I believe Adam's idea below was probably the best to solve this problem. If it works, it will allow me to use 2 welded little boxes completely sealed, one on each door (the only issue is replacing batteries, but depending on their lifetime, I will weld them in there anyway). I will try and put together such a "metal detector", and see how it would work with the metal walls in between. Some observations: 1) Since this is a logger and I will have to read its recordings, maybe I can use the coil itself as an antenna and transmit the data to a handheld unit, very close to the welded box. 2) You mentioned that a "small coil" would do the job. Really ? Any comment or advise here ? I have never built a metal detector before, so I am a bit uncomfortable at first. 3) I have been googling around, I saw freqs from 6 kHz to 400 kHz in metal detectors circuits. 4) In order to maximize battery life, I intend to fire this metal detector every 30 seconds or so and take a read. I am also concerned about how much each firing will drain. Any comments ? Thank you, Pedro. ----- Original Message ----- From: "M. Adam Davis" To: Sent: Tuesday, February 03, 2004 5:06 PM Subject: Re: [EE]: Challenge for keen minds > It would never invalidate it completely, but it would dampen it > significantly, which is why I suggested aluminum for the actual box, or > at least the face. Alternately use the ferromagnetic properties of the > metal to your advantage, and essentially turn the face of the two boxes > into a transformer. The transformer resonates at a certian frequency > without the other box, but when it is brought near it affects the > resonant frequency. > > There will be some coupling through the rest of the container, but that > will be minimal compared to the coupling of the two small boxes next to > each other. > > Do some tests. A weakness is that since it is low power, then someone > from the outside could use a high power electromagnet/transmitter to > force the device to continue to oscillate at the specific frequency, but > this can be fixed by having a seperate coil at away fromt he primary > coil which resonates at a seperate frequency. Any signal strong enough > to trick the first coil into thinking it's still got a closed door will > also affect the frequency of the secondary coil, and you can detect this > trick. Alternately, use a RFID type setup in the other box which > responds to the main box with a sequence of frequencies - much harder to > simulate externally. > > -Adam > > Pedro Drummond wrote: > > >Interesting ! But wouldn't 2 thick metal walls invalidate this ? (one for > >each device, on each door). > > > > > >----- Original Message ----- > >From: "M. Adam Davis" > >To: > >Sent: Saturday, January 31, 2004 1:13 PM > >Subject: Re: [EE]: Challenge for keen minds > > > > > > > > > >>Since this is a double door, you need to be aware of either door being > >>open (ie, they could simply close the one with the sensor but leave the > >>other open) > >> > >>Consider welding two boxes of aluminum on each door opposite each other > >>very close together. When the door opens they are unaligned and/or far > >>apart. In one box you put the data logger with a small metal detector > >>type coil set at a specific frequency. > >> > >>The other box contains another coil and capacitor which, when close to > >>the detector side, modifies the oscillation frequency significantly. By > >>monitoring the actual frequency you can not only get a good idea of when > >>they move apart, but how far apart they are. Since they can be > >>completely sealed inside a metal box (does affect frequency change and > >>sensitivty, but it would still work well) then they could only defeat it > >>if they also had such a coil. > >> > >>The kicker is that they would not only have to have such a coil, but it > >>would have to modify the frequency the same way both your coil and the > >>metal box does. Fuirthermore they'd have to somehow move their coil in > >>synchronization with the door so that your receiver didn't detect the > >>drop and rise in frequency. Since this is all mounted on the inside it > >>would be nearly impossible without quite a bit of additional equipment > >>(measuring the field, etc) and very good reflexes, if possible at all > >>without an active external transmitter. > >> > >>Truck vibration and movement wouldn't affect it. Temperature would, so > >>you may want to look at a temperature sensor so you can compensate for > >>capacitor values changing a bit. > >> > >>Additional security could be provided by using different frequency coils > >>so no one solution could work for all boxes. The circuit could be > >>calibrated when installed, or for higher sensitivity every time the box > >>is sealed/opened/logging data read. > >> > >>Of course, you realize that you should also put in a GSM or GPRS module > >>so it can call or page you when a box is opened with its serial number. > >>Better still, attach a GPS as well and log its movement, transmitting > >>location and time information to your pager when it's opened. Attach a > >>small digital camera and send yourself a picture. Etc, etc, etc. > >> > >>-Adam > >> > >>Pedro Drummond wrote: > >> > >> > >> > >>>Thank you all for the help and interest. > >>> > >>>Ok, time for more data: > >>> > >>>The space being protected is a metallic box, the size of an elevator, > >>> > >>> > >that > > > > > >>>will be towed by small trucks. Its metallic "door" (actually a double > >>> > >>> > >door) > > > > > >>>is the gate I mentioned. During long trips, these boxes are commonly > >>> > >>> > >moved > > > > > >>>from truck to truck, with long periods waiting for the next one, with no > >> > >> > >>>guards nearby. Although there is a very strong lock, theft was detected > >>> > >>> > >at > > > > > >>>final destination in locked boxes (i.e., someone opened it, took some > >>> > >>> > >goods, > > > > > >>>and somehow relocked it). A redesign of the metal box is being made, but > >>> > >>> > >it > > > > > >>>will be a long time until all of them are replaced. It is enough for now > >>> > >>> > >to > > > > > >>>help the insurance company in detecting WHEN and for HOW LONG it was > >>> > >>> > >opened, > > > > > >>>so the responsible will be known. > >>>I do not mind reading its information only when the gate is open, and I > >>>cannot modify the gate, only add some small device close to the door. > >>> > >>> > >>>To comment some of the group ideas: > >>> > >>>Using a contact method for data transfer - I am afraid of dust, grease, > >>> > >>> > >etc. > > > > > >>>But maybe. > >>>RFID - cheap tag, but expensive sensor (I guess). > >>>Gyro/acceleration sensors - can't do, since it will be towed by a truck. > >>>Hinge sensored - can't alter the hinge. > >>> > >>> > >>>BTW, special thanks to Russel ! > >>> > >>>-- > >>>http://www.piclist.com#nomail Going offline? Don't AutoReply us! > >>>email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>-- > >>http://www.piclist.com hint: To leave the PICList > >>mailto:piclist-unsubscribe-request@mitvma.mit.edu > >> > >> > >> > > > >-- > >http://www.piclist.com hint: PICList Posts must start with ONE topic: > >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > > > > > > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.