Audible alarm circuit with silence switch

Discussion in 'The Projects Forum' started by sandstorm, Aug 30, 2010.

  1. sandstorm

    Thread Starter New Member

    Aug 30, 2010
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    Hi everyone!

    I'm a newbie here and I'd like to seek some help.

    I need an alarm circuit (6-12VDC) interfaced with a 220VAC mains. The alarm circuit should set a load (relay) ON and trigger an audible alarm when the mains power is OFF. The audible alarm should be silenced/reset thru a momentary type pushbutton while the load (relay) should remain ON until the mains power has been restored.

    I've devised an SCR circuit as the switching media and a transistor-based beeping alarm and a relay as loads, but I have problem silencing the alarm without disturbing the other load.

    I'm not actually new to controls but basically I'm an "electromechnical relay (EMR)" guy who works on industrial motor controls and can setup the above circuit and practically any control logic thru EMR in no time.
    But i'd like to "downsize" and do it in more economical way. And you guys are the best when it comes to these things.

    Thanks in advance for your help.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It's past my bed time, but here is a quickie. Assuming relay will operate on 200mA or less @ 12V. A = son-alert or other audible alarm. SW NC PB. SCR low pwr sim. 2N5060. 220V dies,triggers 555, in turn triggers SCR. PB-SW only resets A. It might work-untill proven otherwise.
     
  3. sandstorm

    Thread Starter New Member

    Aug 30, 2010
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    Thanks Bernard. But I'm just wondering if the audible alarm will not reactivate since the PB will return to its original position (NC) hence the triac will again be triggered. Kindly note that the reset PB should be momentary type.
    And for the audible alarm, I need it to be beeping since a continuous one won't give much alert compared to an "annoying" beep. Is there available piezo to do this trick or a transistor-based drive circuit is needed?
    Thanks.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The SCR should not retrigger as 555 is in a stable state so no gate signal. I do not know of a beeping package-must be one, or a 556 & speaker. Might move R3 to 555, pin 3,& clamp SCR gate to ground for - pulse.
     
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  5. sandstorm

    Thread Starter New Member

    Aug 30, 2010
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    Thanks for clarifying. I'll get back to you as soon as I've tested the circuit.
    And by the way, would you please resize the components so that the circuit could work on 3Vdc.
    Thanks.
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Here's an alarm-silence circuit I've used. I added in the 240VAC relay to set the alarm on and switch your undefined "load".

    Ken
     
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  7. sandstorm

    Thread Starter New Member

    Aug 30, 2010
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    Hi Ken,

    I've tested your circuit and this works (minus the sonalert- I put LED load for the meantime). But this works the way I need it.
    R2 100K may be omitted. And I observed that the circuit will function correctly as well if R1 1K is not there.
    Now guess what, I tried to replace the SCR with two overlapping NPN-PNP transistors. Without the R1 in place, I noticed that the energization of the load is a bit delayed. However, when I put back the R1, the load was energized as fast as the SCR did.
    The only thing is, using the two-transistor configuration, the load can't be reset.
    Shouldn't the two-transistor be operating like the SCR counterpart in this case? Any idea why shorting the A and K won't reset the load?
    Thanks.
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    R2 is there to provide a path to completely discharge C1. I have no idea why your PNP/NPN SCR-equivalent isn't resetting. Post your schematic. You might try placing a N/C switch in series with the SCR's anode, instead of the N/O switch from anode to cathode.

    Ken
     
  9. sandstorm

    Thread Starter New Member

    Aug 30, 2010
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    Putting an NO across the A-K of the SCR works perfectly well to reset the load. What I mean is shorting the corresponding "A" and "K" terminals of the transistor substitute doesn't reset the load.
     
  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Again, posting your schematic would go a long way in getting an answer from someone who could simulate or breadboard it.

    Ken
     
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