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dc power on-off with single pole momentary switch

Discussion in 'Electronic Design' started by John Bachman, Feb 13, 2008.

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  1. John Bachman

    John Bachman Guest

    The subject tells the goal. Here is the rest of the story. There is
    no micro available. It is preferred to use a dual comparator such as
    393 as one of those is already part of the design and having two of
    one device is preferable to one of these and one of those.

    I have tried and failed. Others care to give it a crack?

  2. Jim Thompson

    Jim Thompson Guest

    Input Voltage?

    How much "stand-by" power can be drawn by the switching circuitry?

    Load Current?

    ...Jim Thompson
  3. Use a 74HC74 flipflop, it has reset for correct power up too.
    Screw the comparator.
  4. John Bachman

    John Bachman Guest

    9 volt battery supply. Load is 12 ma. I figure a pass transistor
    operated by the switch and/or circuitry will be fine.

    Ideally the switching circuitry is on the load side of the pass
    transistor, hence no quiescent draw. That may not be possible. If it
    must draw quiescent current then I will have to assess the feasibility
    of it. Two switches is not the end of the world, just not as elegant.

  5. John Bachman

    John Bachman Guest

    Yeah, I know. Sigh.

  6. Is there any reason why you can't use a straightforward single pole
    toggle switch? It is cheap, ergonomic to operate, draws no quiescent
    current when open, gives negligible volt-drop when closed - and is

    If they had only just been invented, they would be regarded as one of
    the biggest steps forward in technology of the past 30 years.
  7. linnix

    linnix Guest

    The subject tells the goal. Here is the rest of the story. There is
    Why not? Bad design!
    Yes, it can't turn itself off.
    Except for wasting batteries. Auto off is the second biggest step
    forward ...
  8. Go down to your local hardware store. They have lamp switches that push
    on, push off. All done mechanically.
  9. Unless the item is a camera which switches itself off just in time to
    miss as the shot you have been waiting for !
  10. linnix

    linnix Guest

    But many customers just push on and leave on battery devices. On
    second thought, I am going to randomly disable the auto-off feature.
    I am starting a batteries on-line business.
  11. linnix

    linnix Guest

    But the camera button (trigger) should be auto-on as well.
  12. Tam

    Tam Guest

    And if you need to switch more current than the lamp switch can handle, use
    it to operate a power relay. BTW, if you go the 7474 route, be sure to
    debounce the switch.

  13. Arie

    Arie Guest

    Something like this?

    | ====
    100k GND- 10n -------o o------ 1uF -- GND +
    | | | |
    | | |\ 1M 10k
    |-- 100k ----------|-\ | |
    | | }--------------------------------->
    |-- 100k ----------|+/ |
    | | |/ |
    | | |
    | ----- 100k ---

    The comparator is self-latching, after power-on output is high (for minimum
    current drawn).
    Alternative: connect the 10n to +, then it starts with output low.

    Pressing the switch less than 1 sec switches the state.
    Repeating the switch presses fast does not switch state (slow debounce).

    Arie de Muynck
  14. John Bachman

    John Bachman Guest

    Cost is the only objection. A tactile momentary costs .16. A toggle
    is a buck or more. When the total product part cost is $12 then $.84+
    is lot.

    Two momentaries cost $.32 and will do the job, but Oh, that is ugly.

  15. I'd be willing to pay the extra for a decent switch, but I suppose it
    depends on which sector of the market the product is aimed at.
  16. I must say I am impressed, a nice solution.
    As to the issue what is 'nest', this uses 10 components, 2 of which are capacitors.

    Placing, board size, vias....

    I did say 74HC74, but even that requires some debounce, as others have pointed out,
    so at least 2 more components.

    The OP says he has no micro.
    But how a about a simple 8 pin PIC?
    Use the internal osc, internal pullup at an input pin, trigger interrupt,
    flip output, hang some milliseconds in interrupt routine as debounce.....

    It is all there, with *1* component, say a PIC 12F629, only 67 cent in volume.

    And, it also has a build in comparator that perhaps can replace some part
    of the rest of the circuit.....

    I would definitely go that way, have been using that PIC to replace simple circuits
    no several times.
    That way you need to have only 1 part in store.
  17. Tam

    Tam Guest

    Actually, that is not that bad, because you use the other half of the HC74
    package to debounce. However, to do it right you need a SPDT switch: no
    capacitors needed.

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