Usind two LED's in series instead of LED plus resistor

Guys, I need to make a panel for warning lights driven by an Arduino. Essentially the Arduino pin output goes straight to the LED, and I see in normal use you woul add a resistor in series to protect the components. However I plan on using 3.0 - 3.2v 0805 SMD LED's, and require two for each warning light.

The question is, can I wire the two LED's in series, and do away with the resistor?

Additionally, as there will be 12 warning light outputs per Arduino (I was thinking of using Nanos) would it be best to put a 12v power supply to the VIN pin?

Cheers

Les

Lesthegringo:
However I plan on using 3.0 - 3.2v 0805 SMD LED's, and require two for each warning light.

The question is, can I wire the two LED's in series, and do away with the resistor?

Well, it might work. Two 3.0 V LEDs in series would obviously require 6 V and you should always provide current limiting. it is possible that they will in fact light on only 2.5 V each and will then limit their current draw by their own internal "resistance". I suppose you could try it out, but I doubt it would be satisfactory. Better in parallel with a 470 Ohm current limiting resistor for each

Lesthegringo:
Additionally, as there will be 12 warning light outputs per Arduino (I was thinking of using Nanos) would it be best to put a 12v power supply to the VIN pin?

You need to understand something basic about Arduino boards.

The on-board regulator is absolutely not provided to power external equipment. The exception may be sensors which require only a few milliamps to power, and a few indicator LEDs at 20 mA each (with a current limiting resistor).

No motors, no LED strips, no relays or solenoids. The USB input is able only to supply just a little more current.

To power these things at 5 V, provide a proper power supply. Almost always this means a regulated switchmode mains power supply (such as a "plug pack") or "buck" regulator from a supply such as a car battery.

Lesthegringo:
In normal use you would add a resistor in series to protect the components.

Hello Les,

The resistor isn't there 'to protect the components' (although that is one effect of it being there), it is there to limit the current through the LED because LEDs on their own do not control current very well. LEDs have a voltage (which is different for different colours) at which they start to conduct. Below that voltage they don't conduct at all, slightly above it and they become almost a short circuit. Because of this it is not possible to just connect them to some particular voltage and have them work properly. Small variations in voltage and temperature will cause big variations in current. The usual solution is a resistor to control the current to the value required for the LED.

Thanks guys.

I have a couple of unos and some 9v ac pwer packs that have the right plug for the unos ( I think they actually came with the Unos as part of a starter kit) so I assume that they would be ok?

As for the LED's, I will do a break out board for the resistors, it's messier than I would have liked but there you go. How many outputs for LED's can each Uno do?

Cheers

Les

Lesthegringo:
I have a couple of unos and some 9v ac pwer packs that have the right plug for the unos ( I think they actually came with the Unos as part of a starter kit) so I assume that they would be ok?

I repeat - OK only if you do not need to connect anything to the UNO that draws significant power.

Three LEDs at 20 mA each probably OK. 12 LEDs all lit; almost certainly trouble at 240 mA.

How many outputs for LED's can each Uno do?

There are current limits and can get complex. But basically all the current for the outputs come through the 5V pin on the processor, ( this is not the same thing as the 5V pin on the Arduino board ). You should not ask more from the 5V pin than 200mA but other things come into play as there are limits to groups of outputs as well. The data sheet says:-

Although each I/O port can source more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
ATmega48PA/88PA/168PA/328P:
1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150 mA.
2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150 mA.
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.

Also consider the question how many glasses of water can I get from a three pint jug?
The answer depends on the capacity of each glass.

In the same way your question depends on how much current you want to drive through your LEDs.

Use the minimum Vin voltage possible, 7v is great.

Always use current limiting with LEDs.

As mentioned, follow the safe current levels recommended when powering outputs with your Arduino.

A L W A Y S
Show us a good schematic of your circuit.
Show us a good image of your wiring.
Give links to components.
Posting images:

Guys, I'm reviving this as there is a different tack now due to suggestions by other people to use a MAX7219 to control the LED's as a matrix. This means only having to use one Nano instead of four minimum

I'm good with the use of the matrix and the sketches, but looking at the MAX7219 datasheet it shows that the pin output voltage is 3v, and that is the inpiut voltage of the 0805 SMD LED's. If I want to use just one, easy. Unfortunately, as the LED's are required as back lighting I really need three for each light. As the voltage is already 3v out of the MAX chip, it means I can't put resistors in. The current rating of the green 0805 LED's is pretty tiny so I don't believe it would overload the MAX chip, but am wary about other issues mentioned above. Will three 0805 LED's in parallel on one output pin cause problems?

Cheers

Les

As the voltage is already 3v out of the MAX chip, it means I can't put resistors in.

There is no need to put any resistors in line with the LEDs when using a MAX chip. There is a single resistor that sets the current for all the LEDs. The MAX chip contains a constant current drive and there is software control on the brightness. Not on each LED but on groups of them, see the data sheet for what.

Will three 0805 LED's in parallel on one output pin cause problems?

Yes don’t do it.

How many LEDs do you want to control? Each MAX chip can control 64, and you can chain MAX chips together almost without limit.

Lesthegringo:
but looking at the MAX7219 datasheet it shows that the pin output voltage is 3v,

Where do you see that? As far as I know the max voltage is Vcc.

Lesthegringo:
it means I can't put resistors in.

You don't need to. The MAX7219 is the current limiter. From the datasheet:

Only one external resistor is required to set the seg-ment current for all LEDs.

You - OP - have failed to specify your requirement. This always makes it much harder to explain things to you. You said:

Lesthegringo:
there will be 12 warning light outputs per Arduino (I was thinking of using Nanos)

So how many is it?

If you need three LEDs per warning light, then a MAX7219 can drive 64 LEDs, so you can have 64/3 = 21 warning lights (per MAX7219) with one resistor (and a couple of capacitors), controlled by just three pins for any number of MAX7219s.

So how many is it?

Nanos are of course, the most practical Arduinos for a real project unless you need WiFi.[/b][/b]

It is a matrix of 4 x 12, which gives 48 'lights' in total, however each light covers a broad rectangle, so to ensure even light coverage I wish to use three SMD LED's for each light, all illuminating together.

Initially I believed I had to use four nanos due to a max of 12 outputs per nano, but by using the MAX7219 setup, it can be controlled using one. My problem is the multiple LED's per light.

Hope this helps explain

Cheers

Les

My problem is the multiple LED’s per light.

No it is not. Just use the MAX to turn on several LEDs at the same time.

So as I interpret that, you require 48 times 3 - 144 LEDs. Three MAX7219s would do that, you would program each of them for a 6 by 8 matrix so that they multiplex by 6 rather than 8, giving you slightly more current drive. And the "6" of the 6 by 8 would be two groups of three and so sharing a common pin.

Thanks, a bit more complicated for the PCB, but I see what you mean. I wish I was able to get PCB’s made locally here like you can in other countries, but alas Hungary is not well set up, and even Ebay don’t like sending stuff here. I will have to try and produce my own

Cheers

Les

Lesthegringo:
Thanks, a bit more complicated for the PCB, but I see what you mean. I wish I was able to get PCB's made locally here like you can in other countries, but alas Hungary is not well set up, and even Ebay don't like sending stuff here. I will have to try and produce my own

Cheers

Les

Sounds like a business opportunity...

Why use a PCB? Strip board is just fine if you need to make one or two.