PSN-L Email List Message

Subject: Re: Seismic intrusion detectors part 2
From: "David H. Youden" dyouden@.........
Date: Thu, 10 Apr 2003 06:06:20 -0400

Mr. King

Are you aware that the National Institute of Science and Technology has a mechanical engineering laboratory (MEL) which, in turn has a sensor laboratory studying exactly the subject of your question? The laboratory chief, E. Clayton Teague is a friend of mine, and I suggest that you get in touch with him. They are working with intrusion sensors that are smaller than a cigarette pack, have multiple sensors, and are networked via, I think 2.4 GHz radio. The idea is to spread a bunch of these along a trail and watch for intruders. If you need Clayton's e-mail address, and it's not on the website, please ask me.


At 10:02 AM 4/9/03 -0700, you wrote:
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.
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. 
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

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