Electronics advice pls.

[Mr Sandman]

Hi all,

its been a while since I played with electronics, so could do with some advice.

I have 2 magnetic door locks, keypad operated to control access into my house. I also have a mains power smoke alarm system.

what I want to do is use a relay operated by the smoke alarm when sounding to turn off the mag locks to allow people to leave with out having to get their fobs out.

system was working fine with one mag lock, but now I have two I have a problem. the smoke alarm when operating shorts out 2 wires going to the one maglocks power supply. the electricians has simply connected those wires to to the same connections in the new power supply. the problem is now that if someone opens the back door with a fob, the front door also unlocks! and vice versa. I was thinking about using diodes to try to get round this problem. I'm not sure if I have explained the set up very well..so ask away if you have any questions!

Cheers

Steve

[Clive George]

Mmm. So to break into your house, I stick a smoke bomb in and I can open the doors?

cheers, clive

[Mr Sandman]

Yep, a very common method of breaking into houses I'm sure.

Steve

[dennis]

Two pole relay.

[Andy Cap]

If it's a DC sensing circuit, then yes a diode in one leg of each branch will do the trick. If it doesn't work at all one way, reverse it.

If it's AC sensing it wouldn't make any difference. I'd guess and you'd have to go the relay route.

Be careful though, it's might be mains voltage !!!! Get something sturdy, 1A rated at 400v. They only cost pence anyway.

Andy

[Mr Sandman]

Hi Andy. thanks for the useful response! Its just 12 Volts DC, so I can dig a couple of diodes out of an old PCB somewhere I'm sure. Where do you suggest I put them? one on each of the wires to the power supplies? I'm sure I can have a play, cant do any harm. I suppose there must be some electron transfer in some direction across the sensing terminals, I just need to find out which is positive I imagine?

cheers!

Steve

[Andy Cap]

12v is ideal. Just put diode into one of the wires arriving at the lock.

Repeat for the other lock, same wire with diode in the same direction, (ring nearest lock for example).

If locks fail to work, reverse the diodes.

Andy

[meow2222]

Your diodes may need a high voltage rating plus a snubber to survive. Too tired to think it thru properly.

NT

[mick]

So the maglocks de-energise to release, I assume. That's normally the case.

So you have separate power supplies for each maglock now?

the electricians has simply connected those

So both maglocks have now been put on the same power supply, fed from the alarm panel, or both maglocks are fed from the same alarm panel terminals?

I was thinking about using diodes to try

To use two maglocks you need two sets of terminals in the alarm panel, one pair for the front maglock and the other for the back one. They have to be wired to separate terminals otherwise your system won't work - as you've found out. If you only have a single set of terminals then you'll need to look for a new panel or a suitable second-door adaptor (if one is available for that unit). It's quite possible that the electrician has connected your panel in the only way possible!

For the smoke alarm you need to add a 2-pole relay, operated by the signal from the smoke detector. Each pole of the relay is then connected in series with a maglock. From your description it sounds as if the alarm has a single volt-free contact, so you need to power the relay from somewhere - probably at 12 or 24v via a transformer as such contacts are rarely rated for mains voltage. You would use the normally-closed contacts on the relays so that they cut off the maglock supplies when the relay operates.

[Owain]

What happens if the power supply fails?

If they're on an exit route then locks should have a mechanical release, or be fail-safe ie release autoimatically when the power is off (ie magnetic locks, rather than magnetic lock releases)

Owain

[Mr Sandman]

yes, i added a new seperat set up to the back door. The sytem i am using in both cases is an EZ-tag from TLC -

the keypad sends a signal back to the power suply to tell it to energise or de energise the mag lock. both systems have to be seperate or when someone opens the back door, the front door also deenergises.

There is no alarm panel.

it is a possibliity to use a relay yes, but i was hoping a couple of diodes would also work.

Steve

[Mr Sandman]

From: "Owain" Newsgroups: uk.do-i-y Sent: Tuesday, April 15, 2008 10:38 PM Subject: Re: Electronics advice pls.

if the supply fails, the locks release because they no longer have any power.

Steve

[Mr Sandman]

Thanks Andy.

Just to clarify then...or try to! I have two sets of terminals one on each powersupply, these are called P and B. The keypad shorts P & B together to switch the power on to the mag locks. So i have a wire coming from the keypad to both terminals. Both power suppys are set up the same. Then i have two wires coming from the smoke alarm activated relay. these two wires go short curuit when the alarm sounds. if i conect these two wires to one of the power supplys P&B terminals, it all works as planed, ie the door unlocks, but the electrition has put a wire from the terminals on that power supplys P&B terminals to the new power supplys P&B terminals. this causes both power supplys to cut power to the maglocks when the smoke alarm sounds, but also when one keypad operates it also causes the other doors maglock to yurn off. So where do i put the diodes so that the smoke alarm relay operates both power supplys P&B terminals, but dosent allow one power supplys keypad to short the other keypads power supply P&B?

Hehe

Steve

[Peter Parry]

You should have an emergency exit switch on the wall by the door which de-energises the lock when pressed. To have a final exit door which requires either a carried device to open from the inside or a smoke alarm to function is not terribly bright.

[Mr Sandman]

There is a final door exit button, but its pretty useless if you cant see it. Some of us have visual impairments.

Steve

[dennis]

I'll try again.. two pole relay.. connect the NC contacts, one to each door magnet.. energise the relay from the alarm.. alarm off the system is as before.. alarm on the relay breaks contacts to both doors. What could be easier?

[Andy Cap]

Just try and get one working first. Remove the wire from the Smoke Alarm that connects to the lock P terminal and connect the diode. Reconnect the wire to the other end of the diode. Test to see if the SA still operated the lock. If not reverse it. If neither way works, as dennis@home says, the relay route is probably easiest. If just one direction works, then connect the other diode in EXACTLY the same place and orientation.

HTH Andy

[Mr Sandman]

Yes the relay is an option, but I am asking for assistance with the possibility of a diode based solution, I.e. no mechanical element.

Steve

[Mr Sandman]

Cheers Andy, that's what I was thinking too, will have a go and let you know how I get one.

cheers

Steve

[Peter Parry]

In a fire at night (the most common time for fatal fires) everyone has visual impairment. If you are used to it that gives you an advantage. The three most common places to find bodies after a fire are under beds and in wardrobes (usually children) and against the front door (mostly adults looking for a key to get out).

If you want to wire things up to smoke alarms that's fine - but don't do it at the expense of far more useful functions like exit releases. If they are not easy to find where they are now then moving them and making them easy to find in the dark by feel should be a much higher priority than playing with smoke alarms.

As for the smoke alarm wiring - personally I'd put a single relay controlling a no volt release switch on the input side of the power supply or mains feed to the door magnets. That way once the smoke alarm goes off the doors open and can't re-lock. Using relays alone on the smoke alarm circuit to control the locks simply means that when the smoke alarms stop working the doors can re-lock - not a good idea as the fire is developed by then.

I assume you are talking of magnetic plate door locks such as:-

not the sort shown here :-

An NVR switch unsets on power failure and won't restore the lock supply until manually reset by physically pressing the on button.. There is therefore no possibility of the door lock re-energising when you don't want it to after the alarm has gone off.