Monostable Multivibrator

Discussion in 'The Projects Forum' started by ChemEnger, Oct 6, 2009.

  1. ChemEnger

    Thread Starter New Member

    Oct 6, 2009
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    I have an application where I need to send just a pulse output from a varying (sometimes semi-permanent) HIGH signal (from a reed switch in a water meter).

    I therefore think that the best way to achieve this is with a rising edge trigger circuit using I think a monostable multivibrator.

    From various musings around the internet, including this site, it has all the features that I need and I can set at least an approximate pulse length with a suitable resistor and capacitor.

    My main question is what mv to buy, especially given that I would first of all make this up on a strip board. Secondly, is the wiring as easy as it looks?

    I have a 12V battery supply that is part of a keyfob RF transmitter and one of the reasons for wanting a short pulse ouput is to preserve battery life.

    Please excuse any ignorance above - I am only a lowly Chemical Engineer...

    Hope someone can help...
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The attached circuit triggers the 555 monostable on a zero-to-positive voltage on the input to the transistor. The output pulse width is set by R4-R5-C2 component values.

    ken
     
  3. ChemEnger

    Thread Starter New Member

    Oct 6, 2009
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    Hi Ken,

    Thanks for a quick reply! I think (as a chemical engineer) that I can understand most of the circuit. May I ask just a couple of questions?

    Are the capacitor values given in uF?
    I'm not sure exactly what C2 is - I think it's an electrolytic and therefore polarised capacitor and the curve on the symbol makes no difference?
    I presume that R5 is a variable (pot?) resistor?
    The signal comes in through R1?

    Also, I don't know whether it's relevant, but ideally the circuit would be non-retriggerable so that if I got a continuous output from the water meter, the 555 would not continue to pulse out.

    Thirdly (cheeky I know), I presume that I can feed the whole lot (through R1 and +12V connections) 12V only when the switch is closed, again to avoid a power drain?

    Simon
     
    Last edited: Oct 7, 2009
  4. Wendy

    Moderator

    Mar 24, 2008
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    So far you are mostly describing a 555. Check the link out.

    555 Monostable
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,577
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    Yes.

    Yes. The + sign is an indication if the polarity. The curve is the negative on polarized capacitors. On the old foil wrapped capacitors it was the outside foul.

    Yes. The R5 resistor limits the range of the "pot". If you were going for a nonadjustable pulse width, you could eliminate R4 and select the exact value you need for R5.

    Yes.

    The coupling capacitor C1 provides a brief negative-going pulse to the trigger input of the 555 when you switches closed, starting the monostable period. It also couples a brief positive-going pulse when your switch opens, but this is ignored by the 555.

    Yes. the amount of current during that period is limited by R1. I(mA)=12v/10,000Ω But in that circuit, the 555 is powered buy 12v all the time.

    If power usage needs to be limited, you could use a power-up reset monostable circuit, attached. It would only be powered when your switch is closed. If your switch stayed closed for a while, it would be powered during that time, but it would draw no power when the switch is open. There may be a more elegant way to draw no current except for the monostable period...but, I'll have to ruminate on that. ;)

    Ken
     
  6. ChemEnger

    Thread Starter New Member

    Oct 6, 2009
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    0
    Ken, Bill,

    Thanks for all your help on this one. I think I have identified all the parts that I need and will go about building this over the next few days. Having mooched around a bit more, I have found, in addition to the LM555C, a pre-built Monostable Multivibrator IC (Texas Instruments CD4047BMG4), which looks like I only need to add the RC setup to for timing. I think I will try both, as the suggestions here will almost certainly work and I'm only guessing at the IC!

    Simon
     
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