Comparing LED brightnesses (prob a simple question)

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paradigm x
 
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Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

All

Thinking of having various different LED colours in different locations, so trying to test first to get about the same brightness. But having a weird issue with test circuit (prob something stupid im doing).

I have my variable PSU giving 5v on the breadboard and have the blue LEDs with 1k resistors in the kit i have, no worries. But when i wire up a green on in parallel, the blue dims significantly. But if i remove the blue, the green stays the same. :? I.e. the green is the same brightness whether or not the blue is in parallel, but the blue changes if the green is in parallel.

Not quite sure what going on there.

The green arenet diffused but the blue are.

Not quite sure what to make of this seems a bit odd. Will something similiar happen in the x0x itself?

Also the green LEDs seem to need a 150ohm resistor to match the blue (by quickly touching on and off to compare, does this seem a reasonable value?

ive read all the threads i can find relating to leds in here.

MAny thanks

Ben

Luap
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by Luap »

I don't recall the specifics as to why, but it not good to wire LED's in parallel. Particularly when they are different kinds. I would try it again, giving each LED it''s own resistor.
Blue's (and whites) are usually more current hungry than your regular green's & reds too I believe.

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

yeah sorry if unclear, each did have its own resistor.
edit;

Image

this was my test 'rig'

Weird thing is; green led stays same brightness whether or not blue is connected or not. Blue changes (dims) when green is connected.

not sure i understand why. Was hoping to get a few diff coloours in a row to make as similar brightness as possible.

Cheers

Ben

Luap
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by Luap »

Aha, I have a x0x with some substyler sourced blues in it too (Nice LED's). I still don't know the specifics of them though! But I would guess they still draw more current than the greens. Hence the odd behaviour? Incidentally, 150 ohm for the green? That seems very low.. Most LED's seem to want something around 1k & 2k in a x0x. Anything outside of that is usually down to odd spec LED's, like the retina burning ones etc. If your greens are pretty regular, then they'll almost certainly want more resistance than that..
Sometimes the LED's look ok with those low value resistors, but you'll find that with 40 of them, they will draw so much current when you power up the x0x, that it won't boot properly, as the resulting power sag starves the MCU. So it might be worth rechecking what those greens need.

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

yeah i thought the 150 was a bit low but thinking about it i used 220ohms for my shruthi LFO leds.

Theyre only 5mcd, so prob right (?)

Using online calculators, and the LED specs (green are 2.25V, 20ma) 137 = 150 is calculated. Just seems a bit odd. Most people in threads on here seems to be using super bright purple/wite etc leds tho so maybe right. it looks about the same :)

But now im not sure.

Argh. torn. Half thinking just solder in all the blues as so close, half thinking well might as well do it exactly how i want since im building it! Just a bit worried about this current draw now. And how diffused and nondiffused will look together.

Oh well, a few more components to do on the main board while i think about it.

Cheers

guest
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by guest »

repeat your test
but put a multimeter on your 5v line
and see if it changes any when you put the green on
my guess is that the 5v line drops 100mV or so
as you draw more current

most blue leds have a forward drop of 4v or so
so 5v-4v/1k=1mA
whereas greens are around 2.5v
so 5v-2.5v/.15k=17mA
so the percent change in current is large
when putting the greens on the line
but very small when taking the blues off

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

Hmm, interesting stuff, and makes sense.

But now i have another wierd issue; my variable psu wont put out more than 3.75v - ive changed batteries too. Ill recharge them and have another go. Do these LEDS drain current a lot then. They were probably on for half an hour or so while i was playing with the LEDs.

Ill give it another go later once the batteries have recharged for a while.

But ultimately, apart from my testing, is it ok to use small resistors if it gives the same brightness? I see what you mean, taking 17ma against 1ma. Should i aim for the same current draw for all LEDs?

Many thanks guest as always :)

Ive done eveything else now, apart from the switches ! So close :D

Cheers

guest
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by guest »

you should test with a good 5v supply
and match the brightness

you can use the 5v from your x0x
to run the tests
just clip some wires to it

ive seen some problems
with the leds drawing too much current
but that is usually for all the leds being at 20ma
if you have half of them at 16ma
it shouldnt be a problem

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

Thanks

No im only planning 4x green ones on 1, 5, 9 and 13 of the sequencer lights, and possibly a couple of other different ones on rest accent slide and tempo. so thats only 8 out of 40 i think.

Forgive me, where do i get 5v ? I just reread the Power supply build notes and can only see 5.33 and 12V

Many thanks

Ben

guest
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by guest »

if you have disconnectable jumpers
than you can get it directly off j3
pin3 for 5v and pin8 for ground

otherwise follow those traces
to find an easy place to clip to

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

excellent ive not soldered j3 yet, should be a doddle!

Nice one

Thanks

Ben

:)

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

hi

been playing with some resistors and LEDs and have come up with some nice matching brightnesses :) It all works fine taking the power from the x0x :D

What i need to know is what is the maximum current drain allowable without problems.

Using your formula above ive made a spreadsheet to calculate the overall current draw using various types of resistors/LEDs.

From What youve said half of them green would be OK, which implies that 20x17+20x1 = 360mA should be ok. (?)

Cheers

Ben

Luap
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by Luap »

That will largely depend on your power supply for the x0x of course.. 500ma should work, 300 might not!? I've had exactly this happen before, due to power hungry LED's. So long as you get it work ok on 500ma or less though, then it's all good! (Much more than a 500ma supply is not recommended)

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paradigm x
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by paradigm x »

Hmm cant beleive i never put two and two together... ive got a 300ma wallwart.

I was going to ask but forgot too that hopefully/surely any over current drain issues could be resolved with a beefier wall-wart, rather like the shruthi with certain LCDs.

But only about 5 or 6 LEDs are ever lit at any given time, so maybe not an issue ?

God i feel stupid sometimes. :oops:

Also have spent far too long worrying about a completely trivial things (colours) when i could have had it done by now. But then part of the whole synth DIY stuff is in the visuals.

Cheers

Luap
 
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Re: Comparing LED brightnesses (prob a simple question)

Post by Luap »

Not that stupid ;) You are right, ordinarily only a few LED's are ever lit at any one time during normal operation. Perhaps no more than 4 or so? However, thats not always the case when you first switch it on. You can get a random selection of LED's for a second or so. Which could be anything from 1 of them to all 40. The more come on, there more current is drawn, and the more the power rails can sag and starve the MCU, preventing it from booting.
When I had this issue on one of my x0x's, perhaps 4 times out of 5, it wouldn't boot with a 300ma power brick. But always booted with an 800ma power brick. I fixed it not by staying with the higher rated power brick, but by replacing all 40 resisters for the LED's with ones of a more suitable resistance.

Power bricks of more than 500ma put some of the power supply components within the x0xb0x at the very limits of their tolerance, and sometimes beyond. Hence more than 500ma not being recommended in the case of the x0xb0x.

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