Parallel LEDs

[Screaming_Rabbit]

Hi there

Is it possible to connect LEDs parallel in a matrix... for example: In the MidiBox LC application, having the matrix for the LED meters. Then connect for the clip LED a second one to mount right above the fader and connenct it parallel to the clippinfg LED of the meter bar?

If possible, would it also work with buttons?

Thx and greets

Roger

[Duggle]

hi,

the thing to do when running leds off a common source is for each led that is in parallel to have its own resistor.

This stops one from being brighter than the other.

The only problem would be that these "clipping" leds would be duller than the rest on the meter.

solution: use a high intensity led for both the "clipping" leds, each with their own resistor wired in series. You will need to experiment with the resistor value to match the relative brightness with rest of the meter. My experience with high intensity leds is that they are many,many times brighter than standard leds.

cheers

[moebius]

Hi.

As leds shouldn't be connected parallel without own current limitting resistors - I would recommend just wiring connection from DOUT board before the current limiting resistor there, and using another resistor on that connection to the parallel leds.

And buttons are normally open type, so putting those parallel works.

Bye, Moebius

[Screaming_Rabbit]

Thanks Duggle and Moebius

solution: use a high intensity led for both the "clipping" leds, each with their own resistor wired in series

... the problem with that is, that I'm using square LEDs for the meters and don't have a clue, where to get brighter ones. But since the clipping LED is the only RED one in the meterbridge, I wouldn't care to much when it's duller than the GREEN and YELLOW ones.

Do I understand it right, that I could wire them in parallel, use the regular square one for the meter and buy high density ones for the clip-LED next to the fader.

I then could raise the resistor value for the high density LED. As higher the resistor is, the less dull the one in the meterbridge would get?

As leds shouldn't be connected parallel without own current limitting resistors - I would recommend just wiring connection from DOUT board before the current limiting resistor there, and using another resistor on that connection to the parallel leds.

... I don't really understand your description (language barrier or electronic dummy)  :-[

Greets

Roger

[moebius]

AArgh! ;D

I just meant duplicating that part of the matrix, you want to. But not powering those directry from DOUTs connector, but taking signal out before resistor on DOUT board -> running that signal thru another (220ohm) resistor for those "paraller" leds.

And for brighter leds, you need more current running thru it, so you put in smaller resistor.

Try: http://www.mhsoft.nl/LedResCalc.html

Bye, Moebius

[moebius]

Whoops..

I forgot that 74HCT595 outputs can sink or source max. +/- 35mA current.

So with those normal 220ohm resistors on DOUT: U/R=I, 5/220 ~22,7mA, You have only about 12,7mA  more to spend. That means about 393ohm (I think you can use 390ohm) current limiting resistors for additional leds.

Bye, Moebius

[jimhenry]

You could use 330 ohm current limits on all the LEDs to bring the current down to 15 mA (5/330) from 23 mA (5/220).  Then you can run 1 or 2 LEDs from a single output and all the intensities will be the same but lower.  Try the LEDs with 330 ohm current limits on a breadboard before you commit to this approach but usually LEDs are acceptably bright over a range of currents.  

[Screaming_Rabbit]

Thanks a lot!

Roger

[Duggle]

hi all,

Don't forget to include the effect of LED forward voltage drop in your current calculation. It means that less current is drawn than what you have indicated.

typical values:

standard red: 1.2 to 1.5V  

hi intesity red: 2.0V

blue, white:3.2 to 4.0V

So for a standard led (probably what your using S_R):

(5-1.5)/220 = 0.016 = 16mA

I had another look at the schematic. Mobius' suggestion is ideal:  

All "clip" leds are sourced through R28 which is driven through D4. Therefor you should replicate the line which connects the annodes of the "clip" leds.

Do this by connecting another resistor (call it R28a ) to driver output of D4 (IC side of R28), the other side of this resitor is the common anode line of the row of your extra "clip" leds. There will be virtually no interaction between the two sets of leds in this configuration. This means you can have different led types and brightness settings.

I suggest a high intensity 3mm led for your local "clip" indicator. With 330R for R28a this means

(5-2)/330 = 0.009 = 9mA

16+9=25mA total for output D4 (within spec!).

I agree that setting and balancing the percieved brightness is subjective and needs to be experimented with once you have it built. After all there are only 12 (+1) resistors that set the brightness for the entire row of meters.

I am building an LC over my holiday season and I think this is an interesting idea to incorporate.

Another I idea is to have a meter with fewer leds (say 4) by coupling the cathode common lines with diodes.

this would have the interesting effect of modulating the brightness of the leading led segment as the input level changes!

cheers

[moebius]

Don't forget to include the effect of LED forward voltage drop in your current calculation. It means that less current is drawn than what you have indicated.

:-[ Diode voltage drop.. feels ashamed... :'( Bangs head to the wall..

[jimhenry]

Quote:

Don't forget to include the effect of LED forward voltage drop in your current calculation. It means that less current is drawn than what you have indicated.

 Diode voltage drop.. feels ashamed... Bangs head to the wall..

:-[ Head banging in stereo.  :-[