LED Luminous Intensity for back panel illumination?

Hi,
I’m planning on a building a (new) midi controller. I plan to have illuminated buttons in in it. My question is, what is a suitable luminous intensity for an LED to properly illuminate a say ~6mm thick piece of clear plastic button? I ask this because I’m trying to get as many LED’s as I can with still powering arduino off a single USB port, so I’ve been looking at low current LED’s (ie. leds than can run off <10ma, usally SMD types).
It might sound ambitious but ideally I’d like to have about 50 leds (mainly red and blue) powered off arduino. If each one is connected in parallel to the power supply, and rated at say 5ma, then 50 of them should draw about 250ma if I use a suitable resistor in front of each one. This should be ok as USB ports can usually supply about 500ma at absolute max?
My only issue is these types of LED’s tend to have low MCD’s, my question is what is the lowest mcd led I should pick to properly be able to light up a button made of clear acrylic?

Multiplex the LEDs so that only one or a few are on at a time to limit total current. Use SPI shift registers or I2C expander chips to minimize pin count.

groundfungus:
Multiplex the LEDs so that only one or a few are on at a time to limit total current.

That may limit total current in one sense, but it will not affect the relationship between current consumption and overall brightness. Eight LEDs drawing 5 mA continuously are the same brightness as eight LEDs sequentially multiplexed at 40 mA.

groundfungus:
Use SPI shift registers or I2C expander chips to minimize pin count.

No, please don’t do that!

50 LEDs? In fact, what you do want is to multiplex them using a MAX7219. One resistor (and a couple of bypass caps) controls them all, excellent brightness, multiplexed eight at a time with about 40 mA each. Three wires to interface. Pity multiplexing the buttons is not quite as easy.

All depends on the expected illumination conditions.

In a dark environment (ie, outdoors at night or indoors with lights off), putting 20ma through a modern high brightness LED is probably brighter than you want, unless you're losing a ton of light in the plastic. I'd expect it to be uncomfortably bright in pitch darkness.

For one project - amusingly enough, a 150W flashlight, after we got the reflector light-proofed, we found that the power LED at 10-20mA was uncomfortably bright, and hurt our night vision. So I used 1k resistors, driving the ~3v blue and green LEDs in one of those 5050 rgb leds off 5v (ie, 2ma), and that was perfectly good for an indicator for use in dark environments (passing through a 1/4th inch diameter acrylic light pipe), and is clearly visible indoors. I found I needed more current through the red led to make it look about as bright, though - I think I'm using a 470 ohm resistor, and it's a 2.x volt LED.

In a brightly lit room, you'd probably need something brighter, and for broad daylight, a 20mA LED has no hope of being bright enough - but then, you probable don't need to backlight the keys then.

I've refrained from listing MCD numbers because I have no way to measure actual light output, nor do I even have trustworthy spec sheets on the LEDs I used.

My point is, you're going to need to experiment with the correct current to put through the leds to provide acceptable illumination under the expected operating conditions.

Ah well, and that is another reason to use the MAX7219 (rather than mucking about with shift registers) because while it does not facilitate individual LED brightness control, it includes a 16 step PWM overall brightness control and the maximum brightness is controlled by a single resistor which can be changed or made variable if necessary.

Human eye sensitivity and color make uniform current difficult:

If you are interested in this kind of thing, Google for "LUX"

Wow, thanks for the input!

50 LEDs? In fact, what you do want is to multiplex them using a MAX7219. One resistor (and a couple of bypass caps) controls them all, excellent brightness, multiplexed eight at a time with about 40 mA each. Three wires to interface. Pity multiplexing the buttons is not quite as easy.

Interesting, if I did this would I still be able to have say all 50 on together? My plan is to only have the LED's on at all times, not controlled on/off.

So I used 1k resistors, driving the ~3v blue and green LEDs in one of those 5050 rgb leds off 5v (ie, 2ma), and that was perfectly good for an indicator for use in dark environments (passing through a 1/4th inch diameter acrylic light pipe), and is clearly visible indoors

Yeah, to be honest, the only purpose of backlighting the buttons is to see them in the dark, maybe not pitch black but say in a room with no main lights switched on, otherwise, like you said, in broad daylight I wouldn't need to backlight the keys.

My point is, you're going to need to experiment with the correct current to put through the leds to provide acceptable illumination under the expected operating conditions.

True, I'll probably breadboard some with a variable resistor and see what it best.

Ah well, and that is another reason to use the MAX7219 (rather than mucking about with shift registers) because while it does not facilitate individual LED brightness control, it includes a 16 step PWM overall brightness control and the maximum brightness is controlled by a single resistor which can be changed or made variable if necessary.

I definately like the sound of this, if I can controll the brightness of all the LED's with one resistor this could be helpful. I could use something like a trimming potentiometer to control the overall brightness.

operators123:
Interesting, if I did this would I still be able to have say all 50 on together? My plan is to only have the LED's on at all times, not controlled on/off.

Not really, but human eye will be tricked to think so. It is called Persistence Of Vision (POV) and it means you turn some LEDs on, then you turn those LEDs off and turn on another set of LEDs, then you turn them on.... well, you get the idea. If you do it quick enough the eye will see them as all turned on. The technique is called Multiplexing and the eye effect is called POV.

The advantage is you'll consume only a fraction of the current, the disadvantage is the LED brigthness will be dimmed a bit. If you don't do it fast enough you'll see the all LEDs flicker.

Multiplexing is commonly used in electronics, including TVs, both modern ones and old ones.

operators123:
I definately like the sound of this, if I can controll the brightness of all the LED's with one resistor this could be helpful. I could use something like a trimming potentiometer to control the overall brightness.

Yes, with one MAX7129 you can drive up to 64 LEDs (8 rows of 8 LEDs each), it will take care of multiplexing for you as well by turning on one row at a time. That is not the only IC out there with this capability, they are called LED Drivers and you can find many with different characteristics, but the MAX7219 is cheap and easily available.

operators123:
My plan is to only have the LED's on at all times, not controlled on/off.

Ah! Well, in that case, you do not need an IC at all, do not need multiplexing, just 50 resistors. The red LEDs can be connected in series pairs to economise on current (and resistors!).

If you then need brightness (or simply on/ off) control, a transistor controlled by a single Arduino pin to use PWM, switching them all on and off by a variable duty cycle.

rlogiacco:

operators123:
Interesting, if I did this would I still be able to have say all 50 on together? My plan is to only have the LED’s on at all times, not controlled on/off.

Not really, but human eye will be tricked to think so. It is called Persistence Of Vision (POV) and it means you turn some LEDs on, then you turn those LEDs off and turn on another set of LEDs, then you turn them on… well, you get the idea. If you do it quick enough the eye will see them as all turned on. The technique is called Multiplexing and the eye effect is called POV.

The advantage is you’ll consume only a fraction of the current, the disadvantage is the LED brigthness will be dimmed a bit. If you don’t do it fast enough you’ll see the all LEDs flicker.

Multiplexing is commonly used in electronics, including TVs, both modern ones and old ones.

operators123:
I definately like the sound of this, if I can controll the brightness of all the LED’s with one resistor this could be helpful. I could use something like a trimming potentiometer to control the overall brightness.

Yes, with one MAX7129 you can drive up to 64 LEDs (8 rows of 8 LEDs each), it will take care of multiplexing for you as well by turning on one row at a time. That is not the only IC out there with this capability, they are called LED Drivers and you can find many with different characteristics, but the MAX7219 is cheap and easily available.

Paul__B:

operators123:
My plan is to only have the LED’s on at all times, not controlled on/off.

Ah! Well, in that case, you do not need an IC at all, do not need multiplexing, just 50 resistors. The red LEDs can be connected in series pairs to economise on current (and resistors!).

If you then need brightness (or simply on/ off) control, a transistor controlled by a single Arduino pin to use PWM, switching them all on and off by a variable duty cycle.

Thanks both for your input :slight_smile:
Multiplexing definately sounds like a great idea, I’d have to now look at either multiplexing the LEDs as an array or simply just connecting them all normally. Obviously the advantage of multiplexing is each LED can have more current through it (making it brighter) since the overall current draw is less (as less LED’s are on at a single point in time).

operators123:
Thanks both for your input :slight_smile:
Multiplexing definately sounds like a great idea, I'd have to now look at either multiplexing the LEDs as an array or simply just connecting them all normally. Obviously the advantage of multiplexing is each LED can have more current through it (making it brighter) since the overall current draw is less (as less LED's are on at a single point in time).

Once you get to 10-15mA you pretty much reached the maximum light intensity of an LED, pushing it higher is not going to give you much more light. I'm obviously talking about common LEDs, power LEDs can absorb much more current than this, but they need an heat sink.

What am I trying to say? Keep your LEDs current in their tolerance range and add multiplexing to save on current at the price of a little intensity. Don't try to compensate that little intensity reduction by increasing the current as that is only going to shorten your LEDs life.

operators123:
Multiplexing definitely sounds like a great idea, I'd have to now look at either multiplexing the LEDs as an array or simply just connecting them all normally. Obviously the advantage of multiplexing is each LED can have more current through it (making it brighter) since the overall current draw is less (as less LED's are on at a single point in time).

Hang on - what I am saying is that there will be minimal (visible) difference between powering a LED continuously at 5 mA, and at 40 mA for one eighth of the time. There is no "saving" in brightness for a given current either way. Multiplexing is really a way to save on the hardware needed to individually control a lot of LEDs. That said, other than my suggestion that the red LEDs can be in series pairs sharing the same current and a single resistor, you might even consider a MAX7219 more practical than up to 50 separate resistors - but it then must be controlled by 3 Arduino pins, if only to switch it on (and set intensity).

rlogiacco:
Once you get to 10-15mA you pretty much reached the maximum light intensity of an LED, pushing it higher is not going to give you much more light.

You may be confusing here, the logarithmic response of the eye. Doubling intensity achieves a certain perceptible difference in brightness (not that you cannot see smaller steps). So when you go from 4 to 8 mA, it seems to get substantially brighter. When it goes from 8 to 16 mA, it again seems to get substantially brighter. But then you have to go from 16 to 32 mA for it to again seem substantially brighter.

It is a common misconception that multiplexing, or flashing an LED on and off saves current for a given brightness.

Even professional engineers fall for that one. It is not true as Paul__B says. There is no pecieavable difference.

I would experement at what current you can run at but 5 to 10 mA should be fine even for a normal LED.

Paul__B:

rlogiacco:
Once you get to 10-15mA you pretty much reached the maximum light intensity of an LED, pushing it higher is not going to give you much more light.

You may be confusing here, the logarithmic response of the eye. Doubling intensity achieves a certain perceptible difference in brightness (not that you cannot see smaller steps). So when you go from 4 to 8 mA, it seems to get substantially brighter. When it goes from 8 to 16 mA, it again seems to get substantially brighter. But then you have to go from 16 to 32 mA for it to again seem substantially brighter.

That’s a far better explanation than the one I gave! Without being backed by scientific proof I was trying to say that increasing the current is not going to provide a linear increase in (perceived) brightness and by increasing the current on multiplexed LEDs is not going to compensate for the perceived brightness either. On top of that he might be tempted to run the LEDs at a current higher than their rating based on the fact the current is flowing only for a short time: while that might be ok for a while (the short bursts will allow the LED to cool down between the bursts) it’s not going to be nice (as the LED will be subject to an higher thermal stress) and it will shorten LEDs life.

I personally run my LEDs at about half their rating when not multiplexed and near their rating when multiplexed: I stopped to run LEDs above their ratings once I understood (or had the presumption to understand) the diode.