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Turn off delay circuit

Discussion in 'Electronic Basics' started by vic, Mar 26, 2009.

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  1. vic

    vic Guest

    Hi,

    I'd like to design a circuit such that it is powered on by a short press
    of a switch, performs some action, and then turns itself off when it has
    done its purpose.

    I suppose I could do this using a FET, but it has surely been done
    millions of times and I'd like not to reinvent the wheel. Could you give
    me a suitable example of such a circuit, or resources to inform me on
    the subject ?

    Thanks.
     
  2. Phil Allison

    Phil Allison Guest

    "vic"

    ** Google " NE555" .

    Gets about half a million hits.


    .... Phil
     
  3. Guest

    Depending on the volatge and current involved, you could be talking
    about a CMOS 555 timer or as thermal delay relay.

    What voltage?

    How much current?

    Batteries or mains power?

    John
     
  4. Allen Bong

    Allen Bong Guest

    Take a look at the recent discussion on SEB on 24th Feb. The link is
    at

    http://groups.google.com/group/sci.electronics.basics/browse_thread/thread/e2f9392cdea86c89#

    3 versions of solutions using transistors, NE555 and PIC are
    available.

    Allen
     
  5. Allen Bong

    Allen Bong Guest

    Further down on post #88, the problem can be sloved by op-amp as well
    as suggested by John Field.

    The PIC software posted by nobody for the delay timer was for
    something else and has to be reworked.

    Allen
     
  6. Jamie

    Jamie Guest

    Look for the 555 timer, It has 2 inputs, trigger and threshold that are
    part of a RS (Reset/Set) flip flop circuit.
    Pulling the trigger low will latch the output on, pulling the
    threshold high will unlatch the output. etc.

    It's smaller and more versatile than employing a flip flip chip when
    you only need one switch. On top of that, you can also place some timing
    activity in that same lay out.


    http://webpages.charter.net/jamie_5"
     
  7. vic

    vic Guest

    Thanks for your precious advice. I was not clear enough. Let me explain
    again : at first, the circuit is completely powered off. The user pushes
    a button, which injects power into the circuit. Then a mechanism
    maintains the power active, even if the switch has been released. At a
    later undetermined moment, when the circuit does no longer need power,
    it cuts its own power and sleeps until the user chooses to push the
    button again. I'm pretty sure a 555 can't do this.

    vic.
     
  8. Phil Allison

    Phil Allison Guest

    "vic"

    ** YOU are bout as clear as mud - fuckhead.

    And that situation is certainly congenital.


    ** **** knows WHAT THE HELL you are blathering about.

    Whatever it is - you are way off topic here.

    This NG is only about BASIC electronic concepts, hardware and practices.

    Try " alt electronic.gadgets-wankers " instead.




    ...... Phil
     
  9. vic

    vic Guest

    Sorry I really did not phrase my question well it seems. By "power on"
    and "power off", I meant really the power to the circuit, maintaining
    power while needed and cutting its own power after the desired action
    has been performed. It is intended to save as much power as possible
    when the circuit doesn't do anything.

    A 555 circuit would not do since it would need to be powered on permanently.

    That's why I said a FET would do. Probably any other transistor, but the
    FET is closer to my energy saving goal. Here's what I imagined :

    .------------------------.
    VCC o-+------+ +----+ |
    | --- | "Slave Circuit" |
    | ^ | |
    | | | |
    | / | | Something here keeps |
    +--/ o--+----+ the FET conducting |
    | | as long as needed. |
    .-. | |
    | | | |
    | | | |
    '-' | |
    | | |
    | | |
    GND o----------+----+ |
    '------------------------'

    When the user presses the switch, the circuit is powered and in turn
    pulls the FET gate up. The user can release the switch at this time. At
    some other time in the future, the circuit decides it does no longer
    need power and lets the gate go down, cutting power and hopefully
    consuming zero current. I need to figure out if this would work and what
    kind of transistor to use.

    vic.
     
  10. Nobody

    Nobody Guest

    Version 4
    SHEET 1 1100 680
    WIRE 192 -544 64 -544
    WIRE 384 -544 272 -544
    WIRE 208 -448 208 -496
    WIRE 256 -432 256 -496
    WIRE 256 -384 256 -432
    WIRE 256 -288 256 -304
    WIRE 64 -176 64 -544
    WIRE 176 -176 64 -176
    WIRE 384 -176 384 -544
    WIRE 384 -176 256 -176
    WIRE 560 -176 384 -176
    WIRE 768 -176 560 -176
    WIRE 800 -176 768 -176
    WIRE 384 -112 384 -176
    WIRE 560 -32 560 -176
    WIRE 384 -16 384 -32
    WIRE 800 48 800 -176
    WIRE 560 144 560 48
    WIRE 64 272 64 -176
    WIRE 384 272 384 64
    WIRE 560 320 560 208
    WIRE 560 320 448 320
    WIRE 64 496 64 352
    WIRE 384 496 384 368
    WIRE 384 496 64 496
    WIRE 800 496 800 128
    WIRE 800 496 384 496
    WIRE 64 528 64 496
    FLAG 64 528 0
    FLAG 256 -288 0
    FLAG 208 -448 0
    FLAG 256 -432 push
    FLAG 768 -176 out
    SYMBOL voltage 64 256 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 12V
    SYMBOL res 368 -32 R0
    SYMATTR InstName Rcoil
    SYMATTR Value 100R
    SYMBOL voltage 256 -400 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V2
    SYMATTR Value PULSE(-1 1 1s 0 0 0.2s)
    SYMBOL csw 176 -176 R270
    WINDOW 0 32 40 VTop 0
    WINDOW 3 -32 40 VBottom 0
    SYMATTR InstName W1
    SYMATTR SpiceModel Vsense
    SYMBOL voltage 384 -128 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName Vsense
    SYMATTR Value 0
    SYMBOL res 784 32 R0
    SYMATTR InstName Rload
    SYMATTR Value 1K
    SYMBOL sw 176 -544 R270
    SYMATTR InstName S1
    SYMBOL npn 448 272 M0
    SYMATTR InstName Q1
    SYMBOL res 544 -48 R0
    SYMATTR InstName R2
    SYMATTR Value 10K
    SYMBOL cap 544 144 R0
    SYMATTR InstName C1
    SYMATTR Value 470µF
    TEXT 176 536 Left 0 !.model CSW CSW(Ron=1R,It=50mA, Ih=10mA)
    TEXT 176 584 Left 0 !.model SW SW()
    TEXT 174 632 Left 0 !.tran 10s
     
  11. bw

    bw Guest

    My microwave oven does that. It uses no power when off. The timer is
    mechanical.
     
  12. ehsjr

    ehsjr Guest

    Here's a generic circuit to show the idea:

    N/O N/O
    +12 ---+---o--- Rly1-1 Rly1-2 ---o---+
    | ^---o------+ +---^ |
    o | | +--- To your
    o | +-------------- circuit
    | |
    +----->|----------+
    | ___|___
    +---[R]---+---| 555 |
    | | | |---+----+
    | | | | | |
    | | ------- | |k
    | /c | [Rly1] [D1]
    +--[R]---| | | |
    \e | | |
    | | | |
    Gnd -------------+-------+-------+----+

    Use a 555 as diagrammed (not a schematic, just to present
    the idea) above. The pushbutton causes the npn to put
    a negative on the 555 trigger, and provides plus to the
    555 while it is pressed. The 555 output goes high, and
    operates the relay, which takes over the role of providing
    + to the 555. The 555 stays high for however long you
    set it, then drops, removing power. No reason you have to
    use a relay - you can use the idea with a fet or bipolar if
    you want.

    More important: the asumption is that you want a timed delay.
    However, that may not be the case. You might, for example,
    have a motor that opens and closes a window, and you want the
    circuit to run until the window is fully open (or fully closed),
    then shut off. Or a circuit that is indended to bring temperature
    up to some point, then shut off. The problem is that the subject
    of your post says "delay", but the requirement specifies that
    some action has been completed. So to make it a bit more
    generic:
    S1 or Sensor
    +12 ---+---+ N/C
    | |
    | V
    | -------o---- Rly1-1 N/O
    | o---^
    | |
    +----o o---+
    |
    [Rly1]
    |
    Gnd --------------+

    S1 is a microswitch or a sensor that interrupts the circuit
    when the action is complete.

    Ed
     
  13. Phil Allison

    Phil Allison Guest

    "vic the prick "

    ** So it's a toaster - right ??




    ..... Phil
     
  14. Use a PMOS fet as a pass element. Connect the source to the input
    voltage, and the gate to the input voltage through a 1MEG transistor.
    Use the drain as the supply for your circuit.

    Have two connections to the gate of the FET, one to a normally open
    switch connected to ground, and the other to a port on your
    microcontroller.

    Push the button to power up. When the uc initializes, have it set the
    port to 0 output.

    When you want the circuit to turn off, have the uc set the port to
    high impedance.

    Regards,
    Bob Monsen
     
  15. MooseFET

    MooseFET Guest

    PNP or PMOSFET
    Vin--+---- ----------
    ! e\ /
    ! ----
    ! !
    -/\/\-+
    !
    \
    /
    \
    ! N-MOSFET
    ----+-!!
    ! !!--------------Port on micro
    PB ! O -!!
    --! !
    ! O GND
    !
    GND
     
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