Multiple LED's, one drop-down resistor?

Hi everyone,

I wonder if it's permissible to use one resistor to allow multiple LEDs sourced from separate Arduino pins to light up. I'm using 3mA LED's and 2.2k resistors. I'd like to attach one end of the LEDs to individual digital output pins while the other ends are attached to the resistor, followed by ground. By sharing a single resistor, one can reduce the component count a bit. But is there is a practical limit to the number of small LED's using one common drop-down resistor from the LEDs to GND? Or is there a reason one should only use series LEDs and resistors in pairs?

As I recall, 0805 resistors can handle 1/16th of a Watt so at 3mA that would be about 20 LED's. My application would allow me to 'bundle' 3 LEDs with one resistor, at most...

If only one of those LEDs/outputs were on at any time, that would be OK. Some people here argue against resistors altogether. What's a guy to do? Try it without an Arduino - just have LEDs in parallel, commoned to one resistor, same thing pretty much. How does that work out?

Better read this about LEDs, resistors, etc.

http://forum.allaboutcircuits.com/showthread.php?t=19075

you can but you will not get even brightness, ie if only two or three are on, it will be brighter than all of them on

You can reduce your apparent component count by using a DIL resistor unit such as shown here

This comprises 8 discrete resistors built within a standard DIL package

[quote author=Nick Gammon link=topic=99623.msg747168#msg747168 date=1333422670] Better read this about LEDs, resistors, etc.

http://forum.allaboutcircuits.com/showthread.php?t=19075 [/quote]

Mr. Gammon, Many thanks for the link! However, I see no information there on what I'm considering, i.e. having three LEDs with separate 5V current sources drain into one resistor. Each of the 5V pins can be turned on and off individually. Please see the below.

OK, well check reply #3.

Constantin: [quote author=Nick Gammon link=topic=99623.msg747168#msg747168 date=1333422670] Better read this about LEDs, resistors, etc.

http://forum.allaboutcircuits.com/showthread.php?t=19075

Mr. Gammon, Many thanks for the link! However, I see no information there on what I'm considering, i.e. having three LEDs with separate 5V current sources drain into one resistor. Each of the 5V pins can be turned on and off individually. Please see the below.

[/quote]

Those are voltage sources, not current sources.

Thank you both for your replies, you are both correct! Brightness was unlikely to be even in the first place - the voltages sources (thank you, MarkT) are usually at slightly different potentials. I was just trying to think of a way to reduce the part count on my board without affecting performance... so I'll drop the idea and go back to the previous configuration, i.e. individual resistors for each LED, just like the Arduino boards.

Some people here argue against resistors altogether.

That's crazy talk... not using resistors where current limiting is needed is how components stop working. Having devices share a resistor means that you allow no more than a specific current to flow for all paths so each path, if conducting, only gets a portion of what is available. Translates to brightness changes with leds

So, resistors are cheap, buy a lot of them, use them as needed. Don't be lazy. Lazy circuits usually make smoke.

pwillard: ...resistors are cheap, buy a lot of them, use them as needed. Don't be lazy. Lazy circuits usually make smoke.

For sure. Though I have seen LEDs marketed that incorporated a drop-down resistor for 12V use, among other voltages. I guess they were aimed at replacing other types of indicators such as incandescent light bulbs.

At 2.4 volts you can actually use LEDs without resistors. There isn't much opportunity for high currents to develope. Moreover, having resistors, you don't get enough brightness. But that's a special case. Also many ICs seem to have some kind of current limiting built in already.

There are nice SIP resistor networks, 4 resistors, one common ground. Futurlec has them.

It's OK for indication LEDs to share a resistor, however, this isn't professional (unless you use some tricks). You get brightness variation if more than one LED is turned on. Also I never really see it in circuits or on PCBs. If you only light one LED, it's the same as having 3 resistors. You could even multiplex 3 or more LEDs only for the purpose to save resistors (using software).

Using high efficiency LEDs for indication, it is not much sense to leave out the resistor- you get disturbing high brightness, and with some ICs, the LEDs will burn out.

What people normally do is:

-use resistor networks (SMD), however, can be hard to solder. -use SIP or DIL resistor networks, some vendors have them at good prices. -use smaller 1/8w resistors not much difference to 1206.

The 74hc164 actually has 120 Ohms built in (Texas Instruments datasheet). It's still too much brightness for indication (can be disturbing), unless you switch off (reset) the serial register very quickly after loading data.

How many LEDs do you use on your PCB? And what they are used for, actually?

Removing components isn't lazy. Examine a common "old world" VCR, and you see PCBs actually having many empty spaces, originally intend for various components.

I see it repeating endless again and again mysterious lazy hobby tinker people using LEDs, and the same popping up in smoke, or burning out quickly.

I ask to people who write such replies: How many real world cases actually do exist, where people made PCBs without LED resistors, and they popped up? And what's the matter if you fry one 5 cents LED? How many people actually have done it again after they burned one LED?

I guess it's more kind of a rethoric reply, and such hobby tinker people are only (forum based) fiction.

I made some CPU bus panels, having 32 small LEDs altogether, plus 32 resistors. I wasn't interested in high brightness, only to see the binary signals resulting in LED lighting up. The resistors are required since a CPU bus does not have so much power. It's not really serious point of discussion to leave out the resistors here.

Can't we establish one simple single line forum rule here: "Fry LEDs at your own risk"? Or someone could create a web tutorial only about LEDs and resistors, and how to drive LEDs using various ICs/controllers. Seems to be a popular topic on many forums.

pwillard:

Some people here argue against resistors altogether.

That's crazy talk... not using resistors where current limiting is needed is how components stop working. Having devices share a resistor means that you allow no more than a specific current to flow for all paths so each path, if conducting, only gets a portion of what is available. Translates to brightness changes with leds

So, resistors are cheap, buy a lot of them, use them as needed. Don't be lazy. Lazy circuits usually make smoke.

Of course a lot of LED driver chips are constant current drivers in order to reduce the part count.

Actually I tried connecting various LEDs without resistors. I measured the currents, which range from 40mA to 80mA. All too much for regular LEDs. And too much for the Arduino. Especially so if you use more than one LED this way. I wrote up the results into a new thread.

Again all what I can say is the best way usually is to examine existing circuits, and then do what the mayority of people are doing. Or use some chips in a clever way, if you know what you are doing.

It is really not wrong to use resistors, if you have many LEDs, organize them in a matrix, and multiplex them. Sharing LED resistors like the OP suggested, I never saw this in an existing circuit.

What about specific to the arduino itself? If you say, wanted 4 separate indicator leds on 4 output digital pins? Brightness not being as important as showing on or off.

(and yeah, I realize this is an old thread, but redundant threads bother some, zombie threads bother others. Tomato, tomahto)

Use a resistor per LED with each output.

(and yeah, I realize this is an old thread, but redundant threads bother some, zombie threads bother others. Tomato, tomahto)

So we should read through the whole thread to understand your question?

Why not just start a new thread and ask your question?

.

Also many ICs seem to have some kind of current limiting built in already.

My car has a speed governor in it as well, also known as “breaking” (not braking).

I know this is a zombie thread, but I just can’t leave a statement that stupid go unchallenged.