Multiple leds on one arduino pin?

The specification of the LEDs on that first link suggests that the maximum continuous forward current is 20mA, and 30mA is the peak rating for short pulses (the second link doesn't give any current ratings). So I suggest you choose the series resistors for 20mA per LED, unless you know that your LEDs are rated at 30mA continuous. The BC337 is likely to drop 0.2v or a bit more and you can allow for this in your calculations. You may find that the red one has a lower voltage drop and needs a higher value resistor.

I have a couple of similar LEDs in 6-pin mini-DIP packages, which I think are probably these http://www.rapidonline.com/Electronic-Components/Full-colour-6-pin-LED-ndash-Chameleon-71502. Unfortunately the lead spacing is 2mm rather than 2.5mm so they won't fit in a breadboard or stripboard.

Hi dc42, yep after even more googlin' on theses leds i had settled on resistors rated for 20ma I also intend to test a few for forward voltages etc just to make sure. I'll also most likely end up putting a higher rated resistor on the green as it tends to take over when mixing yellow (from experience of other RGB's). I've actually produced my own pcb to put these on (toner transfer method and ferric chloride etch) so they fit beautifully on the board. This was my first first experiment with SMT components and it has worked well in my tests.

Thank you for your help with this I really appreciate it and will post pics and maybe a vid when I'm finished on my blog.

Well, those will be bright at 20mA!
Luminous Intensity:
RED:6000mcd;
BLUE:5500mcd;
GREEN:6500mcd

Will have to adust resistors for the colors not only for even current draw, but perceived brightness
RED:1.8-2.4V;
BLUE:3.0-3.6V;
GREEN:3.0-3.6V

Figure on calculating current based on the lower voltage drop too, for example with 5V source, 0.5 across a transistor, and 120 ohm resistor:
(5-2.4-.5)/120 = 17.5mA
(5-1.8-0.5)/120 = 22.5mA << so a higher value resistor is needed

Similarly for a 50 ohm resistor:
(5-3.6-.5)/50 = 18mA
(5-3.0-0.5)/50 = 30mA << so a higher value resistor is needed

and the data says that at 30mA
*Pulse width?0.1msec duty?1/10

Also, many datasheets (not found for this one) say that the life expectancy of LEDs drops when they are driven at the higher currents.

Looks like 1.8mm pitch on the pads, very workable by hand (vs 0.5mm on the FT232RL chips!)

The 2nd link shows different specs

Quantity:50pcs

Emitting Colour:Red/Green/Blue

Forward Voltage:3.4~3.6V

Luminous(MCD):
Red 1000~1200
Green 3000~4000
Blue 1000~1500

Not nearly the brightness, probably why they are less expensive.

Hey CrossRoads, great info thanks. I'll get calculating straight away.

I'm not overly worried about the brightness especially with 10 on each board.

I'm looking at running around 12 to 14 of these boards using shift registers. Am I gonna need a high current power supply or am I right in thinking that the transistors are doing the work?

Looks like you have a maximum current consumption of 10 x 20mA per colour per board, total 200mA (which is just fine for a BC337 to switch). Times 3 for the colours and you have 600mA total per board. Multiply that by 14 and you get 8.4A. So yes, you will need a high current power supply. One possibility is a standard ATX computer supply.

It would be easier to use higher voltage and lower current. If the LEDs drop 3.2v each, then instead of 10 led/resistor combinations in parallel, you could have 2 sets of (5 leds in series + one resistor). This would drop around 16v, so you would need (say) a 20v supply to drive it, and drop 4v in the series resistor. But it would consume 40mA per colour per board instead of 200mA. That brings the total power requirement to 1.68A at 20v, which is more manageable than 5v 8.4A.

Another option is 5 parallel groups of 2 leds in series, which would need about 9v @ 4.2A. If your boards had 12 leds instead of 10, you would have even more options, i.e. 12x1, 6x2, 3x4, 4x3.

Thanks dc42, I think a big revision is order, but its good to learn. I do have a spare ATX power supply but it is a little unsightly, so maybe a trip to Ebay is order to source a possible supply.

Can't thank you enough for your help.

The transistors just switch. The power supply gives them something to switch.

Hey Grumpy_Mike, yeah felt a little stupid after asking and not thinking about it properly lol. Thanks.

Edit: So been thinking about what dc42 said and rummaging through my box of hoarded power supplies I have found an old laptop power supply rated at 20V 3.25A DC does anybody think this might be ok?

Edit 2: Final circuit being added below! (hopefully!!!!lol)

Add up the volts drop, I think there are too many LEDs. Anything with a current limiting resistor under 50R is pushing it for constant current like behaviour.

Ahh ok so green and blue look ok but red need a rethink?

No other way round, green and blue have bigger forward volts drops so you reach 20V with less LEDs than the red.

Awww man thought I had it!

Ok so the LEDs in my circuit:

Blue Vf = 3.1V:
So 3.1 x 5 = 15.5v

Green Vf = 3.07V:
So 3.07 x 5 = 15.35V

What am I missing?

What am I missing?

The fact that on that schematic you showed 10 LEDs in series not 5. This changes the maths. :slight_smile:

Oh I thought I had done the red as 10 in series and the blue and green as 2 x 5 series each, I will check!

Edit: I've check my circuit and the red led's have 10 in series and the blue and green have 2 x 5 led series in parallel each.

The green and the blues look ok, but the voltage drop across 10 red LEDs is too close to 20v for the series resistor to regulate the current well enough. Go for two chains of 5 red leds too.

Thanks dc42 so should be good with this final schem!

Yes looking good. :slight_smile:

Okey dokey, off to produce a few pcb's. Can't thank everyone enough for your patience and help.