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Triggering 555 Monostable

Discussion in 'Electronic Basics' started by Animesh Maurya, Jan 21, 2004.

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  1. Hi everybody

    I know that when a positive going pulse falls to zero, it triggers the

    It is very obvious that the time period for which the triggering pulse
    goes zero should always be less than
    time period of positive going pulse at the output (pin 3),

    otherwise multivibrator will not return in its stable state after
    (1.1* R*C) sec.

    Which is causing problem to me. Please don't ask me why is this
    happening so.

    Till now I was performing simulation, but when I put my hands in
    designing it's a problem to me.

    I need your valuable suggestions.

    Thanks for the time

    Animesh Maurya
  2. Tim Dicus

    Tim Dicus Guest

    Hi Animesh,

    You may want to consider AC coupling the trigger line to the 555. Use a cap
    in series with pin 2 (trigger) and a pull-up resistor on the 555 side of the
    cap from pin 2 (trigger) to pin 8 (power supply).

    You might try a .1uf cap and a 4.7K resistor to start. I don't know what
    your output pulse width is, or how fast the trigger source high-to-low
    transition rate is, so you may need to play with the values of the cap and

    Hope that helps,

  3. Tim Dicus

    Tim Dicus Guest

    In addition to my earlier post about AC coupling the trigger input, I
    usually include a diode in parallel with the resistor, anode to pin 2
    (trigger) and cathode to pin 8 (power supply).

    This will prevent the trigger input from overvoltage during the trigger
    source low-to-high transition.

  4. The trigger input overrides the threshold input, so if the trigger is
    held low while the threshold rises, the time-out function of the
    threshold input is defeated, till the trigger input voltage goes above
    1/3 of the Vcc voltage. If your trigger signal lasts longer than the
    timing process of the 555, you will have to shorten its duration by
    some means. Common means are to use a differentiator (series cap
    between trigger signal and trigger input, with pull up resistor to set
    the pulse duration, and diode to Vcc to prevent excessive positive
    excursions at end of trigger signal pulse) or to add a non
    retriggerable single shot chip before the trigger.

    You might make one of those with CMOS gates. For example, you could
    apply your long trigger signal to one input of an OR gate and also to
    an inverter (or three in series). Then connect the output of the
    inverter to the other input of the first OR. The output of the Or
    gate will go low when the trigger signal goes low, but remain there
    only till the transition propagates through the inverter. Then the Or
    gate output will go high. On the low to high transition of the input,
    the Or gate output remains high.
  5. John, Tim:

    According to Jim Thompson on 5th Jan, that limiting diode (which I'd
    been using for as long as I can remember) is redundant. I've therefore
    ommitted it subsequently from a couple of projects. Do you agree this
    is safe?

    From: Jim Thompson <>
    Newsgroups: alt.binaries.schematics.electronic
    Subject: Re: Need to mod door bell
    Message-ID: <>
  6. Tim Dicus

    Tim Dicus Guest

    Hi Terry,

    The trigger input pin 2 on the LM555 is connected directly to the base of a
    PNP transistor with no other connections or devices. See the schematic
    diagram here:

    There was a discussion last year (July 2003) on the
    group about reverse voltage on the base-emitter junction of bipolar
    transistors, in particular a 2N3906(PNP).

    I agree with the following people. I use the diode, and will continue to do

    These are excerpts from that discussion:
    Jim Thompson said:

    It will fail. Devices subjected to this sort of repetitive reverse bias of
    the B-E junction gradually lose Beta.

    It can take awhile, although high temperatures will accelerate the effect.
    For example, under the hood of a car, failure occurs within 1-2 days.
    Jim Thompson later responded to this question:
    I don't know if there's a lower limit where damage doesn't occur. My
    experience was in an automotive application at ~1mA.

    I just make it a rule *not* to breakdown B-E junctions.

    ...Jim Thompson
    Watson A Name said:

    I told a guy that he was full of bs when he told us this in the ngs. He said
    that putting reverse current thru the E-B junction of a transistor will
    degrade the transistor's gain. I didn't believe him, so I tried it with a
    few mA, and found that the transistor went from a gain of about 100 before
    to a gain of less than 30 after just a half hour of reverse current of a few
    mA. I was dumbfounded. It was
    really true. Take a transistor and try it if you don't believe me.

    If you're going to subject the transistor's E-B to reverse voltage greater
    than its max rating, put a diode in series with the base or use a voltage
    divider. But by all means, protect it, or else you _will_ find that the
    transistor becomes degraded.
  7. Thanks Tim, but did you read the more recent thread I referenced? In
    that, I concluded Jim was saying the diode was redundant. Perhaps he
    could comment? (I'd email him to be sure he sees this, but don't
    appear to have a valid email address.)
  8. Tim Dicus

    Tim Dicus Guest

    Hi Terry,

    I take the word "redundant" to mean there is already this kind of device or
    protection provided internally in the device, and that adding the diode will
    duplicate its function. In the case of the LM555 from National
    Semiconductor, this is not the case.

    You can see in the schematic diagram there is no other device or connection
    from pin 2 to the base of the PNP transistor.

    I could not find the thread from January. If you can post the relative part
    of that message, I would appreciate it. If I am duplicating the protection,
    I would certainly like to know!


  9. OK, here's the thread in its virtual entirety.
    From: John Fields <>
    Newsgroups: alt.binaries.schematics.electronic
    Subject: Need to mod door bell
    Date: Mon, 05 Jan 2004 09:25:08 -0600

    with attachment DINGDONG.PDF

    That showed a neat circuit, but I assumed John had accidentally
    ommitted the usual diode. Like mine, his circuits had previously
    included it. Hence my follow-up post.
    Minor point: diode across R2 to limit +ve excursions?

    Terry Pinnell

    The Trigger Input is to the base of a lateral PNP and can go above
    rail without any harm whatsoever.

    ....Jim Thompson
    On both standard and CMOS 555 types? In which case I'll cease using
    one on edge-triggered 555 monos. Most published circuits I've seen
    appear to do so. Typical text like this one: "A small signal diode is
    connected across the differentiator to ensure the 555 input signal
    never exceeds 9V."

    Terry Pinnell
    The CMOS version specifies Vpin(max) as Vs+0.3V, so a silicon diode
    isn't going to do squat.

    I'd add a series R to the pin to limit current to, say, 1mA, using the
    internal ESD diode as the actual catcher.

    ....Jim Thompson
    Maxim's 7555 data sheet states that it's the CMOS parasitic SCR going
    into latchup which limits the pin's positive excursion to 0.3V above
    positive supply. If that's the case, then I don't see how putting 1mA
    through the ESD diode is going to keep the pin below that 0.3V any
    than an external silicon junction diode would.

    As an alternative I'd try this:

    V+ V+ V+
    | |K |
    [R] [CR] [R] +---
    | | | |__
    | <--O--[C]--+--[<CR]--+-----O|TR
    | |
    O +---
    | 7555

    John Fields
    You're mis-reading the data sheet... they want you to avoid +0.3V
    *from a low impedance source* which could cause latch-up.

    If you'll look at some logic devices they usually specify a max
    current *before* latch-up can occur.

    But your method definitely avoids any forward current.

    ....Jim Thompson
    How about a Ge diode?

    Terry Pinnell
    (Thread then wandered OT to brief discussion about sourcing Ge diodes.
    As far as I know, my question - which on reflection I'd extend to
    Schottky types too - was never explicitly answered.)
  10. Tim Dicus

    Tim Dicus Guest

    So what Jim is saying is that I can connect the trigger input pin 2 to my automobile ignition coil (about 400v flyback last time I
    measured) and it will do ok?

    It is possible that the junction will take a small amount of reverse voltage without harm, but since nobody seems to know where that
    limit is, I will use the protection diode. Even though I know it is almost impossible to do, I try to design my circuits to be

    I have been at this for a while, and I have yet to see a semiconductor junction that does not have limits in both directions.

    But there is one thing I know for certain:
    I know nothing for certain!

  11. Fred Bloggs

    Fred Bloggs Guest

    That is true, but that PNP buffers the input of a differential pair, the
    other input of which is connected to 1/3 Vcc. The IC is specified to
    Vcc,max=18V making 1/3Vcc=6V. If you have no problem with running TRIG
    to Vcc, then this puts 18-2xVeb-6~10.7V reverse voltage across the
    series BE junctions on the TRIG side. Allowing for a shorted CONT input
    would make this 16.7V of input reverse bias. The transistors probably
    take 30V, so unless there is something else going on, the clamp diode is
    a waste of space.
  12. I read in that Tim Dicus <>
    What is on the schematic is very often far less than what is actually in
    the device. This N-type base and its connecting track must be isolated
    from the substrate, I suppose. What isolates them?
  13. Fred Bloggs

    Fred Bloggs Guest

    Connecting track? You must mean metalization- and this runs on top of a
    silicon dioxide layer or similar insulator....
  14. Tim Dicus

    Tim Dicus Guest

    Hi John,

    Good to have your input. I am aware that most of these are "functional diagrams" rather than true schematics.

    But in that light, I would hope that if there were overvoltage protection, that it would be refered to as a "function" by the

    "...isolated from the substrate, I suppose"? Is there a point you are trying to make? Please enlighten me! I want to insure my
    designs are reliable without wasting money on parts I don't need.


  15. I read in that Tim Dicus <>
    Well, in some cases parts are isolated by what is normally a reverse-
    biased diode, and that is exactly what you want, isn't it?
  16. Tim Dicus

    Tim Dicus Guest

    Hi Fred,

    Your argument is very logical. However, I would logically deduce from the Vcc max rating that the transistors are rated at 18 volts,
    rather than "probably 30".

    I agree the trigger input will take more than Vcc. I know that from experience. But how much more (and for how long) is the question
    I have. From experience, I know it will take at least .75 volt above Vcc, at least intermittently.

    A 2N3906 shows a base-emitter junction voltage limit at -6 volts (PNP), but shows no "base-emitter reverse voltage" limit. Can one
    assume it has no reverse voltage limit? Or would one assume the limit would be +6 volts? Or +40 volts?See the following datsheet:

    Once again, my concern here is I want my designs to be reliable without wasting money on parts I don't need.


  17. Tim Dicus

    Tim Dicus Guest

    Yes, sir! That is EXACTLY what I need! If you would be so kind as to point me in the direction of where I can be assured that the
    LM555 trigger input pin 2 has that feature.


  18. Fred Bloggs

    Fred Bloggs Guest

    The National single supply comparators and opamps using the same process
    are spec'ed to 30V and that is where I pulled that number from. For some
    reason they give no spec in their Abs Max rating chart for the 555- so
    you're right in saying it can't be assumed.
  19. Tim Dicus

    Tim Dicus Guest

    Thanks, Fred.

    I have seen two other datasheets on the 555, and none have given the trigger input limit in their ratings.

    I find that a little odd. Seems to me that is one of the most frequently used externally-driven inputs on that IC.

  20. I read in that Tim Dicus <>
    I don't know that it does, I just suggest that you MIGHT be lucky in
    that a parasitic device that isn't on the functional schematic might do
    exactly what you want.
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