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Thread: Basic Stamp to control my Car-Puter

  1. #11
    Constant Bitrate
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    It depends on how the bios is configured, but by default you can turn off an ATX mb by pulling the PS_ON line low for 4 seconds.

    If you already have the transistor to support PS_ON, then you can directly connect to the stamp (or pic).

  2. #12
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    You can't eliminate the relay completely. You need it to switch the high amp 12v source. A transistor can only handle a few hundred milliamps at best.

  3. #13
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    The transistor only needs to handle the PS_ON, everything else would stay connected all the time. If the computer is off then it should not draw any current at all (or at least very little). The question is how long can it be left in this state before the trickle drains the battery?
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  4. #14
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    ATX motherboards do not turn off and stop using power when the PS_ON goes high (i.e. is not being pulled low)

    PS_ON is a signal input on the power supply side. The motherboard simply uses it.

    The Power supply pulls the line high (+5v) by default. To turn on everything, the motherboard pulls the PS_ON line low and holds it there. This causes 12v to flow across the transistor and power the relay coil, in turn powering the various rails.

  5. #15
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    Ok, what about PowerGood then? If that goes low will the motherboard turn off?
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  6. #16
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    You would have to check your motherboard to see if it stops pulling the PS_ON line low if it sees power_good go low.

    I know my Epia M9000 doesn't even use the power good line.

    The ATX PS Spec says that the power supply should assert this low only when the voltage rails are falling to zero.

  7. #17
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    Well according to the ATX PS Spec that you provided, the PowerGood line is supposed to be dropped at least 1ms before voltage levels fall below tolerance (which is typically only 5% below rated output), which means that any ATX-compliant motherboard that has a self-preservation instinct should shut itself off to avoid brownout damage. Thus if we were to artificially drop the PowerGood line then the motherboard should instantly shut off. I suppose though, as you said, not all motherboards use the PowerGood line, but for those that do this might be a neat way to avoid using a relay.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  8. #18
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    I agree with digitallexus, a PIC is the way to go.
    Another member is working on something similar, a PIC-controlled power supply/shutdown controller.

    H e r e

  9. #19
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    Why is a PIC the way to go when it's like 2-4x as expensive as any other method of doing it not using one?
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  10. #20
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    A programmable chip with multiple lines of input and output that only costs a few dollars at most? What's not to like?

    The alternative is logic and timing chips which is going to be much more complex to design.

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