Monostable that recharges quicker than the time it is on

Discussion in 'General Electronics Chat' started by m1tch37, Nov 23, 2011.

  1. m1tch37

    Thread Starter New Member

    Apr 14, 2010
    2
    0
    Hi,

    I want to build a monostable which stays in its unstable state (for examples sake) for 100ms, but can be triggered as frequently as every 120ms (that is to say, if it was consistently triggered every 120ms, the output would be periodic and high for 100ms and low for 20ms.) The reason I want to use a monostable is because I require the high time to be known and fixed, however the frequency of the triggering (and hence the off time) is unpredictable.

    The issue I am having is all the Monostable circuits I have found must recharge a capacitor before the output can be retriggered, and this recharging actually takes longer than the time the output remains unstable. I have not found any Monostables that could achieve an output duty cycle of 50% or ideally even greater.

    Are monostables with a fast retrigger possible? Does anyone know of any example circuits?

    Cheers,
    Mitch
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    I'm not quite sure I understand what you are talking about. Have you tried a 74LS123?
     
  3. m1tch37

    Thread Starter New Member

    Apr 14, 2010
    2
    0
    Thanks for your reply. Reading the following appnote, it does appear this IC will do what I require.

    http://www.ti.com/lit/an/sdla006a/sdla006a.pdf

    My issue right now is I wanted a circuit I could understand, and reproduce. The 74LS123 might be a neat little IC, but as far as I can see is a bit of a black box. That appnote does have a schematic at the end, however it is quite horrid. Furthermore, I will be implementing it using CMOS.

    Edit: Finding other 123 varieties, I'm going to spend sometime looking at this one: http://ics.nxp.com/products/hc/datasheet/74hc123.74hct123.pdf
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    If you want to use a CMOS monostable you can try a CD14538.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here, have a look at the attached.

    It's using a Texas Instruments' TLC555 CMOS timer.

    LTSpice doesn't have provisions for an interactive pushbutton switch, and I couldn't press one that quickly anyway - so that's what the voltage source "PushBtn" and switch "SW" are doing; pushing that button down for 1mS every 11mS. In the 2nd attachment, SW is closed for 50mS every 120mS.

    R2, R3, and C4 cause the input to TRIG to be edge-triggered; your input signal must have a fast fall time or the timer won't be triggered properly.

    D1 keeps TRIG from exceeding the positive supply more than ~0.7v when the pushbutton input goes high. A Schottky diode like a BAT54 (SMT) or 1N5817 would reduce the overshoot significantly.

    R1/C1 provide the ON time; how long the output will be high; the time is ~ R1 Ohms x C1 in Farads x 1.1. 910k x 100nF (0.1uF) x 1.1 ~=100.1mS. Use a poly metal film or ceramic cap, the lowest tolerance you can find.

    C2 keeps the control input stable. If you want the timing to be reasonably consistent, you will use a cap there. 10nF is the recommended value, but anything from 1nF to 10nF will keep the timing more stable than nothing.

    C3 is a mandatory bypass capacitor. You should also have a 1uF or larger aluminum electrolytic nearby; 100uF to 1000uF is suggested.

    This circuit is basically the same as described about 2/3 of the way down on this page under "555/556 Monostable":
    http://www.kpsec.freeuk.com/555timer.htm#monostable
     
    Last edited: Nov 23, 2011
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