.pdf  Figure 2.
> 
>>+COIL-  NmagnetS +COIL-
>>
>>THE left coil - is connected to right coil +
>>which is then the ground.
>>
>>Left Coil + goes to op amp +
>>and
>>Right coil - goes to op amp -
>>
>>The relative motion is magnet fixed to ground
>>and coil fixed to device.
>>
>>Coil is stable and magnet moves right and left
>>between the coils.
>>
>>Possibly +/- 2mm of range of motion.
>>
>>Each coil having 1050 to 2100 turns
>>custom wound to be exactly the same.
> 
> Hope that's a start,
> Brett
> 
> 
> 
> __________________________________________________________
> 
> Public Seismic Network Mailing List (PSN-L)
> 
> To leave this list email PSN-L-REQUEST@.............. with 
> the body of the message (first line only): unsubscribe
> See http://www.seismicnet.com/maillist.html for more information.
>
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)