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Battery saver circuit (updates?)

Discussion in 'Electronic Design' started by RP Henry, Jul 28, 2003.

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  1. RP Henry

    RP Henry Guest

    About 2 years ago, there was discussion on this ng about an auto-off circuit
    to disconnect a battery after a time. This led to the following ascii
    schematic from Win Hill:

    +9V -------+-----+--o o---+--- load
    Vb | | | (<200k to gnd)
    33k | Q2 |
    | e pnp |
    +-- b 2n3906 | Vb
    | c 4403 | T = RC ln --- = k RC
    3k3 | etc | Vth
    C + | | |
    ,----||----------+--------' The k term vs. Vb and Vth,
    | 100uF |
    | .002CV d nmos Vb = 9V 6V
    +----+-- g 2n7000 Vth ----- -----
    R | _|_ s Q1 = 1.5V 1.792 1.386
    680k /_\ D | = 2.0V 1.504 1.099
    | | |
    ---+----+-----+----------------- gnd

    Does anyone have any suggested improvements (because of new parts available,
    more experience, change in the speed of light, etc)?
  2. I missed it, before. But it actually looks decent. Very cheap,
    small and common parts and non-critical layout. New parts would
    just cost you more and be harder to find.

    It doesn't retrigger, if you were looking for that. And you
    can't just build this into a switch replacement, it's not an
    in-line circuit.

    I suppose that you might consider lowering the capacitor value
    to reduce size or cost and increasing that 680k, but if you go
    too far you may have to start worrying more about leakage.

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