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Current sensing, I think??????

Discussion in 'Electronic Basics' started by JSW, Aug 23, 2003.

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  1. JSW

    JSW Guest

    I want to be able to connect smoke alarms to my existing house alarm. At my
    local alarm shop they sell a small pcb which replaces the 9v PP3 battery and
    allows the smoke alarm to be connected to the main alarm. This pcb probably
    has a voltage regulator on it (12v to 9v) and some sort of current sensing
    to trigger the main alarm when the smoke alarm sounds.
    What I want is to be able to make this pcb myself as the shop wanted to much
    for it and a want a few of them, does anybody know a circuit for such a

  2. Fred Bloggs

    Fred Bloggs Guest

    Yeah- it's called a relay. Your house alarm has inputs labeled N.O. and
    N.C.- so you run a wire pair from one of those over to relay contacts
    inside smoke alarm. Your difficulty arises on adapting the existing
    smoke alarm circuitry to activate the relay. This will be over your
    head- so go buy the finished product or drop the idea entirely.
  3. It's a cleaver idea. The sonalert / transducer inside the remote alarm
    will draw enough current to distiguish between alarm and quiescent.
    Try this:

    From Central Control: +12V, Sense, Return (Gnd)
    Use a 3-terminal Vreg to supply remote detector.
    +12V ----+-----/\/\/----+-----| In |---9V
    \ R1=? | | Out |
    Q1 PNP |----/\/\/---+ | Det
    /| R2=1K |
    | +-------Gnd
    Sense -<-+ |
    | |
    / |
    \ R3 = 10K - 22K |
    / |
    | |
    Gnd ----+------------------------+

    Adjust R1 to sense the current. The PNP transistor's conduction is
    temperature and input dependent. Also, R1 cannot be so large that the
    drop between the 12V input and the regulator exceeds the regulator
    lead voltage. You may need additional circuitry between the sense line
    and the alarm activation circuit. The sense line is pulled to the Vreg
    input voltage when enough current flows in R1. You may also require
    some noise filtering, like a cap between E-B of Q1.

    Frank Raffaeli
  4. scada

    scada Guest

    These factory smoke alarms are tested for Heat, Smoke, and whatever adverse
    conditions occur during a fire! Do you realy want to take that chance that
    you may get it all right!!!! Buying a proven unit gives peace of mind!
  5. Edward

    Edward Guest

    You should also check that your insurance is still valid if you "modify"
    your alarm system.
  6. Fred Bloggs

    Fred Bloggs Guest

    Right- and there are models that run off 120VAC with battery backup, and
    they are alarmed as well as alarm- meaning central alarm control can
    cause smoke alarm to sound via NO or NC inputs- as well as other way
    around. Generally you want latched operation requiring manual reset-
    making that transistor thing inadequate.
  7. Terry

    Terry Guest

    It's a good idea: But suppose it misoperates, perhaps due to some
    voltage glitch in the powers supply? Could a 'non standard'
    modification that caused a 'false alarm' to the company
    monitoring your home alarm system then cost you for a 'false
    On other hand if your house is of average size and you are NOT
    connected to an alarm monitoring company you can probably hear
    your individual smoke alarms if they go off anywhere in the
    house? We can.
    Here; for rented accommodation there are some requirements that
    the alarm be a) AC powered; so kids don't steal the batteries for
    their 'toys' and b) With more than one habitable floor, alarms be
    linked between floors.
    Personally I would keep your systems separate with the possible
    exception of an intermediate relay.
    Relays are very good at physically/electrically separating
    different systems.
    Then if a smoke alarm works to activate the home alarm system,
    fine. If it doesn't you still got your smoke alarm/s standing
    alone, not depending on anything else to power them or give the
    alert! Change smoke alarm batteries yearly. Also most smoke
    alarms will give a 'beep' indication if battery goes low.
    If any gas operated appliances (vented or not) also use a CO
    (Carbon Monoxide) detector. Could be cheapest life insurance you
    Last year, near here, someone had a night time fire in their AC
    main fuse panel. ALL POWER went off; no lights no AC operated
    smoke alarms, nothing. They were lucky to get out!
  8. Gel

    Gel Guest

    In the Uk both BRK & Firex sell these
    conversion boards themselves, so one can assume they're kosher.
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