The theory is pretty simple. You form a simple oscillator with a coil, only the coil in this case is very large. http://www.electronics-tutorials.com/oscillators/oscillator-basics.htm The frequency of the oscillator will change quickly as a car drives over the coil. Alternately, measure the inductance of the coil with a PIC and if it changes suddenly, you've got a lot of metal on top of it. You simple need to detect the frequency shift. In our world (PICs) this can be done fairly simply. Of course, the devil is in the details. Signal strength, shift magnitude, etc can be hard to deal with correctly, but for a one-off project that can be tuned, these shouldn't be overwhelming. This might get you started, or at least help you to know what questions to ask depending on how you want to proceed. -Adam On 10/15/07, Brad Stockdale wrote: > Hello all, > > I am working on a little project to help secure my home... I have a long > driveway and would like to know when someone is approaching my house. I used > to use a driveway monitor that had an IR motion detection sensor. This > worked, but produced more false alarms than true alarms. Not that it was > malfunctioning, but since I live in a rural area, it would detect every deer, > groundhog, dog, whatever... > > I would like to design something similar to: > > http://www.smarthome.com/790908.html > > It uses magnetic probes to detect vehicles approaching. > > Unfortunately, I know nothing about metal detection. I've never even used a > metal detector, let alone know how one works well enough to design one. > > Can anyone give me some pointers on how to go about making magnetic probes > such as the ones used in the above system? Maybe a website explaining the > basics of ferrous material detection and some simple schematics for a basic > metal detection probe? Basically I just want to find a circuit that upon > detection of the metal, it sends out a digital pulse. From there I'll use > that to activate the alarm be it over wireless, wired, whatever... > > I'd appreciate any advice. > > Thanks, > Brad > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Moving in southeast Michigan? Buy my house: http://ubasics.com/house/ Interested in electronics? Check out the projects at http://ubasics.com Building your own house? Check out http://ubasics.com/home/ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist