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Power source for smoke detector

Discussion in 'LEDs and Optoelectronics' started by Heimhenge, Feb 27, 2018.

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  1. Heimhenge

    Heimhenge

    13
    1
    Jul 12, 2011
    I have 9 smoke detectors in my home, 3 of which are in the attic (crawlspace) and only accessible with great difficulty. The system is wired as interconnected, so if any unit triggers all units sound. They're all old and due for replacement, but don't contain backup batteries like modern units.

    I don't mind changing batteries downstairs, where all I need is a step stool. But I'm looking for a solution for those 3 attic units. Lithium batteries will give me 3-5 years between battery changes (or so I've read). But I was wondering about a "permanent" solution using the 110 VAC available at each unit and running it through a FWR with a cap to replace the battery. Those two components could easily fit inside the detector housing.

    So my question is this ... just how flat does the 9 VDC input need to be for a smoke detector? I'm guessing that if a FWR and cap work to power a laptop, it should suffice for a smoke detector. Feedback appreciated, thanks.
     
  2. Bluejets

    Bluejets

    3,692
    763
    Oct 5, 2014
    Backup batteries should be replaced every 12 months regardless. If the backup starts to run below acceptable level, they usually start to hiccup( beep)

    There are units available with internal 10 year backup (non-replacable) as after this period the units are out of date anyhow.
    Interconnection is a must and it might pay to check on placement requirements as this is constantly changing.

    Care must be taken in selecting replacements as compatability is also an issue. Best to get an accredited installer to give you local requirement advice and perhaps a quote.
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    The voltage from a loaded battery drops gradually at first then drops quicker and quicker as it becomes dead.
    The voltage of a loaded capacitor drops quickly at first then drops slower as it becomes dead. you do not want that.
    A smoke detector detects low battery (maybe 7V) then causes a beep every minute. The capacitor will cause "low battery" to be detected soon after its charging disappears and it will not be able to power the alarm sound for long.
    A capacitor is a hazard like using a "super heavy duty" cheap Chinese battery.

    If you use a rechargeable lithium battery then the charger might charge it only one time. The charger might assume that you will remove the dead battery and replace it then charging begins.
     
  4. ChosunOne

    ChosunOne

    339
    83
    Jun 20, 2010
    First and foremost, any solutions you may come up with here may be moot, because of Fire Code. What you are allowed to do with smoke detectors is strictly regulated, at least in my part of the country. Before you do anything other than change the detectors, you might want to check with your local AHJ (Athority Having Jurisdiction). I'd start by calling your local fire house to find out how to contact your local Fire Marshall. Modifying the power supply to your smokes is a felony in some (if not all) states---regardless of whether it actually makes the system safer.

    Having said that:
    If 3 of your home's 9 smoke detectors are in a crawlspace attic, your home must have a really unusual layout. In the home alarm industry, we learned decades ago not to place smoke sensors in crawlway attics, because those spaces become very dusty over the years and when an unusually gusty day causes a puff of very fine dust to kick up in the attic, it looks a lot like smoke to a smoke sensor.

    If you haven't had any false alarms from your attic, I would be suspicious that your smoke detectors are not working. If you're using the ionization type, they're famous (in my industry, anyway) for going dead/insensitive when they get old. (The alarm industry uses photoelectric sensors, which do the opposite--they tend to become more sensitive after a few years. I won't bore you with the "why".)

    An attic is one of several places where we always substitute fixed/rate-of-rise heat sensors instead. Some other places being garages, kitchens, laundry rooms. It's just a bad idea to put a smoke detector in a location where smoke (or dust or steam) inevitably occur in the course of living.

    Heat sensors are unpowered and can be interconnected with your smoke detector system without having to pull any new wiring, and unlike smoke detectors, they don't need to be replaced every few years.

    If you speak with your local Fire Marshall/AHJ, you might ask him/her whether Fire Code allows you to substitute.
     
    Tha fios agaibh likes this.
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    There are some immigrants here who have family members living in the garage and maybe even in the attic.
    People born here have raccoons living in these places.
     
  6. ChosunOne

    ChosunOne

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    83
    Jun 20, 2010
    And I've been in some attic crawlspaces where squirrels visit because they find wire insulation tasty. Which is why I had to crawl up there. My solution was to fill a few old socks with (well-) used kitty litter and leave them scattered around.

    But I'm not sure how relevant this is to smoke detectors.
     
  7. Heimhenge

    Heimhenge

    13
    1
    Jul 12, 2011
    Lots of good comments, thanks to all who responded. ChosenOne raises a good point about attic detectors. Those units are close to 20 years old, and they're ionization detectors, so they definitely need to be replaced. And the attic is indeed dusty (we live in the Arizona desert) but the attic detectors have never sounded a false alarm. I looked at heat rise detectors but am concerned that attic temperatures here in Arizona might cause false alarms.
     
  8. Sunnysky

    Sunnysky

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    116
    Jul 15, 2016
    It might be wiser to feed 9V brick DC to all alarms but modern ones now say the CO2 detector is only good for 10 years along with the battery then toss the unit. CO2 detectors are more sensitive than smoke particle detectors.
     
  9. Heimhenge

    Heimhenge

    13
    1
    Jul 12, 2011
    Yeah, that's what I discovered ... can't buy smoke alarms w/o battery backup these days, unlike 20 years ago when I got the first set. Decided to go with a Kidde model with built-in 10 year lithium batteries. I can still get up into the attic just fine these days, but when it comes time to replace this set I may have to hire a more nimble electrician. :)
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    Hee, hee. I am going to keep away from you because you exhale deadly(??) CO2 carbon dioxide. I use CO detectors because carbon monoxide is deadly.
     
  11. Sunnysky

    Sunnysky

    469
    116
    Jul 15, 2016
    you know that's what I meant :rolleyes::)
     
  12. Audioguru

    Audioguru

    2,694
    611
    Sep 24, 2016
    I suppose the "2" was a typo?
     
  13. Bluejets

    Bluejets

    3,692
    763
    Oct 5, 2014
    So what happens if one decides to false alarm? Any I have installed ( many) one has to access the problem unit to hush.
    I have seen commercial detectors with remote head units but never domestic type smoke detectors.
     
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