> 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
> mailman.mit.edu/mailman/listinfo/piclist

> Mike, the front door at my gf's place of business (biotech firm) has the
> same sort of arrangement.  It's on the front door, and let me tell you,
> that thing is so strong that I nearly tore the door off by trying to
> push it open without pushing the release.  The release lever itself has
> wires going to it, so I would expect that it's responsible for cutting
> power.  Something else to consider is that if you lose power, all exits
> are supposed to be openable from the inside.  If they're not, then this
> could be catastrophic (think of a fire).  As such, I would guess the
> system relies on power to maintain security.
>
>
> Shawn Wilton
> http://black9.net
>
> 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

> The nestle factory in York (UK) has a similar system for all external and
> internal doors/gates - all ID cards were magnetic swipe cards which could be
> used to pay for things inside the factory shops, as well as open doors you
> were authorised to open, the doors all had a panel on the inside that are the
> same as the fire alarm buttons (break the glass, which releases the button)
> except they were green - if the fire alarms ever went off the doors were all
> supposed to release automatically, if they didn't, simply break the glass and
> the door would release.
>
> if they were powered constantly I cant say, it would be an enormous waste of
> power, but having a power to open system wouldnt work with the break the
> glass buttons - if the fire took out the power system breaking it wouldnt do
> a thing and you would get toasty, thus they must be constantly powered to
> hold the doors closed
>
> On Monday 27 September 2004 23:40, Shawn Wilton wrote:
>
>>Mike, the front door at my gf's place of business (biotech firm) has the
>>same sort of arrangement.  It's on the front door, and let me tell you,
>>that thing is so strong that I nearly tore the door off by trying to
>>push it open without pushing the release.  The release lever itself has
>>wires going to it, so I would expect that it's responsible for cutting
>>power.  Something else to consider is that if you lose power, all exits
>>are supposed to be openable from the inside.  If they're not, then this
>>could be catastrophic (think of a fire).  As such, I would guess the
>>system relies on power to maintain security.
>>
>>
>>Shawn Wilton
>>http://black9.net
>>
>>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
>>>mailman.mit.edu/mailman/listinfo/piclist
>>
>>_______________________________________________
>>http://www.piclist.com
>>View/change your membership options at
>>mailman.mit.edu/mailman/listinfo/piclist
>
>
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> http://www.piclist.com
> View/change your membership options at
> mailman.mit.edu/mailman/listinfo/piclist

> On Monday 27 September 2004 23:40, Shawn Wilton wrote:
> > Mike, the front door at my gf's place of business (biotech firm) has the
> > same sort of arrangement.  It's on the front door, and let me tell you,
> > that thing is so strong that I nearly tore the door off by trying to
> > push it open without pushing the release.  The release lever itself has
> > wires going to it, so I would expect that it's responsible for cutting
> > power.  Something else to consider is that if you lose power, all exits
> > are supposed to be openable from the inside.  If they're not, then this
> > could be catastrophic (think of a fire).  As such, I would guess the
> > system relies on power to maintain security.

>http://www.securitron.com/documents/prodBrochure/MAGNALOCK.pdf
>
>Above is a link to the brand of lock that I used at a secure facility, where
>I worked several years ago. IIRC I installed it in about 1988. It's not new
>technology; locks of this style have been around at least 20 years.
>
>The model 62 that I used has a holding force of 1200 Lbs. and draws about
>100mA at 24V, less than 3 watts. It used a mag card reader for entry, and a
>passive infrared motion detector for no prior knowledge egress.
>The panic bar on the door also had a micro switch to interrupt the magnet
>supply in case the motion detector failed.
>
>The mag lock used a 24V battery backup which would hold the lock in for a
>minimum of 3 days in the event of a power failure. Unlocking on power fail
>was NOT an option. In fact the supply was monitored by the alarm system, if
>it dropped below 22V an alarm was triggered. The lock would maintain 1200 Lb
>holding force down to 12V.

> 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
> mailman.mit.edu/mailman/listinfo/piclist
>

> 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
> > mailman.mit.edu/mailman/listinfo/piclist
> >
>
>
>
> _______________________________________________
> http://www.piclist.com
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

>>As for electromagnetic locks where there are just two contact plates,
>>as the OP described, they are usually
>>"fail safe" because it's so much easier to do (just an electromagnet
>>and a metal plate) but the current needed
>>is much greater than with electric strikes because the magnetic field
>>has to provide the force to prevent
>>opening.  I believe they can be made "fail secure" by having a
>>permanent magnet as the securing element, with
>>an electromagnet counteracting its magnetic field to release, but this
>>is subject to the "permanent" magnet
>>weakening over time.
>
>
> This is an interesting point:  how long will a permanent magnet remain so?
> I would tend to think that most permanent magnets remain pretty constant
> over a fairly long term, some better than others.
>
> What started all this is that I'm investigating a dumping mechanism for
> time-delay feeding of livestock.  Basically, fill a hopper now, dump it in
> X hours.  No need to dump a measured amount, because the human
> pre-loads it, no need to be able to dump more than once, because the
> animals get fed twice a day and must be checked in the morning, so
> the food can be loaded then.
>
> The mechanism must be sealed, so it can be hosed off regularly.  It
> should be battery powered, so it can be fairly portable.  And it should
> be cheap and easy to make (because I don't want to have to spend the
> rest of my life making the 64 of them that may end up being needed).
>
> I think I'm moving towards a plate on a rocker, with a magnet on one
> side of the pivot axle and a magnet on the other, and using the
> aforementioned electromagnet to repel the magnet.  Since it is a
> first-class lever, making the food side shorter than the magnet side
> will reduce demands on the magnet.  The magnet can be wound
> around a bolt threaded through the wall of the package containing
> the electronics, which will make it waterproof.  Finally, the magnet
> doing the work means that nothing needs to be constantly energized,
> which is good for battery life.
>
> The big question now is, can I make an electromagnet capable of
> pushing the magnet away and still run the thing on batteries?
>
> Mike H.
>
>
>>Cheers,
>>
>>Howard Winter (who managed a building-fitout a few years ago and found out all these things as a result!)
>>St.Albans, England
>
> _______________________________________________
> http://www.piclist.com
> View/change your membership options at
> mailman.mit.edu/mailman/listinfo/piclist

>>As for electromagnetic locks where there are just two contact plates,
>>as the OP described, they are usually
>>"fail safe" because it's so much easier to do (just an electromagnet
>>and a metal plate) but the current needed
>>is much greater than with electric strikes because the magnetic field
>>has to provide the force to prevent
>>opening.  I believe they can be made "fail secure" by having a
>>permanent magnet as the securing element, with
>>an electromagnet counteracting its magnetic field to release, but this
>>is subject to the "permanent" magnet
>>weakening over time.
>>    
>>
>
>This is an interesting point:  how long will a permanent magnet remain so?
>I would tend to think that most permanent magnets remain pretty constant
>over a fairly long term, some better than others.
>
>What started all this is that I'm investigating a dumping mechanism for
>time-delay feeding of livestock.  Basically, fill a hopper now, dump it in
>X hours.  No need to dump a measured amount, because the human
>pre-loads it, no need to be able to dump more than once, because the
>animals get fed twice a day and must be checked in the morning, so
>the food can be loaded then.
>
>The mechanism must be sealed, so it can be hosed off regularly.  It
>should be battery powered, so it can be fairly portable.  And it should
>be cheap and easy to make (because I don't want to have to spend the
>rest of my life making the 64 of them that may end up being needed).
>
>I think I'm moving towards a plate on a rocker, with a magnet on one
>side of the pivot axle and a magnet on the other, and using the
>aforementioned electromagnet to repel the magnet.  Since it is a
>first-class lever, making the food side shorter than the magnet side
>will reduce demands on the magnet.  The magnet can be wound
>around a bolt threaded through the wall of the package containing
>the electronics, which will make it waterproof.  Finally, the magnet
>doing the work means that nothing needs to be constantly energized,
>which is good for battery life.
>
>The big question now is, can I make an electromagnet capable of
>pushing the magnet away and still run the thing on batteries?
>
>Mike H.
>
>  
>
>>Cheers,
>>
>>Howard Winter (who managed a building-fitout a few years ago and found out all these things as a result!)
>>St.Albans, England
>>    
>>
>_______________________________________________
>http://www.piclist.com
>View/change your membership options at
>http://mailman.mit.edu/mailman/listinfo/piclist
>
>  
>

> On Wed, 29 Sep 2004 09:49:38 +0100, Alan B. Pearce <obf> wrote:
>
>>>The big question now is, can I make an electromagnet capable of
>>>pushing the magnet away and still run the thing on batteries?
>>
>>How are you going to charge the battery? This starts to look like a device
>>with a cable coming out, to go to a single central battery that powers all
>>units. They do not all need to let go simultaneously, to lower the battery
>>current demand, if you can use (say) a six conductor cable (I am thinking
>>something rugged like used for trailer connections on cars) which will carry
>>the power and enough communications between devices for a sequencing signal.
>>Now one battery, an SLA or similar in the office can be on float charge,
>>removing maintenance of that from the hassles the worker has to deal with.
>
>
> The original hope was to do this on three or four D-cell alkalines,
> and I may yet
> realize that hope.  It may require a step-up charger into a cap, to get many
> small sips of juice from the battery to end up with a high enough burst into the
> EM coil to release the magnet for the critical instant.
>
> Or I may end up doing something completely different.  Some initial experiments
> with a solenoid yesterday showed that even up to 12V, the solenoid did not
> really want to repel even a small magnet, much less one capable of doing what
> I need it to do.  All in all, it's been a fairly educational process.
> I will certainly
> try to remember to post a result and let you all know which method proved
> most effective.
>
> Mike H.
> _______________________________________________
> http://www.piclist.com
> View/change your membership options at
> mailman.mit.edu/mailman/listinfo/piclist
>

>
> On Sep 29, 2004, at 4:10 PM, Robert Rolf wrote:
>
> > Every look at electric car window actuators, or electric door locks?  :
> > BUT, they're not cheap.
> >
> www.allelectronics.com/cgi-bin/category.cgi?
> category=search&item=DLA-1
>
> $5.50 - cheap enough to experiment with; if they work, I think they'd
> still be cheaper (non-surplus) than most magnetic latches...
>
> I bought one to play with as a linear actuator.  Imagine my surprise
> when
> I saw most of the reviews at All were from people actually installing
> them in car doors!  Go figure...
>
> BillW

>
> Why can't you use a mechanical latch that is armed by the load put on it ?
> Someone mentioned mousetraps. There are other ways to do it. F.ex. if the
> container is balanced on a horizontal axle (above it), then the necessary
> retaining force will be near zero regardless of its weight. Good
> mechanical design principles (I read up once upon a time) demand that you
> do not use levers with a ratio larger than 1:10 (1:20 for ball raced I
> think). So your force will have to be about 1/10th of the weight of the
> hopper unless you use an arming mechanism. F.ex. if the hopper must be
> pushed against a spring to latch it then the spring would have to push
> with 1/10th of the (reduced by lever, up to 10 times, see above) weight of
> the hopper, and your latch would have to handle 2/10ths of the spring
> force (lever again). That would give a mechanical gain of 500. Just how
> heavy is that hopper ? Finding a magnet that holds 2kg on contact and a
> solenoid that cancels it when energised is not so hard. That would allow a
> total hopper weight of 1 ton. Big enough ? Then there is fail safe, fail
> unsafe, whose standing under the hopper when someone is tinkering with the
> wiring or lightning strikes nearby etc. I like the servo motor idea better
> than a solenoid I think.
>
> Peter