CD4047 mono/astable multivibrator

Thread Starter


Joined Aug 4, 2010
Hi all,

just purchased the cd4047b ic and cannot get the monostable operation to work.

i'm using a 33uf cap and 100k resistor for timing. pins 4,8,14 are connected to vdd and 5,7,9,12 are ground.

i should be getting 8.125seconds but it remains on forever.

can somebody help me?



Joined Jul 23, 2009
Attached see the pdf file of the schematic. Pay attention to the negative polarity of the trigger required and the output pins (both pos low & pos Hi) are not the same as the astable output pin.

Also note R2, this gives the trigger pin a normally-high state and a negative pulse, which is required to trigger the IC in the mode you were using. It would probably even work a bit better if you put a 0.01uF cap or so across the S1 switch to reduce switch bounce and transients, although with such a long on-time, switch bounce isn't really an issue as it goes away in less than a second. Also note that you get both a positive going output and a negative going output. Use whichever one suits your needs.

Now, I really can't say why your circuit didn't work, especially with no schematic (and I expect one from you next time--don't ask for our valuable time helping you if you're not even willing to draw a schematic (hand drawn is OK) and scan it, or draw it in one of the several free schematic capture programs out there).

The main thing is to look carefully at the CD4047B datasheet. Not just the first page, but especially the applications or test circuit figures. Not everything is clear about the way it works from the first page of the datasheet. For example, my R2 (10K) may not even be necessary, but it does make the circuit more robust to noise as will adding a cap across the switch.

ALso, you don't need a 12v SUPPLY, anything from about +5 to about +15 will work fine on this chip (at least the Fairchild version).

Well, I think I've beaten this one to death at this point. Good luck and reply if you have more questions.

Kamran Kazem
V.P. & CTO
Magnetic Design Labs, Inc.
I.E.E.E. Senior Member