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simplest push-button power switch

Discussion in 'Electronic Design' started by Pawel Paron, Oct 9, 2003.

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  1. Pawel Paron

    Pawel Paron Guest

    I want to make the simplest power on/off switch, operated with single push
    button, using one MOSFET and some kind of a flip-flop. My question concerns
    this flip-flop, it has to be something really simple and small, most
    desirably a single SMD chip with a minimal number of pins, is there anything
    like that available? I know how to do it with discrete components, or with
    standard CMOS gates, but these are all too big. I'm looking for something,
    that I can solder directly to the BUZ11 pins, and it will not protrude
    significantly, so the volume of the complete switch is not much bigger than
    the transistor itself.

    Or, can I build something like a D flip-flop, using ne555? I have some of
    these chips in SMD version. Just wonder if I can get it to work as a
    bistable flip-flop, without many external components.

    Regards
    Pawel
     
  2. Surely the components needed for this must take more space than a miniature
    switch ??
     
  3. Pawel Paron

    Pawel Paron Guest

    Well, it's not a must, but If I can do it really simply, like soldering some
    tiny chip, then why not. There must be that MOSFET for another reason
    (another circuit, that protects the power supplying battery from complete
    discharge), so using a small switch instead of the whole thing is not an
    option, it would replace just that flip-flop. In my opinion push buttons are
    more reliable, than bi-stable switches, and they are usually smaller (or at
    least flatter)

    Pawel
     
  4. Fred Bloggs

    Fred Bloggs Guest

    If you look hard enough, you can find a PUSH-ON/PUSH-OFF switch. These
    have the same form factor as momentary switches.
     
  5. Active8

    Active8 Guest

    google search this group for mini-gate? i haven't checked 'em but they
    sound like something that will do the job.

    give it some time and JT might chime in to confirm this.2

    brs,
    mike
     
  6. Pawel Paron

    Pawel Paron Guest

    Thanks, this is the solution, there is a single D flip-flop NL17SZ374.
    Although I think these are the "hard-to-find" components, at least around my
    place.

    Pawel
     
  7. Active8

    Active8 Guest

    great! i remembered Win Hill or someone mentioning finding D-FF in
    single gate.

    but i don't think they're that hard to find, it's just that they're new.
    google this group on Mini-Gate and Jim Thompson and Active8. he replied
    to me that if you go to whatever site he mentioned, and click refresh a
    few times or so, the ad will come up for sample kits. 500 pieces IIRC.

    i gotta get a pile of 'em.

    brs,
    mike
     
  8. Tom Bruhns

    Tom Bruhns Guest

    Not too long ago I played with a simple on/off toggle using one power
    mosfet which switched a 12V load, and the pushbutton switch and a
    resistor and a capacitor. It's only useful in an application where you
    will switch at least once a week or so, and where it doesn't get too
    hot. For reliable operation, it should switch a load powered by at
    least a couple times the gate turnon voltage, and if the supply is
    more than the max gate voltage, you have to add another resistor. The
    memory is simply charge on the gate of the FET, and that limits how
    long it can go between toggles, as each toggle resets the gate
    voltage. It requires a switch with low leakage in the "off" state.
    Might this work for you?

    Cheers,
    Tom
     
  9. Pawel Paron

    Pawel Paron Guest

    But how do you turn it on and off with a single button? Or you use two
    buttons, for charging and discharging the gate?
    As I understand it, I would expect a problem when the gate slowly
    discharges, and at some point the transistor becomes a "resistor", heating
    and finally burning?

    Pawel
     
  10. In Active8 typed:
    I found the message and hit refresh many times, but I think the ad
    expired. If they still have any kits left, they might have pulled the
    ad for it.
     
  11. In Active8 typed:
    It appears they have none left, since it is was "backordered" but it's
    not even that now. Google for "minigate sample kit" on the onsemi site
    and they have a cached page that said it was backordered, but the
    current page doesn't list it at all.

    This guy claims to be selling them, but who knows if he even has them.
    http://www.resellerratings.com/forum/t78845.html
     
  12. Active8

    Active8 Guest

    It appears they have none left, since it is was "backordered" but it's
    not even that now. Google for "minigate sample kit" on the onsemi site
    and they have a cached page that said it was backordered, but the
    current page doesn't list it at all.[/QUOTE]

    i take it that means you found JT's article where he gives the link. if
    so, save me some time looking and repost it, please.

    aside from that, I usually pick up the phone for more info.
    i don't think i'd order from him. money order.., he's never heard of
    PayPal.

    brs,
    mike
     
  13. Active8

    Active8 Guest

  14. Tom Bruhns

    Tom Bruhns Guest

    Hi Pawel,

    Yes, there are potential problems with the simplest circuit. If the
    load is heavy, then it has the potential to destroy the FET. But it
    IS simple! Consider a 12V 1A light bulb as the load. Connect the
    load to +12V, the other end to the FET drain. FET source to the minus
    side of the 12V supply (call it ground). Connect a 10 megohm resistor
    from the drain to an 0.1uF capacitor. Other end of the capacitor to
    ground. Switch from the capacitor/resistor to the FET gate. So when
    the FET is "off" the capacitor charges through the load and resistor
    to 12V. Push the switch for a short time: the time must be short
    compared with R*C, or in this case, let's say not more than 1/10
    second. The FET gate is charged to 12V, a little less because the
    resistor starts to discharge the cap when the FET drain voltage drops.
    The FET is now on, and the capacitor discharges to the FET drain
    voltage, which is nearly zero. The next time you press the switch
    button the FET gate goes to that new voltage. So the limitations are
    that you must not press the button too long (t<<RC), that you not
    press it too often (time between presses >> RC), and that you press it
    more often than the leakage current drains off the gate charge. My
    tests with an IRF540 (I think it was) amazed me: it took TWO WEEKS
    for the gate charge to decay enough to significantly affect the
    conduction. BUT this will be affected strongly by the FET and by the
    cleanliness and insulation around it.

    You can modify the circuit fairly easily by adding another PNP
    transistor which will keep the FET gate solidly ON indefinetly, and a
    resistor from gate to source to keep it OFF when it's in that state.
    All this is easiest to implement when the power source is on the order
    of 12V, but higher voltages can be accomodated using resistive
    dividers, adding a couple resistors to the circuit. Because the
    current required to hold the gate in either state is extremely small,
    the resistors can be high values and not dissipate much power. Let me
    know if the addition of the PNP isn't obvious...basically the base is
    driven through a resistance from the FET drain and the emitter ties
    through a high resistance to about +12V. The collector goes to the
    FET gate. You can use the same scheme for toggling that I mentioned
    above. Be sure the cap can't charge to a voltage that can destroy the
    FET gate!

    Cheers,
    Tom
     
  15. Active8

    Active8 Guest

    Actually it was John Larkin. I couldn't find a similar one from Jim.

    The link and the whole message:

    http://www.onsemi.com/site/home/0,4370,0,00.html

    http://groups.google.com/groups?hl=...ki61n3m2ffq49cpkojpkcv3pq1av%404ax.com&rnum=2
    [/QUOTE]

    ok, john, then.

    i'm not having any luck. i should have jumped on it asap.

    that little gator on the PCB ad is nice, though.

    brs,
    mike
     
  16. Pawel Paron

    Pawel Paron Guest

    Thanks for this brilliant explanation. I got the whole idea. But I see the
    problem, that I mentioned earlier: relying on the gate charge may lead to
    destroying the FET, when the transistor enters some state between on and
    off. As I observed, this can happen even with a static electricity, when you
    just approach your hand or something near the gate, sometimes the transistor
    may switch from off state to high resistance and possibly burn. So leaving
    the gate open and "floating" is not an option, I want to use it for my
    bicycle lights (12V/20W hallogen bulb), imagine I'm stuck in the forest,
    because my lights got damaged, and all the wolves are around :) Actually
    I've provided a simple solution against such an accident, all the
    electronics between the bulb and the battery is connected via plugs, I can
    unplug it and connect the battery directly to the lamp. Normally there is a
    circuit, that warns me with sound when the voltage drops to 10.5V, and
    another one, that cuts the power off at 8V, this one has a MOSFET and I
    wanted to use it as an on/off switch too. And this has to be robust, in all
    conditions (wet weather, static electricity - I carry battery with this
    electronic stuff on my belt, and lamp on the helmet). But your solution is
    really interesting and tricky, perharps I'll make some other use for it
    (this version with an additional PNP transistor)

    Regards
    Pawel
     
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