In a message dated 20/12/2008, rlthomasson@......... writes:

Chris,
Thinking about the damping, it seems like a basic coil and  magnet sensor 
like the AS-1 would be self damping due to the movement of the  copper winding in 
the magnetic field.  Or is it a case of it being self  damping but still 
underdamped and needing additional  damping?
Bob



Hi Bob,
 
    The coil + magnet supplied with the AS1 gives very  roughly 0.1 damping, 
when you actually need 0.7 damping, so a lot of additional  damping is 
required. 
 
    If you make you own system with a quad NdFeB  magnet block and a 
rectangular coil, like Bob McClure, you can put a  variable resistor across the coil 
and set the damping that way. This coil can  give 20x the output you get from 
an AS1. 
    
    If you connect the coil between Earth and the +ve  input of an opamp, you 
won't see any damping, since the input resistance is near  infinite. If you 
use an amplifier like Larry's, it already has a 10 k resistor  across it, so 
you could then get significant damping. 
    
    The AS1 uses a twin LF boost amplifier to recover  longer period signals 
out to about 5 seconds, but it is still poor at detecting  20 second signals. 
    John Lahr and I run modified AS1s which work out to  20 seconds 
naturally. Thie involves changing 4 capacitors and three resistors on  the AS1 board, 
maybe $10 maximum
 
    Regards,.
 
    Chris Chapman






In a message dated 20/12/2008, rlthomasson@......... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>Chris,
Thinking about the damping, it seems like a basic coil=20=
and=20
  magnet sensor like the AS-1 would be self damping due to the movement of t=
he=20
  copper winding in the magnetic field.  Or is it a case of it being se=
lf=20
  damping but still underdamped and needing additional=20
damping?
Bob


Hi Bob,
 
    The coil + magnet supplied with the AS1 gives v=
ery=20
roughly 0.1 damping, when you actually need 0.7 damping, so a lot of additio=
nal=20
damping is required. 
 
    If you make you own system with a quad NdF=
eB=20
magnet block and a rectangular coil, like Bob McClure, you can put a=20
variable resistor across the coil and set the damping that way. This coil ca=
n=20
give 20x the output you get from an AS1. 
    
    If you connect the coil between Earth and the +=
ve=20
input of an opamp, you won't see any damping, since the input resistance is=20=
near=20
infinite. If you use an amplifier like Larry's, it already has a 10 k resist=
or=20
across it, so you could then get significant damping. 
    
    The AS1 uses a twin LF boost amplifier to recov=
er=20
longer period signals out to about 5 seconds, but it is still poor at detect=
ing=20
20 second signals. 
    John Lahr and I run modified AS1s which work ou=
t to=20
20 seconds naturally. Thie involves changing 4 capacitors and three resistor=
s on=20
the AS1 board, maybe $10 maximum
 
    Regards,.
 
    Chris Chapman