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Electrical interference, abnormal? — Parallax Forums

Electrical interference, abnormal?

ArnoNLArnoNL Posts: 15
edited 2009-03-15 14:13 in BASIC Stamp
Dear all,
'
I'm now working on several small Basic stamp projects for some time. Unfortunately I encountered problems with all of them.·A few days ago I discovered·what the problems were.·All had to do something with interference or something because I·discovered·that when I move my body, hand etc. near a circuit that a lot of "unscheduled actions" took place. If·I touch a wire, move my hand near wires, the BOE, a breadboard·or components (even though I don't touch them) or sometimes·even when wires are touching each other·I get abnormall readings and/or errors.
Now I I'm wondering what I have to do to overcome and prevent such things from happening. Is this normal? How do you all cope with this kind of things?

Thank you so much for valuable replies as I really don't know what's going on and all things I build till thusfar are completely unreliable due to this phenomenon.

Kind regards,

Arno
The Netherlands
·

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-03-14 21:37
    CMOS inputs are high impedance and have fairly high gain. As a result, they're very sensitive to electrical fields. It is normal practice to either connect unused inputs to some kind of output (which is a low impedance at a defined voltage) or to a pullup resistor 100K or less (typically 10K) which is connected to ground or the regulated supply voltage. For many microprocessors, you can just set unused pins to output mode which will define their state.

    For used input pins, you have to look at how they're connected and what they're connected to. Again, the idea is to make sure their input voltage is well defined and low impedance where possible. If not possible, they may need to be shielded somehow and the details depend on what you're trying to do.

    If you want specific comments and advice, you'll need to make a schematic of your setup and attach it to your reply. An actual picture may also be useful. If you're using any non-Parallax parts, particularly ICs, you should make sure you have their datasheets and possibly attach those as well so we don't have to look them up ourselves.
  • ArnoNLArnoNL Posts: 15
    edited 2009-03-15 12:30
    Hello mike, thanks for the reply,

    what do you mean with low impedance?

    So the first thing what I'm going to do is put all unused·pins to output (is this as good as getting 10k pull up resistors and connect them to ground? Why is a direct connection to ground not advised?).

    Also I'm going to make a ground connection with 100k resistor·from all not connected pins of the·IC sensors I'm using.

    I will let you know the outcome of these actions as soon as I ge the resistors.

    I will also post the schematics.



    Kind regards,
  • Mike GreenMike Green Posts: 23,101
    edited 2009-03-15 14:13
    A direct connection to ground is dangerous because, if you accidently program the I/O pin to be a high level output, there will be a short circuit from Vdd to ground and the I/O pin structure will be destroyed. A resistor limits the short circuit current to a tolerable level.

    Look up the term "impedance" in the Wikipedia (Google "wiki impedance").
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