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Whats the proper way of connecting auxiliary sirens to an alarm?

Discussion in 'Alarms, CCTV & Telephones' started by fex10, 5 Jul 2016.

  1. fex10

    fex10

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    My Scantronics alarm has a bellbox output that is 12v when the alarm is set or unset and the 12v is removed when the alarm is triggered. This is opposite of what a simple 110db 12v siren needs. So when auxilary security devices are wired to an alarm which require 12v when the alarm is triggered, how are people wiring these up?

    I've currently got them wired via a transistor/relay setup but wondering if there's a simpler way.
     
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  3. sparkymarka

    sparkymarka

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    Are you sure ? Most bell outputs are 0v applied or 0v removed in the case of a ( scb ) some outputs seem to be at 12v normal going to 0v in alarm.. What model panel do you have ?
     
  4. fex10

    fex10

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    I have a Scantronic 9651. The manual does say 12v normal going to 0v in alarm, though my multimeter reads 12v normal, nothing in alarm, but I will recheck.. that if it goes to 0v in alarm then problem solved.
     
  5. EightyTwo

    EightyTwo

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    It's an open collector drive - a transistor output that pulls to ground. It's a sink, not a source, and not designed to take the siren's power. If your siren is a sound bomb or similar, the transistorised relay approach is a good one!
     
  6. PaulUszak

    PaulUszak

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    Very probably you can dump the relay and connect directly. The only consideration would be total current capacity of the thing you are trying to connect. Check the panel specification in the engineer's guide or ask here. To confirm operation:-

    1. Set the meter for volts DC
    2. Put the red probe of your multi meter on some part of the panel that you know absolutely for certain is always +ve, say the positive battery feed to the panel pcb, or some terminal that is labelled (+) or 12 /13. DO NOT pick the output of the mains transformer as that will be AC, not DC. DO NOT pick the input of the mains transformer as that would not help at all.
    3. Put the black probe on your bell box output
    4. It should read 0V in NON ALARM states
    5. Set the alarm off
    6. It should now read +12 /13V
    On no account set the meter to AMPS or current reading - you'll either blow up the meter or pop a fuse on the control panel. Hope this helps and you can deduce what is happening. Shout if you need anything else. I've just done the opposite of what you're doing so have some experience (n)

    PS. What's the transistor for?
     
  7. bernardgreen

    bernardgreen

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    0 volts and open circuit are NOT the same thing and measurements of voltage can be confusing.

    Hold off.jpg

    Hold off which stops the bell sounding is in most designs a current operated function. The results shown for the voltage at Y ( relative to 0v in the panel ) are for a high impedance volt meter. Using a low impedance volt meter which will draw some current through the relay coil the voltage at Y will be slightly less than that at X due to voltage drop across the coil.

    Not all bell boxes use a mechanical relay but the basic principle is the same.
     
  8. fex10

    fex10

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    Thanks for all that info. This is all in aid of connecting an aux strobe and siren both of which are simply activated by 12v.

    The scantronic won't power a relay directly. Hence I set mine up that the bell output powers a transistor which then powers a relay.

    My concern is that the relay is continuously energised until the alarm sounds. This could cause the relay to fail from overheating and if it fails the aux siren will come on permanently. I want to rewire it so that the relay is energised when the alarm sounds.

    I can see two ways of doing it:

    Option 1. Connect the transistor/relay to the "comms" output of the alarm. The comms output can be set to go to 12v when the alarm sounds, which then energises the transistor and relay.
    Option 2. Use a different transistor which energises the relay when the bell output goes to 0v. If such a transistor exists..

    I've seen alarm installations with aux sirens so my real question is whether there's a standard way in which installed achieve this? Or do other alarms have an output that goes to 12v and can power a siren directly? DIY alarms do :)
     
  9. bernardgreen

    bernardgreen

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    Pull the relay in with 12 volts then hold it in with a reduced voltage. Simplest is a resistor in series with the coil and a capacitor in parallel with the resistor.

    Normally energised is better, if it fails you will know as soon as it fails.

    Normally de-energised is not the best, If it fails to operate when needed the alarm won't sound and by then it will be too late.

    Generally relays fail to pull in more often than they fail to drop out. Anothe rreason fro normally energised.

    For what it is worth I have had a relay energised for 31 years without a break except for 2 or 3 short periods of a few seconds. When taken out of service it was still fully functional
     
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  11. Europlex

    Europlex

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    As per the 'Connecting Sounders' Instructions in the 9651 manual (you do have one don' t you?)

    Output 1 - Siren
    Both are a neg (-) applied in alarm condition
    Output 2 - Strobe

    What is the problem?
     
  12. fex10

    fex10

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    The outputs won't power a bell, not even a relay, they are too weak, the voltage just drops as soon as a load is applied. So you would need a transistor, but a standard transistor requires positive current to switch on a load, so now you have a relay that is continuously energised.
     
  13. Deryck Tintagel

    Deryck Tintagel

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    What load are you connecting? The panel can sink 500mA - should be plenty for a modern sounder
     
  14. fex10

    fex10

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    I tried connecting a relay! It didn't work until I did it via a transistor.
     
  15. Handymanjo

    Handymanjo

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    Standard wiring connection on scantronic 9651

    Bell box to control panel

    - volts to - volts on panel
    +volts to +volts on panel
    Sw- to output 1 on panel
    Tamper return to TR on panel
    Strobe- to output 2 on panel
     
  16. EightyTwo

    EightyTwo

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    I think that the general recommendation here is to use a conventional alarm sounder/bell and not try to cobble something together from bits that you've already got.

    The +ve side of your relay coil (and the siren) should go to the +AUX and the negative of the relay coil needs to go to the bell trigger (OP1). Run the siren's -ve through the COM and NO relay contacts and on to the -AUX. Use a diode across the relay coil, of course, or buy a "Handy Little Relay" on a PCB. The same goes for the strobe...
     
  17. fex10

    fex10

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    I retried connecting relay direct to the bell outputs and it did work, so I don't know why it didn't work the first time I tried. So everything is solved and no need for transistors. Thanks for the help even though my first premisis were all wrong.
     
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