Excellent information, everyone. I don't know why, but for some reason I was feeling like the current through the magnet would have to be high for a lot of force. Hmmm. That still leaves me with a problem to solve, however. Ah well. I'll get it. Thanks guys! Mike H. On Mon, 27 Sep 2004 21:20:48 -0600, Ivan Kocher wrote: > Number 3 is the number .... !!! :) > > These systems work with an electromagnet at the fence and a ferrous > metal in the door. > > The door has a two piece system, one like a box the is glued/screwed to > the door, and the other that will be atracted to the electromagnet. > > The current is not an issue, because it is very low in the 10mA to 100mA > range, at 12V. The idea is that when the electromagnet enegizes, it > moves the piece of ferrous metal in the door so they both touch, and the > friction there is very high, making imposible to move the door. > > These electromagnets are not very strong, in the range of a few pounds > of force (plain magnetic pull), but once a bar of iron is in contact, it > will not move, due to the friction between the two of them. One will > almost destroy the magnet trying to move the metal. > > Notice that this is the way the system work in the door, you do not > separate the metal from the electromagnet, just move under it when you > open the door. > > In case of a power failure, batteries keep the system going, control, RF > readers and magnets. I think they use 12V at +/- 28Ah. Nothing serious. > > And yes there is a big number of companies offering these systems. > > The RF cards are some sort of smart card with RF link, but there are > about 3 or 4 standards for these, "compatible". > > There are some security concerns I personally have on these systems, but > they go to the RF card. I know one that is just one of those "dumb" > smart cards, like a serial memory, that have no authentication. So I > wonder... what might happen if a go about walking with a mini RF reader > in my pocket, storing the data that these cards have ??? the I can go > any place I want, and the system will register somebody else .... > I know camera security etc might be there too, but ... I _can_ open > doors I should not be able to .... !!! > > Ivan Kocher > ---------------- > > > > > Mike Hord wrote: > > I recently visited a friend out of state, and his apartment complex > > had a pool area which was gated for limited access. One simply > > swipes an RFID card and the gate can then swing freely open. > > > > There were no apparent wires leading to the gate, but it had a pad > > on it which touched a corresponding pad on the fence next to it. > > > > Can anyone shed some light on how this works? I have a few > > theories I'll outline below...as usual Google produces lots of > > companies that want to sell me things, but no info on the system > > in question. > > > > 1. The pad on the gate is a magnet. The pad on the fence is an > > electromagnet with a ferrous core. When the EM is off, the gate > > is held to by the magnet. When the EM is turned on, it is made > > to repel the magnet by just enough to make opening the gate > > easy. I feel like this is the most likely method, since power > > consumption is essentially zero under static circumstances > > (except for card reading mechanism). > > > > 2. The pad on the gate is a magnet. The pad on the fence is an > > EM with no core which is usually energized slightly to pull the > > gate to, but can be switched off to allow the gate to open. Less > > likely, since a power failure means security failure. > > > > 3. The pad on the gate is ferrous. The pad on the fence is an > > EM which is turned on, strongly, at all times. To open, power to > > the EM is interrupted. I find this less likely, since it means a > > power failure will inevitably cause the gate to become insecure. > > > > There could be another option somewhere I'm not considering > > (actually, there certainly are, but who's counting?). I like the > > first method, although it would take a lot of power to counteract > > a magnet as strong as the one in question would have to be. > > > > But, I suppose that counteractive force would not have to be any > > stronger than the static force required to keep the gate shut. So > > that makes 3 less and less likely. > > > > Input? Has anyone any experience with this system of door lock? > > As I said, my friend is several hundreds of miles away so I can't > > just swing round and check it out myself. > > > > Mike H. > > _______________________________________________ > > http://www.piclist.com > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > _______________________________________________ > http://www.piclist.com > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist