decoupling 7555 + 4066

Discussion in 'The Projects Forum' started by Genoil, Aug 29, 2011.

  1. Genoil

    Thread Starter New Member

    Aug 5, 2011
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    Hi,

    I've read the sticky on decoupling, but I still have a question. I have a small circuit with a CMOS 7555 and 4066 placed close to eachother. The frequency of the 555 in astable is between 0.5 - 3 Hz and it switches a single port of the 4066. The signal on the 4066 it switches is in the pA range. Do I still need decoupling with these low values? I've understood the 7555 doesn't require one on the Vcontrol pin so I took that out, but not sure if I need one between V+ and V-. Also, the 7555 and 4066 are really close together (<1 in). If I need decoupling after all, can I use one for both? And is 100nF enough?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    If it's audio you're switching, you're going to hear popping on the signal through the 4066.

    555 timers require a minimum of 0.1uF ceramic or poly metal film and a 1uF or larger aluminum electrolytic. And no, you can't combine the 0.1uF cap on the 4066 with the 555 timer's cap unless you like lots of noise.
     
  3. Genoil

    Thread Starter New Member

    Aug 5, 2011
    21
    1
    Thanks! I'm not switching audio, but a wire running from an iphone's touch screen to ground (dock connecter plug of iphone):

    http://www.youtube.com/watch?v=2-3MjeL1Szc

    I think the phone doesn't care about possible noise. So as long as I can be assured I don't wreck the 7555 or 4066 and don't get any disturbance in the pulse it is sending out, I'd rather not use them at all. The point is that I'm trying to squeeze the whole circuit onto an as small as possible area, to be able to move the thing around the screen like a chess piece. So any superfluous components I can get rid off will help :).

    It's only for a hobby/art project so moving to SMD to reduce the circuit area is not really an option.
     
    Last edited: Aug 29, 2011
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, SMD isn't as "bad" as you might think. Once you get used to it, you'll find that making boards is a lot easier than for thru-hole and DIP IC's; there are few if any holes to drill. I personally don't go below 0805 size components anymore; they require better eyesight than I have.
     
  5. Genoil

    Thread Starter New Member

    Aug 5, 2011
    21
    1
    I'm currently busy getting this on the smallest possible area on a stripboard. Swapping the 4066 out for a 8-pin TI single SPST analog switch, haven't got the part in yet but I reckon it'll do the job.

    I guess it would have been easier to understand the circuit with a normal diagram, but I only have the stripboard layout attached. Basically it is the "minimal component" 555 astable from the doctronics 555 page with a trimpot used as the resistor and the output wired to the analog switch.

    I was wondering if I placed the decoupling caps (c2, c3) right and if I can use the little 0.1" .1μF ones for that? Also the bypass cap on pin 5 shared with the "global one across V+ and GND, can I do this?

    Or should I just not use them if noise is not really my concern?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    896
    The two decoupling capacitors mentioned by Sgt Wookie are for the ordinary high current 555. The Cmos 555 and the 4066 can share a single 0.1uF ceramic disc capacitor from +V to ground near them.

    Your stripboard layout shows pin 5 of the Cmos 555 shorted to the positive supply. Then the Cmos 555 (or the ordinary high current 555) will not work.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Post your schematic so that we can tell if you have wired it up as you intended.

    Without that, we have to guess. Guessing frequently causes the smoke to be let out of parts.
     
  8. Genoil

    Thread Starter New Member

    Aug 5, 2011
    21
    1
    I've updated the stripboard layout to not short pin5, completely overlooked that whilst trying to get under 1 inch :). Now there is no bypass cap on pin5, but most 7555's don't seem to require that.

    But is C2 still close enough to the switch to decouple it over V+? In that case I could potentially get rid of C3.

    I don't have a full circuit diagram at hand, but I attached the one from the doctronics page. The only real difference is the added switch IC and the values of the caps. I should really make a readable diagram of the whole thing, agreed...
     
    Last edited: Aug 31, 2011
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