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Power Latch Circuit Question

Discussion in 'Electronic Basics' started by Larry Brasfield, Mar 2, 2005.

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  1. There is a somewhat simpler approach, at least from
    a hardware perspective, when you have a general
    purpose (micro)computer available.

    Imagine you have these hardware elements:
    1. An electronic switch to connect or disconnect
    power from all of the circuit except as noted here.
    2. A mechanical, momentary switch which, when
    depressed, forces the electronic switch to be on.
    3. A programmable system with an input by which
    to read the state of the mechanical switch and an
    output which can force the electronic switch on or
    off, but which, as reset is asserted, does not force
    the switch on or off. (An output that is tri-state by
    default upon power-up or reset is easiest.)

    Then, use this algorithm:
    Upon power-up reset, wait until the mechanical
    switch appears to be depressed for a time longer
    than any normal bounce, then assert the output
    that holds the electronic switch on. Monitor
    the mechanical switch for non-depression, lasting
    longer than a normal bounce, before beginning the
    next step. Monitor the mechanical switch during
    normal operation, (via interrupt or polling), often
    enough to be able to notice subsequent depression.
    Once depression is observed lasting longer than a
    normal bounce, enter a pre-shutdown state where
    all or most indications of power being on are not
    displayed. In this state, wait for the mechanical
    switch to be non-depressed for longer than a
    normal bounce. Then negate the output holding
    the electronic switch on.

    This may seem involved, but it requires somewhat
    less hardware than the article you cited.
     
  2. Cauley Felps

    Cauley Felps Guest

    Hi,

    I'm building a PIC-based MIDI timecode LED display from the web. I'm
    mounting it in a small enclosure I have, and would like to use the
    existing power switch (a SPST tactile button).

    I found a schematic for latching power using 4 transistors here:

    http://www.edn.com/article/CA472837.html

    I changed the resistor values for my 5v supply and plugged it up on a
    proto board. It seems to work OK, other than the fact that it powers
    up in the "on" state (I didn't think that was normal).

    There's a 7805 regulator in the PIC circuit with a 2200uF and 330nF
    cap parallel across the input and ground, and a 10uF and 100nF
    parallel on the regulator's output and ground. There's also 4- B40C800
    diodes, so it can take AC or DC, but I've been testing it with a nice
    clean 12vDC supply going straight to the 7805 and filter caps.

    I planned to use a small 120AC to 12V transformer when I put it
    together. I may also stick a DPDT 120V slide switch on the back panel
    to kill the power completely.

    Does all this sound like the most logical way to go about it? If so,
    is the transistor/latch circuit OK after the regulator and filters, or
    would it dirty up the power feeding the PIC and LED's?

    I'm open to any other suggestions on the power supply or latch
    circuit.

    Please reply in the newsgroup
    (and thanks a lot)

    George

    FWIW- my values came out to
    R1- 10M R6- 2.3M
    R2- 470k R7- 5M
    R3- 2.3M R8- 500k
    R4- 1k R9- 5K
    R5- 50k

    the cap stayed at 1uF and the transistors were BC547's and 557's.
    R1 started at 2.3M, but it would switch back on if you held the button
    for more than a split second while turning it off.
     
  3. Cauley Felps

    Cauley Felps Guest

    Larry,

    Thanks! I think I get most of what you're saying, but may not be
    capable of pulling it off at this point. You're talking about using
    the same PIC to monitor the button presses right? I guess this would
    still leave the PIC powered up and just turn off the display output.
    I'm also not sure that the PIC and program I'm working with could
    handle any additional functions. I'll save this info for later. I'm in
    the process of getting familiar with PIC code, but it will take me a
    while. All the stuff where I got the circuit is open source and he
    sort of encourages changes and enhancements.

    I'm still wondering about the rest of it, if you or anyone here knows.
    (the location of the latch circuit and the reason it powers up
    latched)

    -Thanks!
     
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