In a message dated 20/09/05, gmvoeth@........... writes:

> >       You can make OK cheap seismic sensors using the 41 / 50 mm dia 
> piezo 
> > sounder disks and a weight, but you need a very high impedance low noise 
> > amplifier, maybe 100 M Ohms input.
> 
> If you were to suspend most of the weight of a 4 or 5LB mass on the end of a 
> spring and then take up the last one ounce of weight upon such a Piezo 
> sounder disk would that improve the signal receiving qualities any over just 
> placing a one ounce weight on it ?
> A strong signal on such a device would probably ruin it.

Hi Geoff,

       Apart from any danger of breaking the device, you would be limited in 
period by the mass and the force constant of the spring. These tend to come 
out at about a second max for simple spring + weight systems. You would also 
have to provide effective damping for the system.
       With the simple Piezo Disk + Seismic Mass, the natural resonant 
frequency is about 170 Hz, so you are operating it as a simple force in / voltage 
out accelerometer well below resonance. You can filter out any impulse responses 
at the natural frequency, or you can provide sorbothane or magnetic damping. 
The lower roll off is due to the RC time constant of the disk capacity and the 
input impedance of the opamp. With a simple piezo system, I get about 5x the 
voltage output of a 380 Ohm 4.5 Hz geophone. 
       You do have to thermally screen the device to prevent drifts due to 
temperature changes and you need a couple of low leakage diodes, FDH300 or 
1N3595, across the input to protect the opamp. Loaded Piezo disks will happily 
generate many 10s of Volts if knocked, quite enough to damage the input protection 
circuit in an opamp. Zeners have relatively high leakage currents in the 
microamp region and are likely to limit the max RC response time.

       Regards,

       Chris Chapman
In a message=20=
dated 20/09/05, gmvoeth@........... writes:



>    &nbs=
p;  You can make OK cheap seismic sensors using the 41 / 50 mm dia=
 piezo=20

> sounder disks and a weight, but you need a very high impedance low=20=
noise=20

> amplifier, maybe 100 M Ohms input.



If you were to suspend most of the weight of a 4 or 5LB mass on the end=20=
of a spring and then take up the last one ounce of weight upon such a Piezo=20=
sounder disk would that improve the signal receiving qualities any over just=
 placing a one ounce weight on it ?

A strong signal on such a device would probably ruin it.



Hi Geoff,



       Apart from any danger of breaking t=
he device, you would be limited in period by the mass and the force constant=
 of the spring. These tend to come out at about a second max for simple spri=
ng + weight systems. You would also have to provide effective damping for th=
e system.

       With the simple Piezo Disk + Seismi=
c Mass, the natural resonant frequency is about 170 Hz, so you are operating=
 it as a simple force in / voltage out accelerometer well below resonance. Y=
ou can filter out any impulse responses at the natural frequency, or you can=
 provide sorbothane or magnetic damping. The lower roll off is due to the RC=
 time constant of the disk capacity and the input impedance of the opamp. Wi=
th a simple piezo system, I get about 5x the voltage output of a 380 Ohm 4.5=
 Hz geophone.=20

       You do have to thermally screen the=
 device to prevent drifts due to temperature changes and you need a couple o=
f low leakage diodes, FDH300 or 1N3595, across the input to protect the opam=
p. Loaded Piezo disks will happily generate many 10s of Volts if knocked, qu=
ite enough to damage the input protection circuit in an opamp. Zeners have r=
elatively high leakage currents in the microamp region and are likely to lim=
it the max RC response time.



       Regards,



       Chris Chapman