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Modified time delay circuit help needed

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supernoob

New Member
Not exactly sure how to go about this and days of searching existing electronic diagrams has given me a vague idea of what components to use, but nothing concrete yet.

My idea is to intercept the wire coming from a smoke alarm that is used to link it to another smoke alarm and use the signal to activate a relay to physically switch off the mains power to the stove in the kitchen.

I think it would be best to put in a delay of about a minute or so to give the cook a chance to evacuate the smoke before turning off the stove power in the kitchen.

My initial design thought was just to use a simple time delay circuit with a 555 ic, but that will activate the relay after a minute even if the smoke has cleared and the smoke alarm is off and there is no fire danger.

Any help is much appreciated
 

Sceadwian

Banned
I sure hope you chose your relay carefully, you need to break a HUGE amount of current on an electric stove that's running and the current rating of a relay is NOT what you can safely break with it.

If you've searched for days for an existing circuit you need to refine your search methods. I found this my first search on Google.
Time Delay Circuit Using 555 Timer
It should be trivial to add a push button to that circuit to disable the output pulse.
 

supernoob

New Member
thanks for such a quick reply! As I mentioned, I already have looked at the time delay circuit using 555 ic timer, but i can't use a push button override as I intend to use the relay to turn off the stove from the power board rather than near the stove or inside the kitchen.
 

Sceadwian

Banned
Why can't you use a push button reset? I think you would just ground the reset line?
 

Sceadwian

Banned
Like blueroom said, what relay are you using? You can't just 'turn off' a few dozen or hundred amps of current from a normal relay, they arc and could catch on fire themselves.
 

supernoob

New Member
Sorry, i wasn't quite clear with what I meant. My idea is for the relay to power a solenoid or some other mechanism that physically turns off the power switch.

Clipsal sell a product that goes into the power board that has a little arm that protrudes and physically turns off a switch adjacent to the module.
 

supernoob

New Member
Why can't you use a push button reset? I think you would just ground the reset line?
From what I understand, Smoke detectors send a constant 9V signal over a copper wire so that all your smoke detectors in the house can activate should one detect smoke.

Once the smoke has cleared, the constant 9V signal stops and the smoke detectors stop sounding.

I want to use the 9V smoke detector signal to activate the timer delay, and the termination of the 9V signal to deactivate the timer delay, should the 9V signal last for less than a minute.

If there's no other option, I guess I will have to go the push button reset route.

My intention of using the timer delay circuit is if the stove is unattended while in operation and a fire starts, that the stove will be switched off if the smoke alarm goes on for more than a minute. If the stove was attended, I don't think there will be enough time to clear the smoke as well going to power board to push the reset button on the timer delay circuit.
 

Sceadwian

Banned
Technicalities aside I think it's a bad idea to try to automate basic safety like this. If a smoke alarm goes off a person needs to check it out immediately. If the smoke alarm goes off so much that you need to do something like this there is something wrong.

A breaker should not be a primary safety shut off, they're a BASIC over current fail safe nothing more and should not be externally automatically triggered. If you need the ability to shut the stoves off to be nearby but out of the room get an electrician to install a full sized disconnect rated for the oven's current use.
 
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colin55

Well-Known Member
I have a circuit that detects when a smoke detector is activated.
Furthermore, you can use a RCD (earth leakage detector) to cut the power as these will trip with very little current.
 

Sceadwian

Banned
Let me try to put this in common sense terms. If there is a fire and the insurance company finds out you have a circuit like that hooked up to a smoke alarm they'll throw your insurance claim out the window. Modifying the smoke detector invalidates it's UL certification. Even a halfway intelligent person shouldn't even think about taking that kind of chance. You screw something up and there's a static shock on your control lines that feeds back into the smoke detector that you don't account for and it doesn't go off... Lives are not something that should be experimented with. If you want something like that done get it done by someone FULLY certified to do so.
 
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colin55

Well-Known Member
My circuit merely monitors the current taken by the smoke alarm.
This is quite legal as that it the way they are montored for a large fire-detection system.
 

Sceadwian

Banned
When installed by a licensed professional. Not a weekend warrior =)
 

colin55

Well-Known Member
If you are so worried, put in parallel detectors. They only cost $8.50 each.

You always make mountains out of molehills.
 
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