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Can anyone assist?

Hi chaps, hope someone here can help me either design a unit or build one for me.

i have mains powered interlinked smoke detectors in my flat and an key fob operated access control unit on my front door that allows me quick entry in, rather than fiddling with keys. The front door is very strong and not easily forced open in the event of emergency.

What i want is to connect the 2 together in some way so that if there was a fire in my flat and i was unable to get out, the front door will unlock to allow someone in to rescue me.

the access unit operates a 'fail open' lock on the door, so all i need to do is interrupt this wire with a relay that would operate when the smoke alarm is sounding.

so... there are 2 ways i can see of doing this....

1...use a sensor that will sense voltage or activity in the cable that interlinks the smoke alarms in the flat and operate a relay in the presence of that voltage.

2...use a sensor that will listen out for the sound of the alarm and operate the relay is its presence.

the relay will need to be held on for a period of time after the initial activation too. allowing access to the flat even if the smoke alarm stops sound ing, and have a reset button on the unit to return unit back to normal operation for use after false alarm i.e. burnt the toast!

Can anyone help me design this unit or better still build it for me?

I am obviously willing to pay!

Many thanks!

Steve

Reply to
R.P.McMurphy
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If the power goes out the door unlocks? That's peculiar.

Reply to
Homer J Simpson

id rather that than be locked in.

Steve

Reply to
R.P.McMurphy

Well, there should be a wire that links the two detectors together. I assume that is hot when the alarm sounds. If so, connect a 240 VAC relay from that to neutral and use that to cut the door power.

Reply to
Homer J Simpson

how can i then delay the reconnection of power?

Steve

Reply to
R.P.McMurphy

It depends on how much isolation is needed. That depends on the detectors.

Add a changeover contact to the relay which transfers the coil from the detectors to 240 VAC. Add a pushbutton in series which opens that connection.

When the fire alarm sounds, the door power will be cut and the relay will transfer to mains AC and will lock on, thus open. When the fire alarm goes off it will stay on. When the push button is pressed the relay will transfer back to the alarms which will be off thus the relay will stay off.

You need one NC push button and one 240 VAC relay with DPDT contacts. Any electrician can wire this.

Reply to
Homer J Simpson

OK, will these do?

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What do you mean by a change over contact? how does this switch the live power supply to the relay that was originally being powered by the smoke alarm interlink wire to a permanent source?

cheers for your help

Steve

Reply to
R.P.McMurphy

I'd use

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I don't like miniatures for AC wiring. Ideally find one with a 240 VAC coil.

The switch is OK.

There's enough inertia in the relay for it to switch over from one source to another.

Reply to
Homer J Simpson

One thing to consider for the fire alarm detector is to aquire an additional one of the linked smoke detectors and open it up and remove its detectors. The electronics chip can still stay active and pickup the link signal from your other detectors and fire off the piezo sounder in this stripped down unit. It is relatively easy to use a small resisor-capacitor-NPN transistor circuit to filter the AC drive waveform from the chip to the piezo element into an On/Off type logic signal. This logic signal could then be used to operate the relay with the changeover contact (latch contact scheme) on the door access lock mechanism.

Use care to not modify any of the fire/smoke detectors that you depend upon for their native function because improper re-work could render them non-functional when you need to depend upon them the most.

- mkaras

Reply to
mkaras

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