# How to start a LED using a TTL circuit

Discussion in 'Homework Help' started by ADHD, Jan 24, 2011.

Jan 18, 2011
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The LED has 3V @ 60 mA.

As far as i know, the TTL has at least 2,4V @ Voh, but i don't know the current.
I also remember that a transistor is a current amplifier, but what about the voltage ?

2. ### Wendy Moderator

Mar 24, 2008
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Generally it is better to use the open collector version of TTL, that way you get the entire power supply voltage to play with. Something like the 7447 for example, it was meant to drive LEDs.The 7405 is an open collector versions of a hex inverter.

If all else fails you can use a transistor. Basic TTL circuits are not meant to drive LEDs.

How much experience do you have?

Jan 18, 2011
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I'm just a programmer enrolled in a Digital Circuits class and i have a final in less than 24 h.
I only need to know this stuff theoretically ( circuit diagram ).

4. ### Wendy Moderator

Mar 24, 2008
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Here is how you would use a TTL gate open collector...

Thing about open collector, Vcc can be up to 15VDC.

You can do something similar with regular TTL outputs. You need to consult your notes and materials, since they may have their own ideas, but personally I wouldn't try to drive a LED directly using this (Wikipedia TTL schematic)...

The TTL current output would be 6ma. Hmmm, maybe it would work. Not something I would do, but plenty good enough to light an LED.

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5. ### Georacer Moderator

Nov 25, 2009
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The 74XX familly has outputs that reach almost always the HIGH level of Vcc and have a maximum current output of 40mA.
Voltage ratings drop when the load exceeds the 40mA.

So that particular LED you are referring to wouldn't work with a 74XX IC. A normal LED with 10mA @ 2V or so would need a current liming resistor when hooked in a 74XX output.

6. ### Wendy Moderator

Mar 24, 2008
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I believe you could use the ground side similar to how the open collector works, but not the positive side. It very much depends on the exact family (datasheets hold the answers there), but I believe most TTL come out with 3.4 as a high.

7. ### Audioguru AAC Fanatic!

Dec 20, 2007
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No and no.
The minimum output high voltage of old TTL is only 2.4V.
The minimum output high current is only 40uA and the max allowed output low current is 16mA.

8. ### Wendy Moderator

Mar 24, 2008
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I suspect AG and I have had to deal with a lot of TTL. When I was going to college in the late 70's they were cutting edge, held it longer than DTL and RTL, but are pretty well obsolete. There were a lot made however, so the schools are busy burning up the remaining supplies.

That 3.4VDC is a pretty hard number for a high for TTL, it is obvious why if you look at the schematic on post #4. A low is any voltage 0.7V or less, again the schematic tells the tale.

Thing I find interesting about TTL is that, unlike most other logic, if you don't tie the input to a logic state it will pretty reliably assume a high for the open input. I wouldn't recommend doing it, but it will.

Since there is a transistor going to ground (the usual output is much closer to 0.1VDC than 0.7VDC) you can use it to sink very well.

9. ### Georacer Moderator

Nov 25, 2009
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My bad. Seems I have to make a serious revision of the 74XX family characteristics.

I re-read the 74LS00 datasheet (http://www.datasheetcatalog.org/datasheets/70/232209_DS.pdf) and re-state:
A 74LSXX can typicaly hold 3.4V at its HIGH output (2.7 minimum),
and can give 0.4mA on HIGH output or sink 8mA on LOW output.

It can safely sink no more than 0.34mA on its input.

Other variations of the 74XX family might have slightly different characteristics.

Sorry for the misinformation.

Apr 5, 2008
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11. ### Audioguru AAC Fanatic!

Dec 20, 2007
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I learned RTL then I used DTL at my second job.
I worked with a huge office computer in 1970? It used punched cards for its program and core memory (thousands of little ferrite donuts with a grid of wires). The computer was connected with wire-wrapped joints.
The computer had a intermittent serious problem that I fixed by piggy-backing a DTL gates IC on top of another IC and a couple of connections.

I remember the spec's for a 74xxx TTL was a minimum high of only 2.4V. The minimum high for a newer and better 74LSxxxx was 2.7V. That was before blue and white 3.5V LEDs were invented.

12. ### Georacer Moderator

Nov 25, 2009
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Great find bertus! I didn't know the 74HCXX had such a strict output profile!

13. ### Audioguru AAC Fanatic!

Dec 20, 2007
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74HCxxxx outputs have lots of available current (about 60mA) but the max allowed output current is 25mA.

I have used many 74HCxxxx ICs and all of their outputs go to the positive supply and to ground when their output current is low or nothing.