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'[EE]: Noise problem with schmidt trigger'
2003\05\11@204924 by Lim KK

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I have a schmidt trigger connected to my microcontroller counting the number
of pulses coming from the schmidt trigger. I notice that whenever any power
supply is turned on (DC power supply unit, any step down transformer), the
counter value jumps unpredictably. When I remove the input to the schmidt
trigger(microcontroller to schmidt trigger wire is still there), everything
is fine. I replace the input to the schmidt trigger with just a long wire
and the problem appears again.

It seems to me that the schmidt trigger has very low threshold value, to the
extend that it interprets these noise as a signal. Is there anyway I can
solve this problem? I thought of a rather stupid solution as to wrap the
input wire to the schmidt wire with aluminium foil. Will it work?

Thanks
Mark

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2003\05\11@210218 by Tom Messenger

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It sounds like you have a fairly high input impedance to your schmitt
trigger making it sensitive to electrical noise.  If your circuit does not
require high input impedance, try lowering it.

What is connected to the input of the schmitt trigger?


At 12:48 AM 5/12/03 +0000, you wrote:
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2003\05\11@213826 by Lim KK

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Thanks for your reply.
The input to my schmitt trigger is actually pulses coming from an optical
encoder. 2 pulses of different phases are sent to my microcontroller via two
schmitt trigger to decode the position of the motor. My microcontroller is
configure as an event counter to count these pulses. I tried so many
techniques but fail to find a solution to this problem. Whenever any
equipment with transformer is switched on nearby, the counter jumps wildly.

Being a rather lousy student myself, can you explain how can I lower the
high input impedance to my schmitt trigger? The wire carrying the pulses
from the optical encoder to the schmitt trigger is about 2meters long.

Thanks alot!
Mark


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2003\05\11@230145 by Marc Nicholas

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I wonder if a Schottky diode would help?

-marc


On 5/11/03 9:37 PM, "Lim KK" <KILLspammark_limkkKILLspamspamHOTMAIL.COM> wrote:

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2003\05\12@004025 by Tom Messenger

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At 01:37 AM 5/12/03 +0000, you wrote:
>Thanks for your reply.
>The input to my schmitt trigger is actually pulses coming from an optical
>encoder. 2 pulses of different phases are sent to my microcontroller via two
>schmitt trigger to decode the position of the motor. My microcontroller is
>configure as an event counter to count these pulses. I tried so many
>techniques but fail to find a solution to this problem. Whenever any
>equipment with transformer is switched on nearby, the counter jumps wildly.
>
>Being a rather lousy student myself, can you explain how can I lower the
>high input impedance to my schmitt trigger? The wire carrying the pulses
>from the optical encoder to the schmitt trigger is about 2meters long.
>
>Thanks alot!
>Mark

It's difficult to know the best approach without seeing your circuit.
Perhaps if you can make a small gif of it, people on the list can suggest
better solutions...

You also need to verify how the electrical interference enters your
circuit. If possible, try running it completely isolated from AC power -
use batteries or whatever.  See if this works.  It may be coming in the
opto input to the schmitt trigger or somewhere else.  You will have to
become adept at guessing where the trouble is and trying to verify your
guesses!  It is difficult for beginners but after doing so, you will learn
a LOT!

Good luck!
Tom M.

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2003\05\12@013330 by Mike Singer

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"The importance of a properly grounded system cannot
be overemphasized." - from " OIT Ground Wiring and Electrical
Noise Reduction"

Another not-so-bad resource on grounding "ELECTRICAL GROUNDING
ARCHITECTURE FOR UNMANNED SPACECRAFT" (NASA-HDBK-4001)

Or "RELIABILITY PRACTICES ELECTRICAL GROUNDING PRACTICES
FOR AEROSPACE HARDWARE"

Good links:
users.pandora.be/educypedia/electronics/generalelectronicsmis.htm
(Grounding and shielding techniques)

Good Luck.
Mike.

> >>> I have a schmidt trigger connected to my microcontroller
> >>> counting the number of pulses coming from the schmidt
> >>> trigger. I notice that whenever any power supply is
> >>> turned on (DC power supply unit, any step down
> >>> transformer), the counter value jumps unpredictably.
> >>> When I remove the input to the Schmidt trigger
> >>> (microcontroller to schmidt trigger wire is still there),
> >>> everything is fine. I replace the input to the schmidt
> >>> trigger with just a long wire and the problem appears again.
...

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2003\05\12@031054 by

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Pull up or pull down resistors on the input to
the schmitt trigger ? What is the output from the
encoder ? Just a open or closed switch ?

Jan-Erik.


Lim KK wrote:
>The input to my schmitt trigger is actually pulses
>coming from an optical encoder.

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2003\05\13@065838 by Lim KK

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yup, I tried to pull it up, but the problem persist. The output of the
encoder is pulses (0V to 5V). I think the encoder circuit producing the
pulses are fine. When I replace the input of the schmitt trigger with a long
wire(1m), the problem appears.

mark


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2003\05\13@075758 by

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And the long wire replaces what exactly ?
What's in the other end of the "long wire" ?
How long is the "not-so-long-wire" ?
Jan-Erik.


{Original Message removed}

2003\05\13@080613 by Lim KK

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oh, the long wire is connected to nothing. It's around < 1m. No, there's no
"not-so-long-wire". I'm trying to find out how long does the wire need to be
in order to cause problem.

mark


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2003\05\13@081300 by

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Well, out in the woods, you could probably have a
fairly long wire, in a noisy workshop a much
shorter wire. Of course a long un-terminated
wire will cause this kind of problems. You
have just connected an antenna to your input...

Jan-Erik.

Lim KK wrote:
>oh, the long wire is connected to nothing. It's around < 1m. No, there's no
>"not-so-long-wire". I'm trying to find out how long does the wire need to be
>in order to cause problem.

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2003\05\13@082335 by Lim KK

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But when connected to my pulse encoder using a 5m wire + tying the input of
the schmitt trigger to a +5V via a resistor, the problem persist. The wire
itself is already shielded with the exception of the exposed end (approx
3cm). I'm suprise that such a small transformer can cause these problems.


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2003\05\13@082739 by

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I'd check the signal quality with a o-scope...
Jan-Erik.

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2003\05\13@083552 by Lim KK

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Thanks, I tried that, but everything happen so fast, my uni antique o-scope
cannot capture anything. I simulate the noise by flicking the switch of a
AC-DC power supply. But I'm sure something came out of the schmitt trigger
cos the micrcontroller counter changes.


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2003\05\13@091544 by Mike Hord

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How exactly do you have the wire shielded?  Is it coax, or twisted pair, or
shielded untwisted, or what?

You may be able to fix it simply by using a twisted pair wire.  I had a
project go south when I ran 8-strand untwisted cable, but replacing it with
CAT-5 ethernet cable fixed it right up.  Without the CAT-5, I was getting
peak noise of two volts on the end of the wire, and that was with it running
through a grounded metal conduit!

Mike H.


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