Supervisory circuit for fire alarm panel

[alah]

Hi, everybody; I have a few BRK 9v batttery operated smoke detectors lying around and decidec to give them some use.
I designed a circuit to interface them to a central panel; till now the ALARM function works OK.
I have problems designing the TROUBLE  function, which should signal if any wire of the zone gets disconnected ( meaning loss of detection
I´m attaching the ALARM circuit. Any ideas, please. THANKS, Alberto

[C]

Change your logic.

Think of your smoke alarm as having an output of NOT Alarm.
You have a signal from each smoke alarm that states it is working & not alarmed.
A broken wire then becomes ALARM.
A dead smoke alarm becomes ALARM.

[NiHaoMike]

Change your logic.

Think of your smoke alarm as having an output of NOT Alarm.
You have a signal from each smoke alarm that states it is working & not alarmed.
A broken wire then becomes ALARM.
A dead smoke alarm becomes ALARM.

I don't think calling the fire department just because a wire came loose is a desirable effect.

[floobydust]

To detect cut/open wiring, alarm panels use end-of-line (EOL) resistor. Usually 2k or 4k7 ohm. So the alarm panel sees either 2k ohm (loop normal) or a short (when alarm contact is closed).

[C]

I would think that a house burning down do to a failure is a bigger problems then a false alarm.

Creating a signal only on alarm is a failure as you you do not know of failed communications. A signal that gets removed on alarm cures this problem.

The need to separate an actual fire alarm from other failures says that the connecting wires have many states.
Open connection is one state.
A Short is a state.
Small current could be actual fire alarm.
Larger current controlled by smoke alarm not reporting fire a third state.
This larger current could be an energized relay or an on transistor. Both would fail to lesser current reporting the actual alarm.
The important point is that the smoke alarm has to actively state it is not reporting a fire.

[KD4PBS]

I don't think calling the fire department just because a wire came loose is a desirable effect.

Except that this is exactly how every commercial and monitored fire alarm system works. (and also monitored for "trouble" as floobydust wrote)

[alah]

Thanks to all for your replies. I know about EOL Rs, ALARM and TROUBLE signals.
TROUBLE would mean loss of comms or battery backup failure.
I was trained at SIMPLEX facilities but for maintainance purposes, not designs.
Ill stress my brains with floobydust in the meanwhile.
BTW, I have some new modules for SIMPLEX 4100 panels which I would like to sell.
If anyone is interested, please let me know, THAnKs again.