Yea, that's a great one! How about taking a 22 shell casing (EMPTY, FIRED AND DEAD!) and filling it with powered carbon, solder a wire to the brass case and take a brass rod about 1/8 inch in diameter and suspend it in the empty case with the powdered carbon making sure you do not touch the sides or the bottom of the case build a device that will hold the wire with out touching the case and solder a wire on to the brass rod. now that this is all done take an VOM, VTOM, DVOM meter and hook one lead to the rod wire and one to the case wire. If you tap on the case the resistance will change. A mini seismic detector. If you need more sensitivity use a 1 foot length of 1/2 inch rubber tubing build a device that will allow you to stretch the tube two feet and attach the ends in the middle of the tube. Attach a 2 inch brass rod vertically to the center of the stretched tube.
We used tie wraps and bent the rod in an L shape so we could attach it to the rubber tubing. We mounted the rubber tubing dead center inside a 3" PVC pipe and caped the ends. Each end held the tubing. I drilled a hole in the PVC and mounted the syringe that is filled with carbon capped with a brass plug with a wire soldered to the brass plug. Aligning the brass rod into the syringe tip filled it with carbon capped it off with a brass plug and soldered a wire on it. We made several of these different ways until we got one we liked. These were very sensitive and changed resistance when shook or bumped. I used them as a driveway alarm and buried them under the ground and placed them 50 feet apart. We had an alarm circuit t6hat looked at certain set resistance and if it changed the alarm beeped. Using different densities of carbon by packing it tighter allowed different resistances.
The PVC pipes were sealed with automotive silicone sealer and are still in the ground working after 2o years. They are very sensitive and can detect a deer at 100 feet, so we had to be adjusted with a screw that loosened or tightened the rubber tube. This has to be done evenly so the rod will not bind in the end of the syringe. I had to fiddle with the carbon and rod setting for a while to get optimum performance, but at the time a decent seismic detector cost several hundred dollars, however now they are under 50 bucks I'll bet. It also helped I had a friend who is an electronic genius, he is a walking circuit encyclopaedia, his mind is truly amazing. I built the detection circuit from a Radio Shack book of electrical projects on security systems and the ideas from my friend. It was fun at the time and I saved some money.
badmoon
