cd4538 monostable multivibrator circuit

Precision Monostable Circuit

Here is a monostable multivibrator using IC CD4538. It is a precision monostable/astable multivibrator IC free from false triggering. This can be used for various application in which precise timing cycle is required.

CD4538 is the precision monostable/astable multivibrator IC that is free from false triggering. It is more reliable than the popular timer IC 555. Here the IC is wired as a short duration monostable timer using R1 and C1 as timing components. With the given values, output of IC1 remains low for three minutes. By changing the value of C1 or R1 various time intervals can be obtained.

Unlike 555 IC in the monostable mode, here in CD4530, output of IC becomes high at power on and becomes low when the trigger pin5 gets a low-to-high transition pulse. When S1 is pressed, the high going pulse triggers IC and its output goes low. This drives the load through the PNP transistor T1.Load can be an LED, Buzzer etc.

Schematic of the Monostable Multivibrator Circuit

cd4538 monostable multivibrator circuit

CD4538 Pin Connection


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  • sandy

    Hi all..
    I am facing a problem with 4538.
    When i trigger s1 here i am getting two pulse o/p at Q with 150 ms delay.
    Hope to get any suggestions..

    Thanks in advance..

    • Jim Keith

      It is the edge of the clock signal that must be checked –this requires the sweep rate to be increased substantially (1uS /div or so). This is very likely the issue if a shunt capacitor fixes the problem.

      A better way to solve the problem is to use the Texas Instrument CD14538B that incorporates schmitt triggers on the inputs. A dirty clock signal may be cleaned up via a series resistor and a small shunt capacitor. The earlier version (CD4538) may not have this feature.

    • sandy

      Thanks jim for your quick response..
      I have checked the wave forms in oscillpscope. The inupt is good and reset and B is also set high but the o/p pulse is still coming at two times whole on and off time…

      When a capaciter 104 is set at input it is functioning ok..
      But it is not working for fast switching ex..10-15 ms i/p switching

    • Jim Keith

      My guess is that you have a dirty clock pulse –view on oscilloscope to determine this. If triggering via pushbutton, you may be getting a pulse when you close the circuit and another when opening the circuit (150ms later) –pushbuttons generate dirty signals.

  • Aggie

    Can you suggest a timer circuit which behaves in the below mentioned logic.
    0 to 60 seconds adjustable timer with trigger input and reset switch, (case – 1) if timer set for 10 seconds, if triggered timer starts and lapses after 10 seconds, and output goes high for 4 seconds and then goes low.(case-2)if timer set for 10 seconds, if triggered, timer starts and if reset before 10 seconds, the output remains low.
    Please email me the details
    thanks & regards


  • Lio

    thank you for your quick response. Is this scheme might work?

  • Lio

    Hello, I want to make a montage with the 4538 circuit. But I want when I press the S1 button, the output of the circuit starts up position for 1/2 second. Even if the input is more than 1/2 sec. Is that possible?
    Tanks for your answer 😉

    • Jim Keith

      I think I was wrong on this detail –if additional trigger edges occur during the timeout, the pulse is extended.

    • HANAN

      Hi Lio,
      As the title implies: This is a MONOSTABLE circuit. It will make only one Pw pulse when you press S1. If I’m wrong, and the pulse length includes the time S1 is pressed then you can add in series with S1 a differentiation circuit: A 1nF capacitor between S1 and pin#5 and 10K resistors from each side of the capacitor to ground (RREMEMBER: One of these resistors must be already there according to my first remark above !)

    • Jim Keith

      The basic operating mode is retriggerable so if the pushbutton is depressed for an extended period of time, the output pulse is also extended.
      What you want is non-retriggerable –to do this, refer to fig13 of this datasheet:

  • Bonie

    My questions is:
    1. Whats the difference between CD4538 & CD14538? Ive been mentioned that CD4538 is no longer manufactured.
    2. its possible if im using CD14538?
    Reply asap. Ty 🙂

    • Jim Keith

      As far as I know, they are the same. The prefixes “MC” and “MC1” were introduced by Motorola (Now ON Semiconductor) when they entered the 4000 series CMOS logic business some 35years ago. The prefix “CD” was introduced by RCA and picked up by both Fairchild and TI when they 2nd sourced the same products –it appears that the CD14538 is now sourced by TI and is the only DIP version. Check out my article “Tips on collecting ICs” as DIPs are slated for extinction.

      There may be a difference in the calculation of the timing components (Rx & Cx), so it is best to actually check the datasheets. I believe that the monostable 74HC123A & B have different timings.

  • Crady

    ERROR: There is a missing pull-down resistor between pin 5 and ground (5K to 10K would do fine).

    Otherwise nice article.

    • Popescu Marian

      Thanks! It seems that Mohankumar was such a bad designer… Almost all his articles are wrong, that is why I started to test his circuits.

  • Hanan

    Still no reply on “pushing the switch shorts the battery”!

    Also: no precision timing if based on an electrolytic capacitor ! value of ‘C’ is changing with tempereture!

  • jayashree

    an you pls explain the pin configuration

  • padma

    can u piz explain the pin config of ic 4538?

  • Hanan

    I guess between pins #5 and #8 it should be a resistor (10K ?) rather than just wire.
    Please observe also “CMOS unused inputs rules” or else the internal circuitry assumes an undefined state allowing considerable current from the supply, thus draining all energy from the battery in no-time (relative). You loose one of the main advantages of using a CMOS device. Therefore: connect pins 12 and 15 to Vss (or’-‘) and pins 11 and 13 to Vdd (or ‘+’)

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