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Logic gates + noise

J

Jon Slaughter

Jan 1, 1970
0
I was playing around with some logic gates and they seem to be extremely
sensitive.

If I take an AND gate and breadboard it, and put both it's inputs to high(by
a jumper wire of about 1.5in), and touch the jumper wire which is sheilded,
the output fluxuates ;/ I could understand leaving the input floating but
when it's directly jumped to high? Is this normal or something else going
on?
 
J

Jon Slaughter

Jan 1, 1970
0
Peter Bennett said:
What logic family (and complete part number)?

74HCT08

Any chance that the jumper wires are making intermittent connections
when you tough them?

Potentially... I didn't think that maybe there would be minute vibrations
that would be enough to cause contact issues... thats the best explination
at least
 
D

David L. Jones

Jan 1, 1970
0
I was playing around with some logic gates and they seem to be extremely
sensitive.

If I take an AND gate and breadboard it, and put both it's inputs to high(by
a jumper wire of about 1.5in), and touch the jumper wire which is sheilded,
the output fluxuates ;/ I could understand leaving the input floating but
when it's directly jumped to high? Is this normal or something else going
on?

Something else is going on.
Likeliest culprit is a bad connection on the input. Measure the
voltage directly on the chips input pin and see if it really is logic
high.
Check your ground and power connections on the actual chip pins too,
they could be making bad contact.
Try the other AND gates in the package.
Some trickier culprits will depend upon what logic family you are
using.

Dave.
 
D

Dirk Bruere at NeoPax

Jan 1, 1970
0
David said:
Something else is going on.
Likeliest culprit is a bad connection on the input. Measure the
voltage directly on the chips input pin and see if it really is logic
high.
Check your ground and power connections on the actual chip pins too,
they could be making bad contact.
Try the other AND gates in the package.
Some trickier culprits will depend upon what logic family you are
using.

Dave.

Also make sure that the original poster is grounded, to eliminate static.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff
 
R

Robert Monsen

Jan 1, 1970
0
I was playing around with some logic gates and they seem to be extremely
sensitive.

If I take an AND gate and breadboard it, and put both it's inputs to high(by
a jumper wire of about 1.5in), and touch the jumper wire which is sheilded,
the output fluxuates ;/ I could understand leaving the input floating but
when it's directly jumped to high? Is this normal or something else going
on?

Try powering the chip through Vcc... ;)

Sometimes cmos chips will power up through the inputs, and seem like
they work, but it'll lead to strange results. Make sure you have Vcc
firmly tied to your 5V rail.

Regards,
Bob Monsen
 
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