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automatic shut off circuit

Discussion in 'Electronic Design' started by Albert, Jan 4, 2004.

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  1. Albert

    Albert Guest

    I need an automatic shut off circuit for battery powered electronic
    items. Over the years, I've spent way to much money buying batteries
    because the previous user forgot to turn the power switch off. Just
    this week, I threw out a set of C cells from my daughters electronic

    I wonder if anyone has designed something like this before.....

    I want to incorporate it into existing items, so it needs to be small,
    cheap to build and be powered from the existing batteries. Ideally,
    I'd like to have 20 to 60 minutes of on time once the unit is powered
    up. The length of the ontime is not critical, high precision timer
    chips are not needed. Quick and dirty should be ok.

    Any suggestions??


  2. James Sweet

    James Sweet Guest

    Sounds like a good use for a 555 timer circuit or an 8 pin microcontroller,
    the latter offering more flexibility but a steeper learning curve.
  3. Bob Parker

    Bob Parker Guest

    Hi Albert,
    I've seen circuits to do that in some of the books of collections
    of Elektor magazine projects.
    You might also find something suitable at
    Good luck.

  4. You can use a 555 with its output driving a pass transistor that feeds the
    555 supply and the rest of the circuit. A power on switch shorts the
    transistor resetting the circuit.

    I've used this method for a battery operated antenna pointer with the pass
    transistor driven by a micro port. Worked beautifully.

  5. Albert

    Albert Guest

    A 555 and it's support components is a major undertaking.

    I was hoping for something much smaller and simpler, perhaps a
    momentary contact switch, a resistor, cap and a mosfet.

    Would a cap connected to the gate of a mosfet keep the mosfet turned
    on if the cap was charged through a momentary contact switch to the
    plus side of the battery?
  6. Luhan Monat

    Luhan Monat Guest

    I think that 555 circuit is about as simple as it gets.
  7. Rolavine

    Rolavine Guest

    Yes, I've used this before, put an N channel enhancement mode logic level
    mosfet between your circuit and ground. It is a bit sloppy during the turn off
    but maybe that is OK? Test it yourself!

  8. Bob Parker

    Bob Parker Guest

    That's the kind of thing I was thinking of. It depends on exactly
    how the power switching of the existing equipment works, available
    supply voltage, etc...
  9. One problem might be that as it discharges, the MOSFET won't turn off
    like a switch but will gradually reduce the current, possibly with bad
    consequences for both the MOSFET and powered device. So, there should be
    another switch in between the MOSFET and the device such as a relay or
    something else with hysteresis.

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  10. Bob Parker

    Bob Parker Guest

    Happy New Year Sam,
    I agree. However it depends on what's being switched and what
    currents and voltages are involved. Some equipment wouldn't have a
    problem with a very simple arrangement like this, but it could cause
    major problems with other stuff.
    Another difficulty is physically fitting a power-off circuit into
    the device which needs it....

  11. Mike Page

    Mike Page Guest

    Why not a wall wart (battery eliminator) ? Unless it's for fun; but it
    might turn out harder than you think - what's your minimum voltage and
    maximum current ?
  12. GPG

    GPG Guest

    To speed up switch off you could try something like this:
    \ o
    o LOAD-
    | |
    .-. |
    R1 | | |------o
    | | | |
    '|' |-+ .-.
    |-------o>| | |
    | | |-| | |
    | .-. | '-'
    .-------| | |R2 |V- |
    ... | | | |
    | | | '-' -------|
    | | | + | | |
    '-' ### | | |
    | --- \| | .-.
    --------| ||---' | |
    | <| | |
    | | '-'
    | | |
    V- '----o-------------o
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
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