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Toggle power with menbrane switch

Discussion in 'Electronic Basics' started by Joseph, Mar 31, 2007.

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  1. Joseph

    Joseph Guest

    Will this work to toggle power using a membrane switch?
    http://www.flickr.com/photos/[email protected]/441030617/

    To power up: The PWR button brings the SHDN button high, turning everything
    on. The processor keeps the SHDN pin high after after it boots up.
    To power down: Monitor the PWR button, set the SHDN pin low when it is
    pressed. Question. I need the diode to separate the "keep on" voltage from
    the PWR button signal. I'm not sure I have this correct.
     
  2. John Fields

    John Fields Guest

    ---
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    Register?

    Too much of a PITA for me... Why not post your binary to
    alt.binaries.schematics.electronic ?
     
  3. Guest


    %#&$.
    Sorry! I though they didn't make you do that! Of course I'm away
    from my normal PC now and can't post a proper message. Anyway...pls
    try this link:
    http://home.cinci.rr.com/creek/images/max1675.jpg
    j
     
  4. ehsjr

    ehsjr Guest

    I think "The PWR button brings the SHDN button high"
    should read "The PWR button brings the SHDN *pin* high"

    It won't work as you have drawn it. With everything
    off, the out pin will be low. Pressing the pwr
    button will connect low to SHDN - the opposite
    of what you need. I think you need to do something
    like this:

    +3 ---[PWR]---[C1]---+----+----mon[CPU]--------+------> 5V
    | | | | Out
    | [D] [D] | Pin
    | | | |
    [R] +---[R]---+---SHDN [C2]
    | | |
    | [R] |
    | | |
    Gnd -----------------+--------------+----------+

    Sequence from off to on:
    All off, press power, SHDN goes +, 5V turns on, CPU turns on.
    The power on reset condition must put + on SHDN. A power
    on reset routine must loop until mon goes -, meaning
    the power button has been released. Once mon goes -, the
    main program runs.

    Sequence from on to off:
    Press power, mon goes +, program reads mon and toggles
    SHDN low. CPU continues to run briefly via C2 after MAX1675
    shuts down, to hold the SHDN pin - long enough for C1 to
    be discharged.

    Ed
     
  5. Joseph

    Joseph Guest

    The spec sheet says that simply connecting OUT to SHDN will turn the
    regulator on.

    I've posted (I think) my updated schematic to
    alt.binaries.schematics.electronic.

    Ed,
    It looks like you have the 3V going _through_ C1...is that correct?
    I think your explaination will work, I just can't figure out the schematic.
    j
     
  6. ehsjr

    ehsjr Guest

    I suppose it's symantics - the 3V isn't "going _through_ C1"
    per se. You get a pulse charging C1, then the 3V is blocked.

    The cap is there to give you a + pulse to get things
    started and to get things stopped. But I drew it wrong -
    no discharge path. Grumble. The cap needs a discharge
    path on the right:

    +3 ---[PWR]---+---[C1]---+---+----mon[CPU]--------+------> 5V
    | | | | | Out
    | | [D] [D] | Pin
    | | | | |
    [R] [R] +---[R]---+---SHDN [C2]
    | | | |
    | | [R] |
    | | | |
    Gnd ----------+----------+-------------+----------+

    I added a resistor - it allows the cap to discharge
    when the button is released.

    Ed
     
  7. ehsjr

    ehsjr Guest

    ehsjr wrote:

    ^^^^^
    left
     
  8. Bob Myers

    Bob Myers Guest

    Depends on whether you're in the US or the UK...;-)

    Bob M.
     
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