Trying to figure out LED resistor values (and also my MB6582 Lives Again!)

[m00dawg]

Curious what resistor values folks are using with their LEDs? I'm using SmashTV's yellow LEDs and found that with 220 Ohm resistors, they seem to be a tad dim. I can tell which ones are illuminated but it tends to blend in. Since I had to solder up a new baseboard (which is working yay!) I was thinking of using something more like 100 Ohm. These are the values I calculated with the new amber LEDs I bought (in the event I couldn't repair my existing control surface).

I tested the resistor values on both (with a constant 5V rather than a matrix) and it seems to be subtle for both SmashTV's LEDs and these amber ones (they're from Mouser). The right answer is to use pin strips and just try it :) But wanted to see if anyone else was using values this low? I can't find examples of that on the wiki - seems like 220 ohm is the floor for most folks. I think for normal cases that probably might be ok, but since the 6582 is using a scan matrix bumping up the brightness a bit might be nice (assuming it won't significantly reduce the LED lifetimes of course).

Any thoughts? As an aside I was quite happy that, so far, my control surface board seems to be in ok shape. It, like my old baseboard that I had to finally give up on, had some lifted pads due to using the terrible ribbon cables I originally used (twice!) for it. The new JST method of connecting the boards seems to be working wonderfully! So far the buttons and all but one of the knobs seems to work. One of the knobs is counting down no matter which way you turn it (a problem I think I had before) but otherwise, surprisingly given all the pads I've lifted off the CS board, everything seems to be working!

[latigid on]

For time-multiplexed LEDs you can exceed the 20mA current rating or whatever the datasheet says. Some even give a peak current rating that is valid for a given duty cycle (normally 10% rather than our 12.5 or so) and pulse time.

Totally depends on your LEDs. A tip is to use super bright ones, but probably you already have something installed. In the SEQ v4+ we use 47R but these are superflux RGB LEDs designed for more forward current. Generally the higher the pulse energy, the lower the lifetime of the LED. 

Keep in mind that the 74HC595 outputs are overdriven in this configuration. Your +5V rail will sag and in the worse case it won't be enough to power the PIC properly. But many MIDIboxes work fine like this :).

 

[m00dawg]

Ah all good points! I managed to jam the resistors into the holes enough to test the control surface (carefully) and interestingly enough, the LEDs seemed brighter than they were previously. So wondering if something was up with my old baseboard maybe. I'm using the switching regulator often recommended here for the 5V, although I have it in an external enclosure such that voltage sag may be an even bigger issue for me perhaps. I'll have to see. So far so good though, but I don't yet have the baseboard fully populated (which may also explain why the LEDs are brighter perhaps if voltage sag becomes an issue). I also didn't add in C3 and C4 and I might opt to go ahead and do that in case that gives me a bit more oomphf for transients.

[m00dawg]

Just to close the book on this topic, I found 220 Ohm to be quite nice with my old control surface (using SmashTV's yellow LEDs). I would guess my previous dim LED woes were due to some issue with the previous board and/or the power supply.  It looks great and they're plenty bright (but not too bright) to be quite usable.