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one shot timer for alarm bell

Discussion in 'Electronic Design' started by Steveo, Oct 6, 2003.

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  1. Steveo

    Steveo Guest

    I looking for away to make a 110v School Bell ring for 1min to 5min and stop
    and not recyle!

    This for an alarm system.

    Any suggestioms?

  2. Ken Taylor

    Ken Taylor Guest

    Use a student to push a button. If he isn't alarmed enough, yell "Boo!" at
    bell time.

  3. CFoley1064

    CFoley1064 Guest

    I looking for away to make a 110v School Bell ring for 1min to 5min and stop
    Assuming you want something reliable, go to an electric supply store, and ask
    for a delay-off time delay relay with the time range you're looking for.
    Generally, they have 110VAC constantly applied to them, a non-isolated
    pushbutton to trigger, which starts the time delay. One or two form-C relay
    contacts will energize and remain on for your set time (which can be set with a
    built-in potentiometer or with DIP switches.

    For this solution, you will need a remote pushbutton and the TDR, and possibly
    an enclosure for the relay. Pretty simple, not exactly cheap, and as reliable
    as you can get.

    Good luck.
  4. That's straightforward enough, but could you cover a few points

    1. What is your skill level. For example, are you up to building it
    from scratch yourself? And happy working with mains supplies?

    2. Is a DC power supply already available, or would you expect it to
    be derived from the 110V mains?

    3. What will trigger the alarm? It could for example be an input
    signal from a door microswitch, or maybe a mat sensor, smoke alarm,
    etc. Typically, a good alarm would accommodate several such inputs. In
    particular, will the signal be normally LOW and go HIGH when an alarm
    condition arises? Or vice versa?

    4. How will the alarm be reset? IOW, after that initial bell finishes,
    what is supposed to happen? How about another alarm condition arising
    10 minutes after the first, etc?


    Begging the answer to #1, you would simply need to trigger a 555
    monostable with a -ve going input pulse. The input should be removed
    before the mono times out if you want to prevent permanent activation.
    The mono's output would drive an appropriate relay directly, assuming
    standard 555 with 200mA capability. Mains to the bell would be
    switched by the relay. Passive conditions would have to be restored
    manually. One approach would be to arrange for the alarm input to
    switch DC power to the alarm circuit, simultaneously triggering the
    alarm. A circuit using that latter approach is suggested at

    You would of course need to change the RC values. In your case, for C
    you could use say a 560 uF electrolytic, and for R you could
    substitute a series combination of a fixed 100k with a 1M pot.

    On the other hand, if your trigger was say a +ve going pulse, then you
    might use a circuit such as

    again with suitable RC values.

    Or, if you *wanted* the alarm to be re-triggerable (and your input
    signal was suitable), then you could construct something like
  5. Steveo

    Steveo Guest

    My alarm system is a "X-10".
    It will keep flashing a 110v light untill i disable it. I have a school bell
    that I want to use. It uses 110 volts.
    I was wanting to use the same outlett for the bell.
    I just want the bell to ring for a min. are two and than be disable. As not
    have the city come looking for me!
  6. Frank Buss

    Frank Buss Guest

    Assuming you don't want to open the alarm system, but want to use the 110v
    output, a flip-flop is possible, with the monostable used 555. The first
    signal from the 110v triggers the flip-flop, which triggers the 555. The
    light can flash forever, but the flip-flop needs to reset to trigger the
    mono-flop again.

    To convert the 110v to digital input perhaps a diode, a zener diode and a
    resistor should work (be sure to isolate all 110v DC coupled parts from
    touching). The output from the 555 can drive a triac or something like this
    for the bell.
  7. scada

    scada Guest

    This is a pretty much a standard item for industrial applications. Try
    I did a search for Timer AC, and came up with many hits! You need to take
    some time and sort through it. You basicaly want a "One Shot" Time delay
    open. Just don't buy "Agastat" they are Pnuematic timers, that are
    notoriously lousy for time! Their changes with Hot and Cold!
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