Switching high power LEDs

Asked this already in another thread, but I thought it deserved it's own. ;D

I can't seem to find much information on using high(er) power LEDs with the Arduino. For my project, I'm using seven segment displays with 4 LEDs per segment, so I need a forward voltage of around 8V, preferably compatible with my common cathode digits, and I'd like to retain my 7219 for ease of use, if possible. I'm building a clock, but I need large digits (currently trying to use 2.3" digits)

I came accross a few options: 1) a Max7219 with transistors or a transistor array chip. 2) an Allegro combination shift register and transistor array. 3) Building custom digits with only 2 LEDs per segment.

The problem is, I don't have much experience with any of these methods. Does anyone have any experience with interfacing higher voltages with the Arduino? I'm also open to a long LED matrix, but I also can't seem to find anything using more than one 8x8 matrix.

Any insight would be much appreciated!

The transistor metod would be the easiest way to go, while still keeping the 7219

since you are power sourcing to a common cathode display PNP transistors would be the best way to go, oops ;D the 7219 sources some current for the controlls so NPN , even though they are less efficient at power sourcing

Then you would take the Collector of each transistor to the anode of each 4 led segment and the emmiter to the LED's VCC , The base would go to the max 7219 pin with a oh, 100 ohm, reisitor inbetween

and how much current do the led's pull? because if they have too much the max chip may fry, so if its past the data shett specs for simking use PNP transistors to gnd, since they turn on when base is pulled to gnd

Hi

I'm doing a simular project(Clock whit large digits..) my solution is to make each digit out of 20High intensity leds(2700mcd 2V 20mA), it works really really good. The digits is designed like this:

LLLL
L__L
L__L
LLLL
L__L
L__L
LLLL

where each L corsepond to one led. The good thing is that it can be powered from 5V. Each each led on the digits can be invidualy controled so the numbers looks good since it's not the standard 7-segmentlook. I use 3 74HC595 shift registers to multiplex the current time. So, my advise is to use highintensity leds as output rader than a cascade of low intensity leds that are placed closetogether since that is more expensive and uses much much more power...

I will post some pictures once i get home... and maby som code:D

edit: Forgot to say that if you use cascades of leds use MOSFET-transistors to turn off and on the leds else your different segments will have different intensity depending on how meny segments that are on.

/Jon

since you are power sourcing to a common cathode display PNP transistors would be the best way to go, oops ;D the 7219 sources some current for the controlls so NPN , even though they are less efficient at power sourcing

Then you would take the Collector of each transistor to the anode of each 4 led segment and the emmiter to the LED's VCC , The base would go to the max 7219 pin with a oh, 100 ohm, reisitor inbetween

Thanks - forgive the stupid question, but in this configurationm with 7 NPN transistors connected between the 7219 and the anode segments (with emitters tied to the common cathode), where does the higher voltage come in? The 7219 outputs 5V, the display needs at least 8V across the segment to light, where do I "input" this extra voltage?

So, my advise is to use highintensity leds as output rader than a cascade of low intensity leds that are placed closetogether since that is more expensive and uses much much more power...

I agree, I'd love to use indivudually controllable LEDs, however I'd rather use pre-built displays (for the moment, as it (should) save me some time while I'm starting out) and I'd really like to use the LedControl library if possible (and not have to deal with shift registers, at least until I get my feet wet). I'm also building this for a TV studio, so high intensity might not work for me, I just need larger digits. But thats for the input, I'll definitly build one like this in the future.

Thanks - forgive the stupid question, but in this configurationm with 7 NPN transistors connected between the 7219 and the anode segments (with emitters tied to the common cathode), where does the higher voltage come in? The 7219 outputs 5V, the display needs at least 8V across the segment to light, where do I "input" this extra voltage?

The 8v will go to the Emmiter leg of the NPN transistor

I thought this example diagram may help to explain how to connect to a higher voltage External Power Supply.

http://www.gdesign.com.au/arduino/Example1.jpg

Don't forget to tie both the ARDUINO and the EXTERNAL POWER SUPPLY GND's together.

Ooooh alright, Thanks for the diagram. So, as I understand it, this method won't work with common cathode displays, correct?

I'm pretty new at this kind of thing so I'm not sure but you it should be OK with a common cathode display if you swap a few things around.

SEE REVISED DIAGRAM http://www.gdesign.com.au/arduino/Example2.jpg

Best to do a simple test set-up and try. For the test, the external power supply can be anything so best to try with a couple of AA's and an old LED. If it works, you can then scale it up to the larger power supply etc.

Good Luck and let me know how you go.

Hmm… seems to me like you would need to swap out the transistor at the least, but I am also new at this. I just got a bunch of transistors and parts so I’ll definitely give that a shot. Thanks!