Drive a led with NAND gate

Do you think its a good idea driving a led without resistor with a chipe like 74hc14an?

It says it will give up to 20mA per output,

Regards.Aruen

Aruen:
Do you think its a good idea driving a led without resistor with a chipe like 74hc14an?

No.

It's NEVER a good idea to drive a LED without something to limit the current.

Aruen:
It says it will give up to 20mA per output,

But only 50mA for Vcc ... so you can only turn on two outputs at 20mA

http://www.mouser.com/catalog/specsheets/mc74hc14a-d.pdf

HC family outputs drive about 5 or 6mA max, not 20mA, read the specs more carefully.

Anyway which NAND gate? The 74HC14 is a hex inverter.

MarkT:
HC family outputs drive about 5 or 6mA max, not 20mA, read the specs more carefully.

Anyway which NAND gate? The 74HC14 is a hex inverter.

Ok. Sorry you are right its not a nand.

fungus:

Aruen:
Do you think its a good idea driving a led without resistor with a chipe like 74hc14an?

No.

It's NEVER a good idea to drive a LED without something to limit the current.

Aruen:
It says it will give up to 20mA per output,

But only 50mA for Vcc ... so you can only turn on two outputs at 20mA

http://www.mouser.com/catalog/specsheets/mc74hc14a-d.pdf

The chip already limit the current. isnt it?

Thanks for your answer.

Is there a chip i can use to drive 15 leds with 20mA each led? Preferible without resistor.

MarkT:
HC family outputs drive about 5 or 6mA max, not 20mA, read the specs more carefully.

The 74hc14an ("an" variant) can do 25mA. I linked to the datasheet above.

An LED driver chip is the obvious choice(*). Failing that a high current buffer like a 74HC245 would handle perhaps 15mA
per pin without too muh voltage droop.

Most multi-channel LED drivers are surface mount though.

ULN2803 octal darlington driver is another possibility.

(*) TLC5940 data sheet, product information and support | TI.com

Aruen:
Is there a chip i can use to drive 15 leds with 20mA each led? Preferible without resistor.

I prefer chips with "LED driver" in the name.

eg. TLC5916 TLC5916 data sheet, product information and support | TI.com

There's others, too: LED display drivers product selection | TI.com

fungus:

MarkT:
HC family outputs drive about 5 or 6mA max, not 20mA, read the specs more carefully.

The 74hc14an ("an" variant) can do 25. I linked to the datasheet above.

It cannot source or sink anything like that into a real load. Read the output voltage specs and
you'll see the output resistance is about 200 ohms, so at 25mA there is no output voltage at all,
its the short-circuit current. If you want to drive an LED you'll need to provide enough voltage.

The bus-driver outputs on a HC245 have approx 50--100 ohm output resistance so will handle
perhaps 10 to 15mA reasonably.

Electronics technicians and engineers have been getting it wrong for 50 years. All those wasted resistors… the horror.

Yes, I’m being half serious here and not aiming my comments at you personally, but we constantly see people wanting to skip the current limiting part of a design by employing what I see as misguided rationalizations for why a simple and inexpensive part can be omitted.

TTL Logic chips deliver logic level signals to other TTL logic chips. Nothing in the design of a standard TTL logic chip is designed to drive non-standard loads (something other than another TTL logic chip) unless the chip has the word “buffer” or “driver” in its name.

Any other implementation constitutes “chip abuse” and while it may not be obvious right away… you are harming the chip (and likely the LED if it is a typical red one) when you ask the chip output to deliver maximum unrestricted current to an LED.

It’s not best practice. A well trained technician or engineer would not do it. But how you abuse your own chips is your own problem and your own choice… just remember to buy more parts than you need. And while you are at it, buy a big resistor assortment for say $9.00 and you will be all set for YEARS without having to come up with excuses about why you want to skip using a resistor. They exist for a reason… Engineers don’t design them into circuits to make things cost more or to just annoy people.

pwillard:
we constantly see people wanting to skip the current limiting part of a design by employing what I see as misguided rationalizations for why a simple and inexpensive part can be omitted.

That and the 20 mA fixation.

pwillard:
TTL Logic chips deliver logic level signals to other TTL logic chips. Nothing in the design of a standard TTL logic chip is designed to drive non-standard loads (something other than another TTL logic chip) unless the chip has the word "buffer" or "driver" in its name.

I wish you luck in your quest to convince the Internet that the 74HC595 isn't the key to driving multiple LEDs with Arduinos.

You mean LEDs work at less than 20mA??? :astonished:

Heretic!!

I wish you luck in your quest to convince the Internet that the 74HC595 isn't the key to driving multiple LEDs with Arduinos.

One read of the data sheet will explain why we are constantly abusing this poor IC. If you draw 20mA per pin off of a HCT device continuously, you will certainly drive it outside the Absolute Maximum rating listed in the Datasheet and assuming harm is not done is just wishful thinking.

And for the curious, I have a retro collection of 74LS family logic chips... some of which I have "tested to destruction" on purpose to see how far outside Max Specs you can go before death. Most chips are pretty hardy... but I have also seen chips perform ever so slightly differently after being abused if I didn't actually make it release smoke.

There are a number of people making electronics for fan made props whom you could never convince to use resistors with LEDs on CMOS chips like the 4000 series.

MAX7219 can drive 15 LEDs at 20mA with no resistor.
Also TLC5940.
Also WS2803.

All use 1 resistor to set the current that goes to the outputs.

Another useful chip to have in your box is the AN6884 (or KA2284 - same thing).

It has five 15mA current sink pins that you can use to drive LEDs instead of resistors. It's usually less soldering/board space than the resistors

It also has an 'enable' pin so you can connect 5 LEDs to Vcc then switch them all on/off together with a single Arduino pin. PWM works great and you can connect as many chips as you like to the Arduino pin - switch huge banks of LEDs!

You can also use it without an Arduino if you just want to add some LEDs to something for decoration (none of those other chips can do that!), plus it's a VU meter so you can connect a sound output to it and the LEDs will flash with the music.

And ... they're dirt cheap. You can get 100 of them for $18: http://www.ebay.com/itm/291094048277

Those are not 4000 series CMOS, they are made to drive LEDs and have an internal current regulator set by that resistor. A CD4017 has no such thing built into it, and the limit on current is not meant to be used to drive resistors without a current limiting resistor.

The 3914, 3915, and 3916 all work in a similar manner to the MAX7219. The current through the input scaling resistors sets the current regulation in the LEDs.