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555 question

Discussion in 'Electronic Basics' started by kd6532, Feb 27, 2007.

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  1. kd6532

    kd6532 Guest

    I'm designing a circuit, wherein I'm trying to use a 555 operating in
    monostable mode to turn on an array of sensors for about one second.
    I've chosen by exernal R (10k) and C(100u) accordingly. My trouble is
    with the triggering. My triggering will be coming from a close contact
    that will be closed for a long period of time, opened for a unknown
    amount of time, and closed again. I need the 555 to output the 1
    second pulse when the switch closed (it's imperative). What would be
    the easiest method to do this, while consuming the least amount of
    current. Some kind of positive edge triggered component, RC network,
    etc.? One problem is that the input triggering pulse must typically be
    smaller than the output, which is a problem since the door is opened
    for an unknown amount of time.

    Thanks.
    Kris
     
  2. James Beck

    James Beck Guest

    What did someone say, the PIC10 and 12 series are the 555 of today?
    I would just about agree, with a little programming you could do the
    above without all the external support parts. Use the internal RC osc
    and you are on your way. Just look for the edge, do the output "thing"
    and wait for the next edge, rising or falling, it's up to you.

    Jim
     
  3. http://www.unitechelectronics.com/NE-555.htm

    Search on this page for differentiator.
     
  4. John Fields

    John Fields Guest

    ---
    View in Courier:

    +V>----+-----------+------+-------+--------+---------------+
    | | |K | |8 |
    [10K] [100K] [1N4148] [750K] +---+---+ |
    | | | | 2|_ Vcc |3 |
    +--[0.1µF]--+------+-------|---O|T OUT|--->OUT |
    | | 6| | |
    | +----|TH | |C1
    | O | 7|_ 555| [0.1µF]
    S1-->| +---O|D | |
    | O | | GND | |
    | |+ +---+---+ |
    | [1µF] |1 |
    | | | |
    GND>---+--------------------------+--------+---------------+

    C1 goes directly across pins 1 and 8
     
  5. Need some theory help here. :) What's going on with the 10k, .1uF, 100k,
    and diode? While the button is up, I'm thinking that 10K charges that side
    of the cap faster (obviously), keeping the trigger high as the charging goes
    on. Then when the button is pushed, that side of the cap is yanked to
    ground causing the other side to do basically the same thing resulting in
    triggering the 555. While the button is held down, the right side (100k
    side) of the cap proceeds to charge back to +V thru the 100K resistor. When
    the button is released, the left side begins charging towards the V+ rail
    possibly causing the right side to attempt to rise above V+. The diode
    stands in the way of that letting the extra voltage dump into the V+ rail.
    Is that close?
     
  6. Show us your code. :) Them's the rules. If you say "use a pic" you're
    obligated to show how it's done. ;-)
     
  7. John Fields

    John Fields Guest

    ..+V>----+-----------+------+-------+--------+---------------+
    .. | | |K | |8 |
    .. [10K] [100K] [1N4148] [750K] +---+---+ |
    .. | | | | 2|_ Vcc |3 |
    .. +--[0.1µF]--+------+-------|---O|T OUT|--->OUT |
    .. | | 6| | |
    .. | +----|TH | |C1
    .. | O | 7|_ 555| [0.1µF]
    .. S1->| +---O|D | |
    .. | O | | GND | |
    .. | |+ +---+---+ |
    .. | [1µF] |1 |
    .. | | | |
    ..GND>---+--------------------------+--------+---------------+
     
  8. John Fields

    John Fields Guest

    ..+V>----+-----------+------+-------+--------+---------------+
    .. | | |K | |8 |
    .. [10K] [100K] [1N4148] [750K] +---+---+ |
    .. | | | | 2|_ Vcc |3 |
    .. +--[0.1µF]--+------+-------|---O|T OUT|--->OUT |
    .. | | 6| | |
    .. | +----|TH | |C1
    .. | O | 7|_ 7555| [0.1µF]
    ..S1-->| +---O|D | |
    .. | O | | GND | |
    .. | |+ +---+---+ |
    .. | [1µF] |1 |
    .. | | | |
    ..GND>---+--------------------------+--------+---------------+
     
  9. Clever :)
     
  10. Rich Grise

    Rich Grise Guest

    Hear, hear!

    Thanks,
    Rich
     
  11. Of course the next time you suggest a 555, you have to post a schematic as
    well. ;-)
     
  12. Lord Garth

    Lord Garth Guest


    PSST! This is a non binaries group! Redirect them to A.B.S.E. or there can
    be no schematic posted excepting only crappy ASCII art.
     
  13. James Beck

    James Beck Guest

    How's this?

    while(TRUE)
    {
    if(edge_hightolow() && timed_out())
    handle_output();
    }

    or

    while(TRUE)
    {
    if(edge_lowtohigh() && timed_out())
    handle_output();
    }

    Now, just fill in the functions.....
    ;)

    Jim
     
  14. Yeah, why don't I just go ahead and do that while you go outside and play
    "hide and go **** yourself"?

    Cheers!
    Bobo
     
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