555 Monostable low power circuit?

Discussion in 'The Projects Forum' started by Oddy, Sep 28, 2014.

  1. Oddy

    Thread Starter New Member

    Sep 28, 2014
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    Hello all
    I'm trying to put together a low power 555 monostable circuit and have come up with the following design, having modified the circuit from this source. http://www.instructables.com/id/555-Timer/?ALLSTEPS

    [​IMG]

    My thinking was that once the main switch is pressed and power is applied to the circuit, the trigger pin will detect a low signal causing the output to go high; this should supply a signal to the transistor allowing power to remain supplied to the circuit even once the main switch is off but only until the timer has run its course at which point it will switch off leaving the whole circuit in a state where it isn't drawing any power.

    However trying it out on prototyping board isn't giving much success, can anyone suggest what i've done wrong or perhaps a good alternative circuit to achieve the same task?

    Cheers
    Oddy

    p.s. the threshold capacitor is a 220uf and there is a small decoupling type capacitor on pin 5.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here is one that uses almost no current except the LED.
    The switch turns on the FET and it stays on for the time determined by the RC.
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
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    The CMOS 555 uses 1/2 ma max idle current while the bipolar 555 uses 2ma to 10ma depending on the supply voltage.
     
  4. b1u3sf4n09

    Member

    May 23, 2014
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    Is the 555 timer turning off the moment you release the button? Or is it not turning off at all?

    Isn't that the point of this forum, to discuss designs and correct assumptions? Give a man a fish, he eats for a day.
     
    Last edited: Sep 28, 2014
  5. ScottWang

    Moderator

    Aug 23, 2012
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    It depends on how the TS will and wish, does he just want a fish or wanting to learn how to fishing a fish, so you can see some threads only some replies, some threads got a lots of replies and still continuing, but on the homework help forum only providing the hint and guiding.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Below is the circuit modified to work as you want (I think).
    There were at least two significant problems with your circuit.
    One is that the trigger needs to go momentarily low (not stay low which causes a re-trigger) to actuate the one-shot, thus I added R1 and C2 for that purpose.
    The other is that you can't use an emitter follower to drive its input from its own output, thus I added an extra complementary transistor to make a high-side switch which doesn't have that problem.
    Edit: As an alternate you could use the six CMOS non-inverting buffers in parallel (inputs and outputs) from a CD4050 in place of the two transistors and resistors to control the circuit power.

    The sim shows a 2.4s pulse after the switch is momentarily closed with the timing values (R2C1) you used.
    The supply current goes essentially to zero when off (just transistor leakage current).

    One shot low power.gif
    555 one-shot No Quiescent Power.asc
     
    Last edited: Sep 29, 2014
    absf likes this.
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Could you attach your simulation file?
    Thank you.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Sure. Added to my previous post.
     
    Last edited: Sep 29, 2014
    ScottWang likes this.
  9. Oddy

    Thread Starter New Member

    Sep 28, 2014
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    Many thanks Crutschow that's a great help will update once ive got it done on proto board.
     
  10. Oddy

    Thread Starter New Member

    Sep 28, 2014
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    0
    Thanks got the circuit up and running fine; just had a couple of questions...

    Regarding the trigger having to go temporarily low then high at the beginning; this was what I was kind of aiming at with my first circuit, the idea was to feed the output into the trigger, but to also have a pull down resistor connected to it too; this way the resistor would always ensure trigger was at zero when starting the circuit and therefore once powered the threshold pin would be triggered but upon triggering the output would go high providing power to the trigger pin and brining it back up again.
    This was what i was hoping would work but how come it doesn't work out in real world circuitry?

    And also i didn't realise you couldn't keep a transistor turned on from it's own output, is there an explanation somewhere as to why this happens?

    Many thanks
    Oddy
     
  11. crutschow

    Expert

    Mar 14, 2008
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    The problem is that when the output goes low at the end of the pulse period, it will try to retrigger itself. If there is enough capacitance in the circuit it may then turn itself back on.

    An emitter follower has an output voltage that's about 0.7V lower than the base voltage. So obviously it's impossible to power the base from the source voltage.
     
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