In 1991 one of the PSN members build a number of piezo devices. He had some
issues with the output because if you "tap" it hard, the output can be quite
high and damage the electronics. The devices were built from the old Apple
butterfly fans. The original fan units contained two piezo devices and had
two flippers that osculated when power was applied.

The seismic design went something like this. Cut the fan in half. You now
have two piezo devices to work with. Take a three inch length of 1-inch PVC
pipe and cap one end. Put a small hole in the cap, run the wires through the
hole, epoxy the piezo end in the cap with the flipper remaining free. Next,
epoxy a small fishing weight on the end of the flipper. Once the epoxy is
set, drill another small hole in the second cap and PVC glue it to the pipe.
Last, fill the pipe with #30 motor oil and cap the filler hole with epoxy.
The devices worked best for very high freq and did not work at all for low
freq. Because the flippers are ridged, the devices were extremely
directional.

Regards, Steve Hammond
PSN San Jose, Aptos CA

  -----Original Message-----
  From: psn-l-request@.............. [mailto:psn-l-request@.................
Behalf Of ChrisAtUpw@.......
  Sent: Tuesday, August 09, 2005 8:59 AM
  To: psn-l@..............
  Subject: Re: Homade Geophone


  In a message dated 09/08/2005, gmvoeth@........... writes:
    The Piezo Idea is very interesting

    Have you ever heard of a strain guage approach where you just hang a 10#
or so weight from a strainguage and measure the tiny weight changes due to
ground acceleration ??
  Hi there,

      The strain gauge a) will not be anything like sensitive enough b) it
is quite fragile and won't take overloads c) as primarily a resistive
device, it has a fairly high power dissipation d) they are not exactly
cheap.
    It sounds a lot like the piezo idea but a strain guage would be
resistive in nature I believe.
      The maximum output is less that 1% of the input voltage.
    At the moment I am using the top 9 bits of a 12 bit A/D converter
looking at a +/- 2.048 voltage swing through amplifier filter with a gain of
about 90DB will that same setup work with the piezoelement ?
      I don't know your instrument setup and your description does not make
it clear. I would not expect you to need 90 dB gain.
    Do I need a special preamp for the piezo element ?
      Does your amplifier have a low noise CMOS input opamp with a
controlled DC impedance of 100 M Ohms?

      Are you just starting seismology and electronics?

      Regards,

      Chris Chapman






In 1991 one of the PSN members =
build a=20
number of piezo devices. He had some issues with the output =
because if=20
you "tap" it hard, the output can be quite high and damage the =

electronics. The devices were =
built from=20
the old Apple butterfly fans. The original fan units contained two piezo =
devices=20
and had two flippers that osculated when power was applied.=20

 
The seismic design went something =
like this.=20
Cut the fan in half. You now have two piezo devices to work with. Take a =
three=20
inch length of 1-inch PVC pipe and cap one end. Put a small hole in the =
cap, run=20
the wires through the hole, epoxy the piezo end in the cap with the =
flipper=20
remaining free. Next, epoxy a small fishing weight on the end of the =
flipper.=20
Once the epoxy is set, drill another small hole in the second cap and =
PVC glue=20
it to the pipe. Last, fill the pipe with #30 motor oil and cap the =
filler hole=20
with epoxy. The devices worked =
best for=20
very high freq and did not work at all for low freq. Because the =
flippers are=20
ridged, the devices were extremely directional. 
 
Regards, Steve =
Hammond
PSN San Jose, Aptos =
CA
 

  
-----Original Message-----
From:=20
  psn-l-request@.............. =
[mailto:psn-l-request@...............On Behalf=20
  Of ChrisAtUpw@.......
Sent: Tuesday, August 09, 2005 =
8:59=20
  AM
To: psn-l@..............
Subject: Re: Homade=20
  Geophone


  
In a message dated 09/08/2005, gmvoeth@........... writes:
  The Piezo Idea =
is very=20
    interesting

Have you ever heard of a strain guage approach =
where you=20
    just hang a 10# or so weight from a strainguage and measure the tiny =
weight=20
    changes due to ground acceleration ??
  
Hi there,
  
 
  
    The strain gauge a) will not be anything =
like=20
  sensitive enough b) it is quite fragile and won't take overloads c) as =

  primarily a resistive device, it has a fairly high power =
dissipation d)=20
  they are not exactly cheap.
  It sounds a lot =
like the=20
    piezo idea but a strain guage would be resistive in nature I=20
  believe.
  
    The maximum output is less that 1% =
of the=20
  input voltage.
  At the moment I =
am using=20
    the top 9 bits of a 12 bit A/D converter looking at a +/- 2.048 =
voltage=20
    swing through amplifier filter with a gain of about 90DB will that =
same=20
    setup work with the piezoelement ?
  
    I don't know your instrument setup and =
your=20
  description does not make it clear. I would not expect you to need 90 =
dB=20
  gain.
  Do I need a =
special preamp=20
    for the piezo element ?
  
    Does your amplifier have a low noise CMOS =
input=20
  opamp with a controlled DC impedance of 100 M Ohms? 
  
 
  
    Are you just starting seismology and =
electronics?=20
  
  
 
  
    Regards,
  
 
  
    Chris Chapman