Here is the overall design.

The sensor consists of (2) geophones.  They are places aprox. 30-40 feet =
apart on a known trail that illegals use.=20

The prototypes we are using have (2) LT1677 opamp circuits that amplify =
the geophone outputs and feed the signal into a basic stamp 1.  The =
level of signal isn't converted to digital, the signal when large enough =
(about 1.67 volts) will drive the input pin high and thus we have a =
logic switch.

In order to limit false hits the software in the basic stamp listens for =
a hit on a geophone.  When there is a hit it basically says if the =
second geopghone is hit within the next 2 seconds cancel the entire hit =
becuase it is most likely rain or some other seismic disturbance.  If =
the gephone is hit after that to seconds the it is a real hit and send a =
signal back to us through our repeater.  The data coming to us tells us =
what "port" hit first and what sensor it is.  By knowing what port was =
hit first we can tell what direction the contact is going. =20

During initial powerup we callibrate the instrument by "walking the =
line" or walking by both geophones.  By doing this and counting the =
number of footfalls on an average man we can the have a rudimentary =
counting of the number of people walking by.  By dividing the number of =
detected footfall by the number of detected footfalls in the calibration =
we have the number of intruders.  It all works but the noisy circuit is =
killing me.

This system works but our circuit is noisy and when I make the software =
just send hits on either geophone as they come in, sometimes, at random, =
 one or both of the "ports" will show hits for minutes on end.  If I =
touch the circuit board with my finger i can kill the noise until some =
kind of seismic activity start the noise again.




Michael S. King
Technical Director
American Border Patrol







Here is the overall =
design.
 
The sensor consists of (2) =
geophones.  They=20
are places aprox. 30-40 feet apart on a known trail that illegals=20
use. 
 
The prototypes we are using have (2) =
LT1677 opamp=20
circuits that amplify the geophone outputs and feed the signal into a =
basic=20
stamp 1.  The level of signal isn't converted to digital, the =
signal when=20
large enough (about 1.67 volts) will drive the input pin high and thus =
we have a=20
logic switch.
 
In order to limit false hits the =
software in the=20
basic stamp listens for a hit on a geophone.  When there is a hit =
it=20
basically says if the second geopghone is hit within the next 2 seconds =
cancel=20
the entire hit becuase it is most likely rain or some other seismic=20
disturbance.  If the gephone is hit after that to seconds the it is =
a real=20
hit and send a signal back to us through our repeater.  The data =
coming to=20
us tells us what "port" hit first and what sensor it is.  By =
knowing what=20
port was hit first we can tell what direction the contact is =
going. =20

 
During initial powerup we callibrate =
the instrument=20
by "walking the line" or walking by both geophones.  By doing this =
and=20
counting the number of footfalls on an average man we can the have a =
rudimentary=20
counting of the number of people walking by.  By dividing the =
number of=20
detected footfall by the number of detected footfalls in the calibration =
we have=20
the number of intruders.  It all works but the noisy circuit is =
killing=20
me.
 
This system works but our circuit is =
noisy and=20
when I make the software just send hits on either geophone as they =
come in,=20
sometimes, at random,  one or both of the "ports" will show hits =
for=20
minutes on end.  If I touch the circuit board with my finger i can =
kill the=20
noise until some kind of seismic activity start the noise =
again.
 
 
 
 
Michael S. King
Technical =
Director
American=20
Border Patrol