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Trying to get alarm horn to sound

Discussion in 'Electronics Homework Help' started by TonsdonSwitch, Aug 10, 2016.

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  1. TonsdonSwitch


    Aug 10, 2016
    Hello all, first post here and new electronics student. I have an assignment that is asking what are the least amount of steps to sound the alarm horn on rail 105 in the attached ladder diagram. I have come up with the below:

    1) Selector switch SS101 to P2 (rail 101)
    2) Momentary Button PB102 (rail 102) pushed and held for 10 seconds to activate on delay relay TD106 (rail 106)

    CR 103 is normally closed so I want to keep it in shelf state and current should flow to AH105, sounding the alarm horn.

    However, I received a hint that 4 steps are required and I will need to switch the selector switch to P3 after activating the on delay relay TD106. I cannot see a reason why this is necessary as CR103 (rail 105) is normally closed and energizing will open the relay, stopping the current from flowing to the horn.

    This is driving me crazy, I've scoured online resources and have been unable to find why energizing CR103 is necessary.

    Another question I have, is once TD 106 On Delay Relay is energized, and MomentaryPush Button PB102 is lifted, will TD106 stay closed? I'm leaning towards TD106 reverting back to open state once no longer energized and current no longer flowing through rail 105.

    The assignment also asks for the steps in shutting the alarm horn off, but I'm hoping I can figure that out once I can figure out turning it on.

    Thank you for any help!
    [2016-12-04, Harald Kapp: attachment deleted upon sensible request by the op]
    Last edited by a moderator: Dec 4, 2016
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to electronicspoint, your one-stop solution center :D

    I think your 2 step solution is fine: CR103 is normally closed, so activating CR102 by performing the steps you propose will turn on the horn. After 10 seconds TD106 will turn on, too and short circuit CR102's contacts, thus keeping the horn alive. I see no need to activate CR103 in this step.

    Turning off the horn will require to interrupt the closed circuit TD106 - CR103 - AH105.

    Probably the "hint you received" is for the whole sequence (horn on - horn off)?

    Re-think this statement. How dos current flow through leg 105 when PB102 is released? Will this influence the state of TD106, and if so, in which way?

  3. TonsdonSwitch


    Aug 10, 2016
    Hello Harald. Thank you for the incredibly quick response. That's a great point you bring in the 4 steps being the entire turn on / shut off sequence. I see now why selector switch needs to be turned to P3 and Pushbutton PB103A will need to be pushed to open CR103, in turn shutting off the horn.

    Last question I have is, how does TD106 affect CR102? Does TD106 closing also close CR102 in leg 105? If so, this would explain why the leg continues to be powered after Momentary PB102 is lifted.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Why should it? Is there a connection that controls CR102 via TD106? Look at how TD106's contacts and CR102's contacts are wired. Is there a dependence between the two or not?
  5. TonsdonSwitch


    Aug 10, 2016
    Great advice. Thanks again.
  6. dynamo


    Dec 2, 2016
    To sound the Alarm Horn:
    1) Place SS102 to P2 and leave it there.
    2) Press and hold PB102 to sound the horn.
    3) Release PB102 to turn the horn off.

    If you hold PB102 "ON" for more than 10 sec, the Horn circuit will seal "ON" by N.O. contact TD106 around CR102, requiring that CR103 be energized to turn the Horn "OFF".

    So, it depends how long you press PB102 "ON" for.
    a) less than 10 sec requires pressing PB102 only to sound the Horn
    b) equal to or greater than 10 sec requires switching SS101 to P3 and pressing PB103A to turn "OFF" the Horn.

    WOW, what an old post, I had better get a coffee!
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