Help with these switches please

Discussion in 'The Projects Forum' started by Metalica, Nov 11, 2018.

  1. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    This circuit is working well and achieving what i want with it but here is a little challenge for me.
    Its actually working well as a burglar alarm with a delay set by C1 but here is a flaw or hack or weak point in this circuit that i needs to rectify.
    That is if a burglar opens any of the 3 S1, S2, S3 doors it triggers the relay after a delay of few seconds (I need the delay her since it gives the owner a few seconds to turn the main switch off which is actually a key activated switch) now fine working well but what if the burglar is a trained person and he shorts the physical switche's two wires connected to the door and fools the circuit and attempts a successful burglary.
    Now thats a hack to this circuit please some one help me to remove this weak point.
    In simple words.
    What i want is that when any of the 3 doors are opened no matter what the burglar do to the switches it should continue its work of triggering the relay.



    2.jpg
     
    Last edited: Nov 11, 2018
  2. dl324

    AAC Fanatic!

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    Use an SCR instead of a transistor.
     
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  3. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    Ok good idea but which one?
     
  4. dl324

    AAC Fanatic!

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    I'd use this one:
    upload_2018-11-11_9-33-24.png
    $0.19 for qty 1 from jameco.com.
     
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  5. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    But its not working under simulation i don't know if it'll work in an actual circuit.
     
  6. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    Can you please create a circuit for me with an SCR.
    It should do these.
    When main switch is turned on it'll have a delay time before Relay triggers (Under condition that all the doors are open), and when non of the doors are closed it'll energize the relay.
    And if after main switch is closed and all the doors are closed with in the delay time then it'll wait for the doors s1 s 2 s 3 to open and when any of the doors are opened the relay will energize and stay that way no matter what ever the input from S1 s 2 s 3 is.
    In simple words the circuit should do this.
    When main switch turned on it'll give a time to the user to close all the doors within a time frame and if non of the doors are closed during that time it'll sound an alarm.
    And if Main switch is turned on and all the doors are closed it should wait for the doors to be opened and when any of the doors are opened it should sound an alarm after a delay of few seconds (This delay is needed for the owner to turn the main switch off with in the delay time)
    Now with a transistor it is doing all that but the weak point of the circuit is that its not hack proof.
    Thank you.
     
    Last edited: Nov 11, 2018
  7. dl324

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    Post your schematic.
     
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  8. Metalica

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    Aug 1, 2010
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  9. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    Its above dear
     
  10. crutschow

    Expert

    Mar 14, 2008
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    You need two delays, an arm delay and a trigger delay:
    1. The arm delay is for the alarm power so that the owner can exit without triggering the alarm.
    2. The trigger delay is for when a door sensor is momentarily triggered after the initial arming delay so that the owner has time to turn off the alarm after entering the house.
    For the trigger delay you could use an SCR door trigger with the SCR output delayed to power the alarm relay transistor.

    For the arm delay, power to the SCR circuit must also be delayed when the alarm power is initially applied
     
    Last edited: Nov 12, 2018
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  11. crutschow

    Expert

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    Below is the schematic and LTspice simulation of a circuit that provides the two delays as stated in post #10.
    I used CD4093 Schmitt trigger NAND gates (one IC quad package) to perform the required latch and delay functions, as that is simpler than any SCR design I could think of.
    And the high input-impedance of CMOS devices allows the generation of long delay times with smaller capacitors.

    The simulation starts with the application of power (power-on switch not shown).
    I initially generated many door opening and closings (red trace) to show that the U1-U2 latch doesn't respond to the door until after about 60 seconds, when U1's output goes low (blue trace) and it's output is latched.
    Following that and another ≈60 second delay, the alarm relay (yellow trace) is turned on, even though the door stays closed for that period.

    The NC SDoor switch can be duplicated as many times as desired with all of them in series.

    upload_2018-11-13_9-52-6.png
     
    Last edited: Nov 13, 2018
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  12. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    Thank you but i want to use only transistors or SCRs to perform this operation to keep the cost of the whole project low.
     
  13. dl324

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    Since the single transistor solution has the timing you want, just add a 2N2907 to the circuit and make your own SCR.
     
  14. crutschow

    Expert

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    The CD4093 goes for less than 50 cents US at a number of vendors.
    upload_2018-11-17_15-14-29.png
    I doubt you can make a significantly cheaper circuit using discrete parts to perform the desired functions with both exit and entry delays.

    Edit: Here's the revised circuit to include some LED indicators with its description.
     
    Last edited: Nov 17, 2018
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  15. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    When i try to open your link for the revised circuit it says.
    Oops! We ran into some problems.
     
  16. Metalica

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    Aug 1, 2010
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  17. crutschow

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  18. bertus

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    Apr 5, 2008
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    Hello,

    @crutschow , When the member is not logged in to ETO, the link will only show that you must login.
    Not every member here has an account on ETO.
    Better show the circuit here.

    Bertus
     
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  19. crutschow

    Expert

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    I had addressed that issue with them and I thought it was fixed but apparently not.
    Here's the latest circuit:

    upload_2018-11-18_9-40-2.png
     
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  20. Metalica

    Thread Starter Active Member

    Aug 1, 2010
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    Thank you for this this is flawless no doubt but i would appreciate if you can create a circuit with same effects using common transistors.
     
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