Not gate 7404 ic

[Huzzi.123]

Hello folks
Just wanted to confirm that the way to glow LED in NOT GATE 7404 IC is to connect input to ground state and output to VCC?

Check out the photo I've attached.

Look forward to receiving your responses

 

[unclejed613]

first, that's not a very readable picture..... so i'm not sure what you drew..... second, you don't tie outputs to VCC except through a resistor or some other load if it's an open collector output, and not at all if it's a totem-pole output.
third, since it's an inverter, a grounded input will give a high for the output. 74xx outputs only sink or source about 10mA on average (some less, some more depending on the TTL "family") so you need to drive a transistor to light the LED. led's will light at 10mA, but if you want it lit brightly, you should drive it with 20mA, and for that you should use a transistor.

 

[audioguru]

Your image has no contrast so it is almost impossible to see. I sketched a correction in Microsoft Paint program.

An old fashioned 7404 has Logic Inverters, it does not have Logic Gates so why call it a "NOT gate"?
With the input of an inverter low then its output is high (that is why it is called an inverter). But since the other end of your LED is also high then the LED will not light.
Your LED is connected backwards so it will not light anyway.

I connected the input of my logic inverter to +5V (not ground) so it is high then its output is low.
Then I connected the LED with the correct polarity so that it lights.

 

[sagy]

hello,
can anyone pls tell me how to use LS 7404(not gate ic) with input and pull down resistors, to on/off a led.
i tried with 220Ω but doesn't seem to help to get the output.

 

[Mike odom]

so you need to drive a transistor to light the LED. led's will light at 10mA, but if you want it lit brightly, you should drive it with 20mA, and for that you should use a transistor.

Not if you pick a 2mA LED (ex: D155, daylight readable at 2mA). Some are even rated at 10mA. You just need to pick the right one. If you're stuck driving a particular LED, then, yes, you may need a transistor.

 

[Mike odom]

An old fashioned 7404 has Logic Inverters, it does not have Logic Gates so why call it a "NOT gate"?

yes, it IS called a not gate. It's a tie over to boolean algebra, and it is how I learned it in the 70's. Any time you "invert" you are doing a logic NOT. Doesn't matter if it's tied to a larger gate, AND, OR, XOR, (NAND, NOR, XNOR), it's still a NOT GATE. The first computer I learned on used TTL modules. You opened up the modules, and it was full of transistors and resistors (and diodes). The invention of the 7400 TTL chips made boolean algebra practically obsolete. Before, you needed boolean to reduce gate count as wiring them and parts count was expensive. One gate might include several AND, NAND, OR, NOR, OR NOT inputs. But with the (relatively) inexpensive TTL chips, if didn't matter if you had redundant gates in your logic. And the very first one, the 7400, was a NAND because with it, you can build ANY other gate, AND, OR, XOR, NOR, XNOR, NOT. (see how I snuck in the NOT into my list of GATES?). And the TTL 7400 series chips still include several AND/OR input gates.

 

[Mike odom]

hello,
can anyone pls tell me how to use LS 7404(not gate ic) with input and pull down resistors, to on/off a led.
i tried with 220Ω but doesn't seem to help to get the output.

here's how I would do it, but I use a pull up resistor. To use a pull down, tie the top of the resistor to ground and the bottom of the switch to +5V... but be careful... on TTL chips, the power must be tied to the same 5V as the power on the chip, or you need to tie it via a resistor. Using a pull down resistor also, this would cause a voltage divider. TTL chips guarentee to read a logic "1" if the input is above 2.0V, so another 4.7k should work, but I'd use a 1K or less, just to be sure.

View attachment 68732

 

[audioguru]

A 74xx gate has a maximum allowed low output of at most 0.4V when it is pulling down 16mA. Pretty good for driving an LED.
It has a maximum allowed output high current of only 0.4mA which is not good for driving an LED. It can drive a transistor but the output current from the transistor will be only 4mA (more if the transistor has very high gain when saturated).

 

[Mike odom]

A 74xx gate has a maximum allowed low output of at most 0.4V when it is pulling down 16mA.

It's only allowed maximum .4V output when driving other logic gates, which take anything under .8V as a logic 0, giving a .4V noise margin. When driving an LED, you don't care if the .4V is violated, all you worry about is the maximum current per pin not being violated (includes power and ground pins).

 

[audioguru]

It's only allowed maximum .4V output when driving other logic gates, which take anything under .8V as a logic 0, giving a .4V noise margin. When driving an LED, you don't care if the .4V is violated, all you worry about is the maximum current per pin not being violated (includes power and ground pins).

The 0.4V-16mA maximum rating for an old 74xx inverter or gate might be for its max allowed heating. Then you should be careful not to exceed them when driving an LED.

NO semiconductor manufacturer calles their 7404 TTL inverters "NOT gates". They call them logic inverters. Teachers made up "NOT gates".

 

[Mike odom]

The .4V was not there to limit heating, the max current per pin is there for that. The spec reads it will still maintain .4V when sinking 16mA. Doesn't mean it won't sink more than that, doesn't even mean it won't sink 18mA and maintain .4V, just means it garauntees a .4V max @ 16mA.

And teachers also made up boolean algegra (or, engineers, actually, but teachers taught it).

And... that is what NAND stands for, Not And, NOR, NOT Nor. The bubble is a NOT function, be in on the input (inverted input and gate = NOR gate, looking for all zeros in to yield a one out,etc) or output. An inverter can be drawn with the bubble on the input side or the output side, but the funtion remains the same (NOT). And plenty of logic diagrams showed them on the inputs as well as on the outputs, it depended on what level you were looking for to activate the gate.

 

[sagy]

sure , would try that... thank you