outdoor displays and multiplexing ideas

Last year I did some experiments with multiplexed LEDs in the sun, and found that 50% on time was the lowest I could go and still be able to see the LEDs satisfactorily .

This was for 2 scoreboards that face the sun ( one faces East, the other West, so there is no respite with the time of day ! )

There was quite a long thread about it where I posted the pictures of results

http://forum.arduino.cc//index.php?topic=135826.0

Now I have to make some more of the same boards, so they cannot be dimmer than the first ones that I used SAA1064 chips to drive 4 columns of 7 LEDs at 50%.

For a 7 ( or 8 ) x 5 matrix, it needs 5 chips for 4 digits, so my boards need 30 chips for the 24 digits.

The main problem is that they are I2C driven, with only 4 choices of address, so I had to break them into groups of 4, and use an analog switch to connect the SDA to the right group.

This doesnt work out well for making a pcb for a group of 4 digits say, as I would need a separate SDA line for the 5th chip.

Does anyone know of a similar chip that uses SPI ( or shiftout ) 3 wires? ( I can get the SAA1064 for about $1.50. )

I use TPIC6B595 for my numeric 7 seg displays, but that only drives 8 segments of 16 LEDs.

I have got quotes for LED panels from China that say they use 1/4 drive for outdoor displays, using 74xx595s but I am not sure how.

Perhaps I should just use one 74HC595 latched for each column, they are only 25c in bulk.
My pitch is 13mm I think, and the chips are 7mm wide so they would fit between the rows.

I could have 5 separate latch lines to each group of panels, so I wouldn't have to send the data through the whole 120 chips.
Speed is not a consideration as the scores change rarely, and the clock every second.

I could turn the 5v down to save power wastage in the series resistors ( the 595 works down to 2v but I would have to consider the atmega328 voltage)

To keep in spec I would have to limit the LED current to 10mA as the max supply pin current is 70mA, but the LEDs I am using now are a lot brighter than last year anyway, and they were running at 18mA / 50% duty cycle .

Does anyone know what is normally used for latched displays ?

Boffin1:
I could have 5 separate latch lines to each group of panels, so I wouldn't have to send the data through the whole 120 chips.

SPI can run at 8 MHz so you could update 8-bit shift registers at 1 microsecond each.

Thats true John, but I tend to run it a lot slower ( shift out usually ) because my display tiles are spread out over several square meters, and connected with ribbon cable which has quite a bit of capacitance.

But then again some of my scoreboards are for cricket, where they time in microweeks

Boffin1:
The main problem is that they are I2C driven, with only 4 choices of address, so I had to break them into groups of 4, and use an analog switch to connect the SDA to the right group.

Acting as an I2C master is very easy. You can do I2C in software on any Arduino pins....you can put each one on a different pin instead of having different I2C addresses.

Boffin1:
Thats true John, but I tend to run it a lot slower ( shift out usually ) because my display tiles are spread out over several square meters, and connected with ribbon cable which has quite a bit of capacitance.

But then again some of my scoreboards are for cricket, where they time in microweeks

I didn't know about the scoring of cricket as I've never finished watching a game. I'm not even sure they've ever finished a game.

Have a look at the Micrel MIC5840/5841. One of those might just do the job.

arduinodlb:
I didn't know about the scoring of cricket as I've never finished watching a game. I'm not even sure they've ever finished a game.

A full game of cricket takes about a month to watch if you can only dedicate a couple of hours a day to it...

They do organize "one day" games for people with short attention spans but real cricket fans aren't in such a hurry.

Thanks guys

Acting as an I2C master is very easy. You can do I2C in software on any Arduino pins....you can put each one on a different pin instead of having different I2C addresses.

Each group of 4 SAA chips will still need its own pins, and wiring, thats 8 groups.

The MIC5841 is very similar to the TPIC6B595 that I use, but quite expensive. $ 4.10 compared to $1.50 at Digikey.

I can get the SOP 74HC595 for about 20c.

Perhaps I will make an 8x8 LED pcb ( in case I want to make a scrolling display later ) and just populate the 7x5 LEDs for this project and five 595 chips per digit.

The display will be completely latched then, but I will have to limit the LED current to 10 mA with the resistors.