Broadcast alarm - TAKE 2 - HELP NEEDED

Discussion in 'The Projects Forum' started by Quantum Flux, May 5, 2010.

  1. Quantum Flux

    Thread Starter New Member

    Feb 19, 2010
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    SCHEMATICS AND ILLUSTRATION UPDATED.
    COMPONENTS WITH MISSING VALUES WILL BE FIXED SHORTLY. PLEASE COMMENT ON DESIGN AND OPERATION


    ~ TAKE 2 ~

    This is my project to upgrade an A/V alarm at my workplace. My knowledge of electronics is still full of holes so your advice and help would be appreciated.

    The design presently in use is simply a mutable piezo alarm. The new design will incorporate some features which have been suggested by my colleagues.

    The port triggers a transistor to activate the alarm.

    Explanation of intended operation:
    The unit is essentially a video black/ audio silent warning.
    The computer in the plant room detects those conditions and closes the circuit between pin 1 and 2 of a DB9 cable.

    A ring illuminated push on/ push off switch acts as a mute button. An internal flasher LED provides the flashing effect in series with the ring LED on the mute switch.
    No alarm. Ring = OFF
    Alarm. Ring = FLASHING
    Mute position (alarm or no) = SOLID ON

    In the mute position, a selectable duration timer activates and provides a 250ms approx. chirp sound at the end of each duration. This is a reminder that the system is muted. It should be active wether the DB9 port is closed or not.

    The mode switch acts as an emergency bypass of the complex circuits. The operator would be able to quickly bypass failed components as a stop gap measure until repairs could be made.

    The piezo is a DC 12V with built in driver cct. The different inputs to the piezo provide a few selectable sounds to differentiate from other alarms in the same room.
    * A 555 generating a DC square wave
    * A 555 generating a DC triangle wave
    * Straight connection to 12v provides a constant tone

    I have not added the rotary switch for selecting the timing interval yet but will do soon.

    RENDERED ILLUSTRATION OF THE 1RU PANEL MOUNT:
    [​IMG]

    SCHEMATIC:
    [​IMG]
     
    Last edited: May 14, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
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    R2 is much too low in value, and C1 seems unnecessarily large.
    Double the value of R2, and cut C1 in half.

    The 556 timer requires at least an 0.1uF cap from Vcc to ground, and another 1uF cap from Vcc to ground.

    The LM7805 requires an 0.33uF cap on it's input to ground, and a 0.1uF cap on it's output to ground.
     
  3. Quantum Flux

    Thread Starter New Member

    Feb 19, 2010
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    I was thinking about using a UTC 1618 as a nice simple way of driving the alarm tone and having several selectable sounds. Am I on the right track? If so, where can I buy only 1 or 2 units?

    Can I ask what this is for? Forgive my ignorance. I'm only just branching out from the fundamentals now.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I am unfamiliar with the UTC 1618.

    They help to assure that the regulator will output a steady 5v and not go into oscillation.

    Without those caps, very small amounts of stray capacitance and inductance present in every circuit can cause the regulator to oscillate at high frequencies instead of putting out nice, clean DC voltage.

    You might "get away" with not using them. Then again, without them your circuit may intermittently "go on the fritz" for no easily explainable reason, or simply refuse to work at all.

    A gram of prevention is worth 10 kilos of cure.
     
  5. Quantum Flux

    Thread Starter New Member

    Feb 19, 2010
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    Can you suggest any ICs that provide a variety of alarm sound outputs?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Searching Google for "Siren/Alarm Sound Generator" will get you a lot of hits.

    Lots of links to siren circuits on this page: http://www.discovercircuits.com/S/siren.htm

    Most analog sound generator IC's have fallen by the wayside. Nowadays, everything's done via digital signal processing.

    One can do a lot with a microcontroller.
     
  7. Quantum Flux

    Thread Starter New Member

    Feb 19, 2010
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    Original post updated
     
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