I have been tinkering with geoseismic detectors (Velleman's kits) for over a year, trying to interpolate with signals to determine the map location of intrusion violations. This system requires 12 volt power, I2C adaptor chip and circuitry and is very vulnerable to EMP. It has it's usefulness but it is otherwise expensive, complicated, difficult to maintain and requires a 12V 1AH rechargable battery with charging circuit and shunt inside the post. I just kept thinking there's a better way to do this and have been trying a number of alternatives out with limited success. Wireless, of course, is out of the question with a system designed to survive nuclear warfare.
I got some of these super cheap sensors ($1.00), a solar powered night light ($2.50), a heavy duty capacitor and a zener diode I pulled out of a discarded circuit board I found in the garbage, PVC pipe (free), steel wire (free), plus some optical fiber cable I had lying around ($2.00 for 150 meters). Using the original BASIC STAMP II design book (no basic stamp needed) I was able to design an extremely simple circuit that replaced the 1.5 volt AA rechargable battery with the heavy capacitor to hold a charge from the little solar panel until it was needed. The capacitor's rating, according to my manual, should hold the charge for 12 hours of darkness at night when the sun goes down. I have not tested that yet but it definitely provides ample power during the day when the sun is out. This has no physical connection with wiring of any kind to the monitoring set other than fiber optic and no power supply or even a battery.
Using a jimmy-rigged steel laundry wire cabling system buried just under the ground in the yard to transmit vibrations, I was able to get this functional in about two hours of work (one post). My first impression was that it was broken, because it was pulsing rhythmically on my simple monitor (an old optical fibre test probe) about every two seconds. I examined my wiring again and again, looked at my circuit to see where I was going wrong, sat there perfectly still and kept watching the pulsed signal appearing on the probe. I figured I had done something wrong that was storing a charge on the capacitor then releasing it at regular intervals. I could not otherwise see what was causing it. I was getting discouraged and was wishing I was a better electronics engineer than I am. It seemed to be another dead end.
Then my next door neighbors laundry machine finished it's cycle. The pulsing stopped. Whenever I took a step or even coughed, I could see the pulse register on the sensor. The biggest problem with this thing was that it was too sensitive for most uses. I am going to work on a way to adjust it's sensitivity with a manual calibration at the post.
It's also zero maintenance and so few moving parts or electronics it barely qualifies as digital. I am confident I can pinpoint somebody moving topside to within a meter with this thing, day or night. This system could be made so sensitve I doubt if a housecat could creep on the grounds without detection.
One drawback in cost is that although it can be adapted to run I2C, at present what is needed is an interrupt based system in which any signal triggers a digital line and is buffered with a timestamp to the millisecond in a dedicated processor. I already bought one of these boards and will be putting this into a dedicated x86 device (likely running a simple DOS PowerBasic program) specifically to watch the seismic grid and then pass the data into the main network with buffering.
I will dedicate an entire chapter to this system in my book on Vault-OS when it is ready. Still testing and debugging but I think I stumbled onto something really good this time.