How to incorporate multiple LED indicators?

[steveyraff]

Hey guys,

This is a question I meant to ask a few times. Speaking purely aesthetically - if I want to make a pedal that has 2 or more LED's to show the pedal is engaged, how would I go about doing that?

At the moment, I just use a standard offboard wiring diagram for my projects, with a single LED using a 2.2k resistor. I have an idea for a build where I'd like 2 LED's. Any help appreciated. Would I be wrong in guessing I just run them in series and use a smaller value of resister?

Thanks,
Steve.

[potul]

That's exactly what you need to do. Connect them in series and reduce the resistor to compensate the additional voltage drop.

If the LEDs are different colour, you might want to connect them in parallel, each one with it's own limiting resistor.

Never connect them in parallel with a common resistor.

Mat

[R.G.]

Unfortunately, the answer is "it depends".

If you have enough voltage to drive the LEDs, sure, series connection and adjust the resistor works fine. If you don't have enough voltage, you need to go parallel.

LEDs use voltage. They will not light up at all until the voltage across them in the forward direction is bigger than their turn-on voltage. This varies with the LED's internal construction/diffusion. Red LEDs may only need 1.2V. Fancy colors, including blue, white, purple, and some greens need 3.5-4V each. If you only have 9V available, you're not going to get three 3V LEDs and a current limiting resistor to work from it. And if you ever want to use a battery in there, batteries are not considered exhausted until their voltage falls to about 7.5V for the 9V pp3 size.

So here's what you have to do.
1. How big is the **lowest** voltage you want to run from? If the answer included a 9V battery, you have to pick something like 7.5V.
2. How big are the forward voltages you want to drive? This depends on the colors as mentioned above.
3. Once you know those two things, you can figure out how many LEDs you can drive from the voltage you have. You need to have some voltage across the current limiting resistor. This ought to not be less than about 25% of the total voltage so it can control the current. Bigger voltage across the resistor is better control. Much less than 25% in the resistor means you need to go to an active current limiter to get control at a lower voltage lost in the limiter.

If you can't get there with series LEDs, you have to go parallel, with fewer LEDs in a string and a current limiter per LED string. This give you back lower voltage capability and control, but means that it pulls more total current out of the power supply because each LED gets its own current, not re-using the current in a series string.

[steveyraff]

Sorry I'm only getting back to this now. I've just got back to trying to finish this build.

I would like to use one 5mm Green LED - but as a secondary indicator, a Blue 5mm or 3mm LED (Still have to figure out which fits best).

Any ideas on exactly what resistor and hookup method this may require?

Thanks.

[samhay]

Blue/UV LEDs have the largest forward voltage drop, but I would be surprised if this doesn't work in series. You will have to audition the current limiting resistor - don't go lower than 330R.

[Phoenix]

Sorry I'm only getting back to this now. I've just got back to trying to finish this build.

I would like to use one 5mm Green LED - but as a secondary indicator, a Blue 5mm or 3mm LED (Still have to figure out which fits best).

Any ideas on exactly what resistor and hookup method this may require?

Thanks.

If you want them all to be the same or similar in brightness, you probably won't be able to run them all in series, at least not in a simple arrangement. Different makes/models/colours/sizes/etc of LED's have different millicandela (MCD) ratings - that is, how bright they are for a given current. This varies wildly with different types of LEDs. If you put all your LEDs in series, they will all receive the same current, and this may result in drastically different brightness. You can of course use resistors in parallel with each LED for current sharing in order to alter the current that each one in the series string sees, so you do still have the possibility of varying their respective brightness, but that would be a tedious process to find suitable values.
There's also the fact that the human eye has different levels of sensitivity to different wavelengths of light, so even if you get each different colour putting out the same level of light, your eye may not perceive the brightness as being equal.
It's much easier using LEDs of the same type in series strings for this reason.

It would probably be much easier to give each LED its own current limiting resistor and tweak their respective values to get the brightness you want, but don't let me discourage you from trying a series string if you want to go for power efficiency/battery life - it just may be more complicated than it first appears.

[steveyraff]

Sorry I'm only getting back to this now. I've just got back to trying to finish this build.

I would like to use one 5mm Green LED - but as a secondary indicator, a Blue 5mm or 3mm LED (Still have to figure out which fits best).

Any ideas on exactly what resistor and hookup method this may require?

Thanks.

If you want them all to be the same or similar in brightness, you probably won't be able to run them all in series, at least not in a simple arrangement. Different makes/models/colours/sizes/etc of LED's have different millicandela (MCD) ratings - that is, how bright they are for a given current. This varies wildly with different types of LEDs. If you put all your LEDs in series, they will all receive the same current, and this may result in drastically different brightness. You can of course use resistors in parallel with each LED for current sharing in order to alter the current that each one in the series string sees, so you do still have the possibility of varying their respective brightness, but that would be a tedious process to find suitable values.
There's also the fact that the human eye has different levels of sensitivity to different wavelengths of light, so even if you get each different colour putting out the same level of light, your eye may not perceive the brightness as being equal.
It's much easier using LEDs of the same type in series strings for this reason.

It would probably be much easier to give each LED its own current limiting resistor and tweak their respective values to get the brightness you want, but don't let me discourage you from trying a series string if you want to go for power efficiency/battery life - it just may be more complicated than it first appears.

Very detailed answer - thank you sir. I think I'll just give each their own resister, sounds more convenient.

In saying that, I've just realised from looking at this build, the limiting resister is already build into the layout - so now I am not sure how to go about it at all.

Any suggestions?

[steveyraff]

How about I make the main LED indicator the one that has the resistor built into the layout, and then I add another LED using the standard off-board wiring method ?

[Phoenix]

How about I make the main LED indicator the one that has the resistor built into the layout, and then I add another LED using the standard off-board wiring method ?

That would most likely be the simplest approach.

[steveyraff]

Yes, that worked. Cheers. 

[Kipper4]

Nice Stevey

[LightSoundGeometry]

I like to use common anode bi colors and tri colors

Barry sells the Bi color ones here:

http://www.guitarpcb.com/apps/webstore/products/show/4429223