this is all very useful info,
thanks again

On 13 Jul 2006, at 14:42, ChrisAtUpw@....... wrote:

> In a message dated 13/07/06, nicholas.ward@......... writes:
>
>> Great,
>>  Thanks for the info.
>>  N
>>
>>  On 13 Jul 2006, at 10:10, james fisher wrote:
>>
>>  > Hi, a piezo disk will pick up the higher frequencies
>>  > you are interested in. use a 100 gram weight on top of
>>  > the piezo disk and bury in a 1' deep hole to isolate
>>  > disc from direct sound pickup. Superglue a strong
>>  > magnet to disk and stick on a iron rod driven into the
>>  > ground. Insulate everything well. See past posts on
>>  > piezo disks. They really work well for picking up
>>  > seismic waves as well as sound waves.
>
>  Hi Nicolas,
>
>  =A0=A0=A0=A0=A0=A0 You will need FET input opamp TL071? to match the =
high input=20
> impedance of the piezo disk.
>  =A0=A0=A0=A0=A0=A0 Superglue will not last long; a modified two part =
acrylic like=20
> Holdtite ST3295 is much better. It is tough and doesn't crack.
>  =A0=A0=A0=A0=A0=A0 Can you stick the head of a 3 mm brass bolt to the =
centre of=20
> the piezo disk and screw a 20 gm disk of brass on top? A high weight=20=

> like 100 gm is OK for very low frequency seismic signals, but it=20
> lowers the resonance of the disk to maybe 200 Hz.
>
>  =A0=A0=A0=A0=A0=A0 Regards,
>
>  =A0=A0=A0=A0=A0=A0 Chris Chapman

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