74HC221 (dual one-shot) sharing timing res/cap?

[composition4]

Hi,

I'm using a 74HC221 dual monostable multivibrator (one shot) in a design. This is basically two one-shots in the one package.  I can't get to my breadboard right now, so have a question if anyone is familiar with it or other dual one-shots...

Can I share the same R/C timing network for both halves of the package? As in, the output pulse for both has the same timing requirements... and there are no circumstances in which both one-shots will be activated at the same time.  That way, instead of using 2 resistors and 2 capacitors to take care of the timing for both one-shots, I can use the single resistor and capacitor to take care of the timing for both..

Hope this ramble makes sense.  I know it seems trivial but my design uses 12 one-shots and board space is a consideration.. I would much prefer to use 6 caps/resistors than 12...

Thanks!
Jonathan

[earthtonesaudio]

I think it would cause troubles.  Plus, I can't figure out how using a dual one shot as a single saves space. 
But of course now I'm very curious about what it is you're building. 

[composition4]

I'm not actually using it as a single.  Basically I have 10 momentary footswitches and when one is pressed, it outputs a fixed length pulse, say 20ms.  So footswitch 1 is connected to the first one-shot of IC1, and creates 20ms pulse at Q(1), the output of the first one-shot on IC1.  Footswitch 2 is connected to the second one-shot on IC1, to create a 20ms pulse at Q(2), the output of the second one-shot in IC1.

Footswitch 3 + 4 would be connected to IC2, 5 + 6 to IC3, etc.

So in effect, because I need pulses at the different outputs depending on which footswitch is pressed, I am using 5 dual one-shots.. but the timing requirements for each oneshot are the same for both halves, so I am wondering if it's a problem sharing the res/cap network for both halves of the oneshot.

Jeez, I hope that made some sort of sense!  It's for a 10 channel, one-channel at a time, latching relay effects switcher... with 6 effects loops, 2 external loops (so it can control effects in the amp's FX loop), controls for an amp's reverb and overdrive channel (both for momentary and latching amp channel switching configurations because some amps use momentary nowadays)...buffered up front then true bypass for the rest.. also includes 10 isolated 9vdc power outputs.  umm, a whole bunch of other stuff too, some of it pretty cool and unique. Have taken some ideas from RG's "programmable effects switcher" and "spyder power supply" and expanded on it a great deal.  Most of the way through planning it (have already bought all the parts and done the schematics) and now creating the PCB layouts.  Has been a work in progress for a long time!

Sorry to rant
Jonathan

[R.G.]

Sorry - it's really unlikely that sharing a single R-C between two or more one-shot packages would work without unwanted interaction. Without delving into the internal circuitry I can't say for sure it won't. But it's very unlikely. In a complicated setup like this, it's best to minimize the complexity by separating interactions whereever you can. I'd advise using the resistors and caps.

[head_spaz]

I don't see how you can share the timing circuit between two individual circuits, unless you use them in tandem parallel, and why would anyone want to do that?

If you're not already committed to a design, you might wanna look into using modern CMOS replacements (i.e. CD4098B) with different pinouts, which might lend itself to a tighter layout and possibly the use of smaller capacitors (depending upon your timing requirements.)
Another advantage of using CMOS is that they operate over a much wider voltage range, varying from 3V - 20V, and require less quiescent current.

[earthtonesaudio]

...you might wanna look into using modern CMOS replacements (i.e. CD4098B) with different pinouts, which might lend itself to a tighter layout and possibly the use of smaller capacitors (depending upon your timing requirements.)
Another advantage of using CMOS is that they operate over a much wider voltage range, varying from 3V - 20V, and require less quiescent current.

74HCxxxx is CMOS, and while the pinout of the 4098 is a bit nicer, the '221 is non-retriggerable, which might be beneficial here.

I totally agree with the suggestion to use smaller caps.  If board space is at a premium, it might be a good time to investigate SMD caps and resistors.  Saves time spent drilling holes too...