button led matrix

[intellijel]

Has anyone inmproved on the design?

I thought a simple obvious way to supply current for higher power LEDs is simply to use a bunch of darlington pair array ICs like the ULN2001A in series with the DOUT module pins driving the LED anodes (the ULN2001A would be sourcing the required current, up to 500mA per output). A simple npn transistor array may also be adequate if each individual led colour (e.g. the "red" led of an rgb led) is drawing less than 200mA current.

You would probably also need a darlington pair to sink all the current at the DOUT pins driving the LED common cathodes. There will be more current to sink here than what is being sourced for the individual led colours which share a common cathode.

Thoughts on this?

p.s. I remember reading a comment before from TK that he was suprised there was so much interest in the led/button matrix. Are you kidding? Name me a commercially available, AFFORDABLE, sequencer that has that feature? It is such an obvious way to provide intuitive and quick interaction with a step sequencer, I would imagine that this is actually the most popular addition to the SEQV3 feature set!

It would definitely be cool if more work was done on this... I had recommended before using a maxim led driver chip (TK pointed out that maxim chips can be too hard to source sometimes and he wants to keep midibox designs easy to source parts for) but there has got to be some commonly available chipsets that can be used to make this task easier.

[Wilba]

I discovered (just in time) that while the darlington array is compact and great for current sinking the LEDs, it's not suitable for current sinking both the LEDs and a switch matrix, like what I do in my new MB-SID, because the voltage drop across the collector-emitter means it will be higher than the logic low threshhold of the DIN. So discrete transistors are a better solution, BC547 for example can sink 100mA and has a much lower voltage drop. More info here: http://www.midibox.org/dokuwiki/doku.php?id=wilba_mb_6582

Also keep in mind that it doesn't really matter if you sink 100mA or 1000mA, because the 74HC595 output current is the limiting factor, you're not just sinking current from the power rail through a LED and through the transistor, the current is coming from the output of one of the 74HC595... you could add a driver to that output too and increase the current output, but this is probably not required, most of the "good" LEDs are bright enough at 10mA.

[intellijel]

I would leave the button matrix as is (they use minimal current) but just use the darlington pair array to sink the current of the led cathodes. The signal source may be from the 74HC595s but they would just be switching on the transistor current sources with minimal current (if you use transistors to source current). You would not need this as you stated if you are happy with 10ma or so per led colour, however I am lighting rgb leds through those thick button pads from sparkfun and I have found I need a lot more current. The issue is that because of the array, one pin of a 74HC595 may be lighting up to 4 leds at a time (granted with PWM this will minimise power requirements). A transitor array instead of a darlington pair array would probably suffice for the LED anodes..

cheers,

    Danjel

[cheater]

Hey guys, thanks for working on this thing. I love that new feature, it makes it worth to build a SEQ just for that. :)

*starts putting away money for a SEQ*

[TheAncientOne]

This is the ST page fro their range of high side and low side drivers

http://www.st.com/stonline/stappl/productcatalog/app?path=/comp/stcom/PcStComOnLineQuery.showresult&querytype=type=product$$view=table&querycriteria=RNP139=1025.0

You can drive both ends of the matrix this way, put the current limiting resistors in the source side, and if you vary the source voltage, or PWM it, there is your dimmer.

Allegro do a range of source drivers with the shift register built in, though I'm not certain if there ia a second source.

http://www.allegromicro.com/sf/65958/

I'm wondering if using the NATO stock guides might be useful: the military demand a long availability and often second sourcing, so we might as well get some of our tax back by using their expertise.

A couple of thoughts anyway.

I used the ST chips to drive an 8 x 8 matrix for a lighting display, using 'frames' of 8 bytes cycled out of an EPROM by a fast counter, then incrementing the  frame count to animate the display.  I put the current sharing reistors on a 16pin DIL header, then I could change the display voltage quickly on site. When I have a couple of jobs done, I'll dig up the proto and post the drawings. I suppose I could have 127 patterns and drive it with a core off a sequencer channel...

Hope this helps, though I hadn't even thought about the combined button/LED system. Looking at the sheet for the sink darlingtons, V_CE(SAT) is 0.9 to 1.6 making it too high for the combined one. Time to look for a MOSFET switch array?

Hope this helps

Mike

[Wilba]

Slightly off-topic, but I need something to source around 200mA at 11v... I have a bunch of LEDs grouped three in series with a dropper resistor for each group going to ground, now I would like to switch the supply to all the LEDs with a PIC output pin. The problem is I need it to switch the supply, not just a current sink, which I know how to do... any suggestions?

[TheAncientOne]

Slightly off-topic, but I need something to source around 200mA at 11v... I have a bunch of LEDs grouped three in series with a dropper resistor for each group going to ground, now I would like to switch the supply to all the LEDs with a PIC output pin. The problem is I need it to switch the supply, not just a current sink, which I know how to do... any suggestions?

ONe trick is to do the switching with a high-side PNP darlington, and do the level shifting with a cheap opto-isolator driven from the PIC pin. That way, if a transistor fails, the PIC doesn't get killed. If you can wait until next week, I'll be back in the workshop and will give it a try for you.

What colour are your LED's, (I'm thinking about their forward voltage drop).

Mike

[Wilba]

They're blue LEDs, each has a 3v drop when three are supplied in series with a 220R current limiting resistor. It was just convenient to solder each resistor to the ground on the PCB instead of wiring it (this is an addition to my control surface PCB). Maybe it's easier to wire it so they all get sinked with  a MOSFET or NPN transistor.