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Simple Alarm Circuit help

Discussion in 'General Electronics Discussion' started by Robsony, Sep 10, 2014.

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  1. Robsony


    Sep 10, 2014
    I have a small problem that i'm sure can be solved easily with some advice off here. Explaining the problem may be the issue though. here goes.

    I have a "normally open" gate contact on a business premises, I also have a "normally closed" set of beams that are situated a few metres past the gate. I have wired both of these in parallel to a cctv system on a "normally closed" circuit.
    The out come works perfectly, if the gate is closed and the beam is broken, the circuit goes open and an alarm is raised (a cctv picture gets emailed). If the gate is open, then this disables the circuit as the circuit remains closed due to the contact. Therefore the business is assumed open and the owner doesn't want constant emails due to the beam being broken.

    The problem I have is if the business is closed, and a potential thief spots the beam, he could jump over it and then break in through the gate. No email would be sent! So I thought I would wire another pair of wires onto the same contact and attach it to a different alarm input on the system (normally open circuit). So if the gate opens then the second circuit would go "closed" and an alarm activation would occur. However, as the second pair of wires is connected to the same contact as the above circuit, (in parallel with the beams), the beams cause the second circuit to stay closed, when the gate gets shut. Ie it never gets to its normally open status. So is there anything I can attach to the second circuit that will stop the beams interfering? would a diode work? The other solution would be to install another gate contact and keep both circuits separate but i'm sure there is an easier way.

    Hope someone can understand this.....

    Thanks in advance

  2. Gryd3


    Jun 25, 2014
    It sound to me like the gate is wired in to bypass the beams when the gate is open.
    The only solution I can think of to detect if a parallel line is opened or closed would require the use of a microcontroller and a unique resistor in series with each parallel leg. This would at least indicate which parallel leg is open by taking an analogue reading of the circuit as a whole. This over complicates things and as it is for an Alarm, is not an ideal solution.
    I do think your best, most reliable option is to use an additional sensor on the gate. This would trigger an 'alarm event' every morning when the store opens though.

    Alternatively, swap out the NO gate sensor for an NC sensor, wire it in series and rely on a schedule for the monitoring program to disable alerts during business hours. Again, this has the drawback of triggering an 'alarm event' every morning when the store opens, and the alarm would need to be forced into being active on holidays where the schedule may change.

    What happens with the system if it is armed, and a beam is broken and held broken for a prolongued period?
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Alan and welcome to Electronics Point :)

    I think I understand your question. You have a single signal that you want to use in two different ways, with two different inputs to the alarm controller. When you connect it to one input, this prevents it from working properly with the other input.

    You may be able to use a relay to provide two electrically isolated circuits that are controlled from a single sensor. You need a DC power source, and you connect the relay coil and the sensor in series across that source. When the sensor output closes, it energises the relay. The relay has two independent sets of contacts that change over when the relay coil is energised.

    Here's an idea of a relay that would be suitable. Relays are available with different coil voltages and mounting styles. This one is a 12V chassis mount one.

    If I've misunderstood your question, perhaps you could try to explain it again...
  4. Gryd3


    Jun 25, 2014
    My current understanding:
    S1 and S2 are beams. Normally closed. (Open when beam is broken)
    S3 is a switch on the gate that is normally open. (Closed when gate is open)

    When the gate is open, the business is assumed open, and the closed S3 will prevent an open S1 or S2 preventing an 'Alarm event'.
    This is desired operation but there is a fatal flaw in that the S1 and S2 optical beams can be avoided allowing the gate to be broken into.
    The op wants to monitor the gate to flag an alarm as well, but is unsure how to keep the current function so that the beams being broken in normal business hours don't cause 'Alarm Events'.

    Can the op please verify if there are additional inputs that can be used on the alarm to prevent the circuits from interfering?
    Can the op please also verify if the alarm has any special features such as scheduled monitoring?
  5. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Could you not have both trigger the alarm when broken or open and then the owner switch off the alarm when he is there?
  6. Gryd3


    Jun 25, 2014
    That would be the easiest solution. Swap out the switch on the door, and manually disable the system when you open the business.
    I'm hoping the alarm system has some extra features built in that we can exploit to help prevent the owner from forgetting to turn it on.
    Also curious if the alarm stays triggered, if it will keep sending alerts, or if it will just send one and thats all.
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