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Unused Pins

Discussion in 'General Electronics Chat' started by prprog, Aug 2, 2008.

  1. prprog

    prprog Member

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    Hello:

    I am using just 2 bilateral switches of a 4066 Ic. Which unused pins do I need to tied to ground? Do I need to do the same for a LM339 IC ?

    Thanks,
     
  2. mvs sarma

    mvs sarma Well-Known Member

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    The switch control pins of unused switches. of course provide a 0.1 uF cap across the Vcc and Vee of the chip
     
  3. prprog

    prprog Member

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    Only the Control pins, to ground?
     
  4. dave

    Dave New Member

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  5. mvs sarma

    mvs sarma Well-Known Member

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    Afterall you need to see that the chip doesn't oscillate and created unwanted noise around. Thus keeping the control pin at ground, the concerned gates will be off (high impedance). I hope this is enough.

    This link answers the LM339 issues.

    http://www.electro-tech-online.com/threads/lm339-do-all-unused-pins-require-grounding.23289/
     
    Last edited: Aug 2, 2008
  6. prprog

    prprog Member

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    Thanks. I tied the control pins ...but still the switching is not working. I am trying to substitute what is normally a keyboard switch with the 4066 but it seems the 4066 is oscillating. I am using +5volts , does this makes a difference? Searching on the web I read that 4066 works better at higher voltage , like +9v.
     
    Last edited: Aug 2, 2008
  7. Mikebits

    Mikebits Well-Known Member

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    What makes you think the device is oscillating, scope? Did you bypass VCC?

    At 5v the Ron of the switch is about 300Ω. Depending on a pullup after the switch, you might not get a good logic 0, assuming active low keyboard.

    At 10v the Ron will be about 120Ω which would be better. You should try to figure out what value pullup your trying to pull low, again assuming active low key.
     
    Last edited: Aug 2, 2008
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Please attach a schematic of what your circuit is doing.
     
  9. WardXmodem

    WardXmodem New Member

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    I liked the words "of course, use a 0.1 uF cap across Vcc and Vee.

    That means, people assume everyone knows about noise, "decoupling" or whatever those terms are. I don't. I'm not complaining. I am just thinking I found someone who really knows and this is the time to ask:

    I have been working with little PIC LED kits, and they need ZERO caps to keep them working well.

    I know this is "on the cheap, and if it works why not?" but I'd like to know a bit more about "best practice" when using caps for noise suppression, etc.

    I know that switching outputs can put a load on power sources, so maybe it is a good idea to just stick a cap across the power going to a pic (is 0.1 a "good choice?")

    I once made my own scratch built computer board, and the power was oscillating like crazy, until I stuck a very very tiny cap across the input (I think it was) of the 7805 regulator, so I learned by experience a cap can help...

    Thanks very much, from a very experienced programmer, pretty good digital hacker, but very green "general electronics" person. I just bought my first handful of transistors and diodes and etc to start experimenting, and so far have only done some "Joule Thief" type boost circuits with various inductors I could find. I would LOVE a 'scope to watch what is going on at various points, see what frequency it is running, see at what voltage the transistor conducts, etc. Real basic stuff!

    Take care.
     
  10. mvs sarma

    mvs sarma Well-Known Member

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    I assume that general filtering of power supply has been done as per standard practice. Then it is necessary and customary to have decoupling caps across Vdd&Vss or Vdd& Vee
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Good luck with your projects.

    Regarding adequate decoupling of electronic circuits.
    The circuit you have just built may work fine on the bench, but in the outside world it could be subject
    to an environment outside your control.

    If the circuit was say 'life or process' critical you wouldnt want a 'glitch' to cause a serious malfuction.
    Especially considering the low price of a cap.

    I am sure in the programs you write, being an experienced programmer, you cover all the users 'trivial' possible
    inputs to your program, just in case.!:)
     
    Last edited: Aug 3, 2008
  12. WardXmodem

    WardXmodem New Member

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    Ah, you explained it well - since the cap-less circuits were run from 4 x LR44 batts.
     

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