Author Topic: 4x5 Charlieplexed LEDs shield (Read 6510 times)

sparr · « **on:** August 20, 2012, 09:28:14 am »

http://i.imgur.com/0U2KY.png

This shield uses 5 pins to drive 20 LEDs. Most charlieplex layouts only allow one LED at a time to be lit. However, if your VCC is less than double the Vf of the LEDs (such as 3V blue LEDs) then you can drive a whole row at a time, which allows you to scan/refresh the display a lot faster.

PS: There's a chance this shield could become official!

potato · « **Reply #1 on:** August 20, 2012, 06:00:07 pm »

If this dose become \"official\" I will get one fore sure. Why use smd LED\'s?

sparr · « **Reply #2 on:** August 20, 2012, 07:12:11 pm »

the linked pcb uses through-hole LEDs and SMD resistors.

pfriedel@gmail.com · « **Reply #3 on:** August 22, 2012, 07:35:38 pm »

Actually, the better question is why have 1 resistor per LED? You can get away with having 1/2R per pin (so typically 75R for standard red LEDs at 5V), because the circuit path is always going to be out one pin and back through another. I haven\'t tracked down the circuit routing on yours 100%, but it\'s similar enough to how I laid out my Tiny85-based charlieplexes.

Although, looking more closely at it, I suppose it\'s sort of a situation where you\'re stuck either way - either have 4x more resistors than you need, or make it wider to permit a column of SMD resistors and lose the aesthetics of an exact-sized shield. Ah well, SMD resistors are cheap enough. And a tiny85 driving a 4x5 grid of LEDs allows for a fair amount of blinkenlights.

sparr · « **Reply #4 on:** August 23, 2012, 10:51:40 am »

We are probably going to go with a wider board (for the resistors) and taller board (to keep the power headers). And with one resistor per pin.

The reason to have one resistor per LED is to allow whole rows to be lit at once, instead of just one LED at a time.

pfriedel@gmail.com · « **Reply #5 on:** August 23, 2012, 03:01:41 pm »

Ah, I suppose that guarantees the same forward drop and therefore the same brightnesses? But yeah, spacing it out a bit will help - the thought of soldering the LEDs while dodging the resistors was making me twitchy.

It\'s silly, because I already _have_ a 4x5 plex that plays life hanging off of a minimal tiny85 board, but this shield makes me want to be a backer.

sparr · « **Reply #6 on:** August 24, 2012, 05:31:37 pm »

You\'re really going to like my next shield design then

http://www.youtube.com/watch?v=_rGxiRtp13I