I have watched few videos from Ben Eater, who is uploading videos about his 8bit computer on youtube and I have noticed, that he is useing the outputs of 74LS chips to directly source current for LEDs. I have looked into the datasheets, but almost all of these chips can source only about 0,4mA and normal LEDs require about 10mA and low power LEDs about 2mA. So how is this possible?

 

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he is useing the outputs of 74LS chips to directly source current for LEDs.

That is not a recommended design practice. I've seen a lot of junk on YouTube; little information is peer reviewed.

I have looked into the datasheets, but almost all of these chips can source only about 0,4mA and normal LEDs require about 10mA and low power LEDs about 2mA. So how is this possible?

LEDs will emit light at much less than 10mA; albeit at lower intensity.

 

Thanks for the reply, anyway, after a long search I have found a thread where someone stated, that those chips can in fact source much more current than 0.4mA, but at 0.4mA the output is guaranteed to output enough voltage to drive another chips. So that explains a lot.

 

I have looked into the datasheets, but almost all of these chips can source only about 0,4mA...

Ummm... not quite; they are not guaranteed to maintain a valid TTL logic "1" output voltage level while sourcing more than 0.4 mA, but they can certainly source a lot more then that when not called on to maintain a valid "1" level-- such as when driving an LED.

EDIT: I see you found the answer while I was writing the above. Good; all is well...

 

I have watched few videos from Ben Eater, who is uploading videos about his 8bit computer on youtube and I have noticed, that he is useing the outputs of 74LS chips to directly source current for LEDs. I have looked into the datasheets, but almost all of these chips can source only about 0,4mA and normal LEDs require about 10mA and low power LEDs about 2mA. So how is this possible?

Not recommended practice.
If you wish to drive LEDs from 74LS chips, drive the LED from the Vcc supply using an inverter as a current sink.

If you wish to go ahead anyhow and drive an LED from a 74LS gate as a current source, definitely DO NOT use the output as a valid logic source to another gate input.

 

I have watched few videos from Ben Eater, who is uploading videos about his 8bit computer on youtube and I have noticed, that he is useing the outputs of 74LS chips to directly source current for LEDs. I have looked into the datasheets, but almost all of these chips can source only about 0,4mA and normal LEDs require about 10mA and low power LEDs about 2mA. So how is this possible?

TTL can sink a lot more current than they can source.

You can order ultra-efficient LEDs that give useable light at 2mA.

LEDs were once only just adequate for indicators - but logic designers seemed to muddle through somehow.

The ideal solution is buffer transistors - there's various DIL transistor arrays that take less board space, or the well known family of Darlington arrays in either 7 or 8 bit versions.

 

I have watched few videos from Ben Eater, who is uploading videos about his 8bit computer on youtube and I have noticed, that he is useing the outputs of 74LS chips to directly source current for LEDs. I have looked into the datasheets, but almost all of these chips can source only about 0,4mA and normal LEDs require about 10mA and low power LEDs about 2mA. So how is this possible?

Realize what that specification is. That is 0.4 mA and keeping the output voltage to a legal high value. If you are not driving logic with that output that spec is meaningless. You can use the current limiting of the high side driver as the ballast resistor. Agreed, "not recommended", yes, I agree, but it works such as it is.
"Not recommended" suggests that this is not recommended for a product for commercial use. I agree. I use it for my own stuff or for test fixtures where techs can replace things if they fail.

 

Thanks for the reply, anyway, after a long search I have found a thread where someone stated, that those chips can in fact source much more current than 0.4mA, but at 0.4mA the output is guaranteed to output enough voltage to drive another chips. So that explains a lot.

Yes, and LEDs labelled "high brightness" are quite bright at just one mA so at least as bright as an old-school indicator LED at 0.5mA. Just add 1k resistor in series with the LED.