Alarm question

Discussion in 'Tech/Mech General' started by XRN, Apr 29, 2016.

  1. XRN

    XRN Funster

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    Can anyone tell me how I can determine whether a door reed switch is a "normally open" or "normally closed" one? The van is fitted with a Cobra alarm and all the doors have reed switches, with the wired section fitted to the frames and the magnetic bit on the door. I have one which doesn't trigger the alarm when the alarm is switched on and the door is opened. I can access the wires on the back of the reed switch but I am not sure what I should be measuring with a multimeter to work out the type. Can anybody advise the best way to test as there are no markings on any of them which would give a clue. I am assuming it is the switch and not the wiring so knowing what I should be seeing whilst testing would be useful to work out what is wrong.
     
  2. Chipster

    Chipster Funster Life Member

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    Use a switch that is known good. Remove one wire from switch. Set alarm. If alarm goes off or doesn't set its a normally open switch ( the magnet pulls it closed). Or with a multi meter set to resistance disconnect switch and put meter leads on switch terminals. Open circuit on meter with magnet away from switch = normally open. Move magnet to switch and meter should make/read low resistance. My money's on normally open (y)
     
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  3. XRN

    XRN Funster

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    I might not have got this right but if the magnet is away from the switch and you have an open circuit, would that not suggest its a normally closed switch, i.e. when the door is closed as it would be when the alarm was set, the circuit would be complete and broken only when the door is opened?
     
  4. tonyidle

    tonyidle Funster

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    Yes. A NC reed switch is more difficult to implement than NO so when the magnet is near the switch (door closed) the reed will be closed.
     
  5. autorouter

    autorouter Funster

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    The term 'Normally Open' is a technical term used by the manufacturer to describe the operation of the switch. If it's open when the magnet is away from the switch, it's a 'NO' (Normally Open). If it's closed when the magnet is away, then it's a 'NC' (Normally Closed). Whether the magnet is usually near the switch or away from the switch in the situation that it is used is of no concern to the manufacturer.

    Generally alarm switches are 'NO', with a magnet holding them closed, so that if the wire is cut the alarm is triggered.
     
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  6. XRN

    XRN Funster

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    That makes sense now, many thanks for the help everyone
     
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