How can an NAND logic gate output 1 when there is no current flowing?

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Jaymie

New Member
How can an NAND logic gate output 1 when there is no current flowing? - As I understand it an output of 1 means that a current is flowing or does it just mean current is allowed to flow? If that is the case would it mean that NAND is pretty much the same an XOR logic gate because it allows the current to flow but there is no current flowing?

The question has also been posted here: WikiAnswers - How can an NAND logic gate output 1 when there is no current flowing? but it is currently unanswered.

- Thanks in Advance, this has got me greatly puzzled -

Nigel Goodwin

Super Moderator
Your question doesn't really make any sense?, outputing a logic 1 or 0 has nothing to do with current flowing or not, as in pretty well all electronics, it's down to what the load draws - so basically no load, no current.

A logic 1 doesn't mean current is flowing, and thinking that way is going to seriously limit your understanding of logic circuits.

Jaymie

New Member
Clarification...

Ok I'm obviously getting this all wrong. Back to basics, if an output equals 1 it means current has the potential to be drawn - is that correct? If so, using this diagram (which forms an AND gate)...

When A and B has a hypothetical input of 1, Q would output/equal 1; meaning current can be drawn - correct? - however, the first NAND gate should stop that because 1 + 1 = 0 so no current can flow ???

- Please bare-in-mind I am just starting logic gates and am a beginner -
- ...so could you explain in simple terms and try not to be too harsh -

ericgibbs

Well-Known Member
Ok I'm obviously getting this all wrong. Back to basics, if an output equals 1 it means current has the potential to be drawn - is that correct? If so, using this diagram (which forms an AND gate)...

When A and B has a hypothetical input of 1, Q would output/equal 1; meaning current can be drawn - correct? - however, the first NAND gate should stop that because 1 + 1 = 0 so no current can flow ???

- Please bare-in-mind I am just starting logic gates and am a beginner -
- ...so could you explain in simple terms and try not to be too harsh -
hi,
Forget about currents when working with logic gates.

Consider the gates as mechanical switches, ie: open/closed or On/Off STATES.

The first gate in your dwg is a NAND gate.
A and B being '1' then the output is '0'

for the second gate also a NAND, if either [ or both] input is '0' then the output is '1'

EDIT: you see in my explanation no mention of voltages or currents, the logic is the same.

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Jaymie

New Member
So then if the first gate/switch is off there would be no purpose off the second because current can't flow through the series of switches. - if this is wrong what/how would you use them in a circuit? Simply put: if these switched don't control current what do they control?

ericgibbs

Well-Known Member
NAND truth table.

Code:
Inputs           Output
B 	A		C
0	0		1
0	1		1
1	0		1
1	1		0
Ok.?

ericgibbs

Well-Known Member
So then if the first gate/switch is off there would be no purpose off the second because current can't flow through the series of switches. - if this is wrong what/how would you use them in a circuit? Simply put: if these switched don't control current what do they control?
hiJaymie,
Whats the point of talking about currents..?

Study the circuit gates in simple LOGIC terms, ie; 1 and 0

Code:
AND gate truth table.
Inputs           Output
B 	A		C
0	0		0
0	1		0
1	0		0
1	1		1

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Jaymie

New Member
yes thank you i understand that but what does the on/off represent a switch of... what? - i'm porb still getting the wrong idea. But how then do you use them in a circuit?

ericgibbs

Well-Known Member
yes thank you i understand that but what does the on/off represent a switch of... what? - i'm porb still getting the wrong idea. But how then do you use them in a circuit?
hi,
Hang in there.

there are a number of different types of logic gates.

These gates are used in combination to give a required OUTPUT when the required INPUT conditions are met.

In your circuit input 'A' could be from a switch and input 'B' from a push button.

The switch must be ON for the push button to work and set the 'Q' output to a '1' .

Jaymie

New Member
ok I am getting this. so in your example how would you get the output Q to i.e. turn on a LED? ...because there isn't any current flowing out of Q.

Nigel Goodwin

Super Moderator
Again, don't think current - it's either logic HIGH, or logic LOW - current doesn't enter in to it really. If you are using logic gates to switch an output, it's probably more common to use them to SINK current rather than SOURCE it, so logic LOW provides the current, not logic HIGH.

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Jaymie

New Member
oh I think I get it now logic gates don't control the current (i.e. on or off) they control the level of voltage - high (1) or low (0) - yes???

Nigel Goodwin

Super Moderator
Yes, that's it - but the voltage level isn't fixed, there's a certain transistion point between LOW and HIGH, with a 'dead band' in the middle (of the specification at least).

Check the first part of this tutorial to see how to drive an LED, but PIC's have FAR higher capacity than logic chips.

http://www.winpicprog.co.uk/pic_tutorial_extras.htm

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ericgibbs

Well-Known Member
oh I think I get it now logic gates don't control the current (i.e. on or off) they control the level of voltage - high (1) or low (0) - yes???
Look at the link in my EDIT, you can press simulated buttons and see the result.

Jaymie

New Member
Thank You very much Nigel Goodwin and ericgibbs. You've really helped me understand the basics - and thanks for the link.

It's so much easier now!

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