# Logic Gates

Discussion in 'Electronic Basics' started by Bart, Dec 12, 2005.

1. ### BartGuest

I look at all the symbols for the different logic gates, NAND, NOR, XNOR,
etc. and on some of them they have a little circle on the output. I look at
datasheets for different IC's and I see the same little circle on some of
the pin designations. My electronics books show these but I can't find what
it means.
Any help is appreciated,
Bart

2. ### RichardGuest

First, if your book doesn't cover this material, you need a different book.
You'll see these circles on inputs and outputs in various schematic
diagrams.

Try a Google search for 'logic gate symbols'.

The first hit I got was http://www.kpsec.freeuk.com/gates.htm, and there
were several others.

Richard Seriani

3. ### John PopelishGuest

A small circle on either input or output means an inversion takes
place, there. So an OR gate with a ball on the output means that the
output is low when any input is high, instead of high when any input
is high. It just takes the output from an OR gate and inverts it.
The result is called a NOR (not OR). It would have been more logical
if it had been called a ORN (OR not) but that is harder to pronounce,
I guess.

4. ### flankGuest

Bart,
An excellent resource when working with a digital IC is the device's
datasheet. There are many places to find the datasheets, but one in
particular is datasheetarchive.com. You will need to do a search for
whichever part you are looking. The sheets are often many pages,
containing quite a bit more information than you might need. But the
first page (sometimes the second) shows the truth table for the device.
The truth table is simply a table indicating all the possible inputs
to the chip, and the resulting output. The datasheet also has the
pinout diagram, which is always nice to have.

Flank

5. ### BartGuest

Thanks a bunch, makes sense now! Oh, if the output is high I assume I
can source an led to turn on, and if an output is low, I can sink an led to
come on?

Bart

No.

RATS

8. ### Big Mouth Billy BassGuest

"Rats" spelled backwards is "Star."

9. ### Rikard BosnjakovicGuest

Source, yes. Usually.

Sink, no. Unless it's an open collector.

10. ### Jasen BettsGuest

yes, depending in the device some devices can't source much or can't sink
much.

11. ### David L. JonesGuest

Usually, yes.
A typical logic gate is capable of sourcing or sinking enough current
to drive a typical LED.
The output voltage may be out of spec when drawing this much current,
but that doesn't matter if you are just driving a LED and not another
logic gate as well.
Some gates have open-collector outputs which can only sink current.

Dave

12. ### flankGuest

Bart,
Not all chips supply/sink the same amount of current. For the most
part, ICs cannot supply or sink much current at all. An led needs
around 5-15 mA. This is stretching it for some ICs. A 7404 (hex
inverter gate) can sink a pretty fair amount a current. I typically
wire my LEDs such that the high pin is at 5V, and the low pin is tied
to the output of an inververter gate through a 330 ohm resistor. Then,
when I send a high signal to the input of the hex gate, the output goes
low and turns of the LED.
Also, it is a good idea to isolate the signal used to drive your LEDs
away from the logic signals. An LED can draw enough current from a
gate to cause the gate to malfunction.

Flank

13. ### Rich GriseGuest

No, it's because it's not an "Or Not". An "Or Not" has only one input
inverted. (X = A + /B) ;-)

Cheers!
Rich