LED grid for work clock

Hey Guys/Gals,

I have been looking at a few tutorials on how to best power a grid of LEDs for a work clock. I was hoping someone could weigh in on a good method to go about it, a balance of cost effectiveness and likely longevity of the circuit. I want to have a PCB made for the project, and don't want to use/buy unnecessary components.

I have seen people use a combination of shift registers and transistor arrays to ground a common cathode for each word of a clock. Though, in these approaches it seems that a resistor is necessary for each LED as to not have varying levels of brightness under different conditions.

I saw this method: http://imgur.com/a/ghTYs. I have honestly no idea what the guy is doing, it seems like he is using alot of ICs.

I was thinking about multiplexing. If I had a 13x9 grid, I think I could use two shift registers, and one transistor array, with 9 resistors. I could use turn 3 pins from my controller into 8 with one shift registers for the first 18 of 3, and then just use 5 IO pins on the controller. I use daisy chain the other shift register onto the first one, and use that to drive the transistor array to ground 8 of the 9 rows, and then just use another IO pin on the controller for the 9th row. If I am not mistaken that brings me up to 9 pins which I could use to drive my entire grid.

I intend to use an ATMEGA 328 for the process, and flash it with a program made in the arduino language.

Can I get some recommendations on which method is likely to be the most reliable and economic to implement?

Would it also be possible to get some recommendations on what ICs to use? I also am having some trouble selecting the smd led. Can any suggestions be made for some diffuse or neutral white smd leds that mimic a typical 5mm led in brightness/size?

Thank you and if I can restructure my question at all or provide more information please do let me know.

I use these in my clocks. SB Projects . Mine are synchronized clocks, well I do make independent clocks also.

This is the general idea about using a matrix:-
http://www.thebox.myzen.co.uk/Workshop/LED_Matrix.html

not have varying levels of brightness under different conditions.

Do you need that? It makes things more complex.

You might like to use a couple of MAX7219 chips, one will do an 8 by 8 matrix and will do all the multiplexing for you, and allow levels of brightness control but not on an individual LED basis but on a block of LEDs basis.

Grumpy_Mike:
This is the general idea about using a matrix:-
Arduino Workshop
Do you need that? It makes things more complex.

You might like to use a couple of MAX7219 chips, one will do an 8 by 8 matrix and will do all the multiplexing for you, and allow levels of brightness control but not on an individual LED basis but on a block of LEDs basis.

I would prefer that functionality, yes.

I had been debating using these too: Shift Register 8-Bit High-Power - TPIC6B595 - COM-00734 - SparkFun Electronics
I figure I could get away with using 3 of them.

Any suggestions on white SMD leds? I have never really journeyed into using SMD technology before, so I don't want to order something that just won't work out in the end.

Why use SMD?

the clock in the link seems to be a neat idea.
the link shows a double sided board with what looks like an 11 x 10 matrix of leds.
on the back is more than a dozen IC's. I suspect many are just darlington arrays, some are step registers.
the link says he had to place and solder the IC's by hand as he could not reflow both sides.

my thoughts would be to buy perf board and use through hole for the LEDs
the SMD LED's may have a wider field so they may be required. but it us clear that he has 23 separate chambers, some longer than others. one would need to fill each with an equal amount of light per square area of the surface for the display to look right.

Make a logic board for the signal side of things. two boards would be much easier and not having to pay for the large board to be made would save a lot on the wallet.

I had been debating using these too: Shift Register 8-Bit High-Power - TPIC6B595 - COM-00734 - SparkFun Electronics
I figure I could get away with using 3 of them.

Those chips will only sink current, that is switch an output to ground, they will not source current, that is supply a voltage.

Any suggestions on white SMD leds?

Surface mount is nothing to be scared of, as long as you have a fine tipped soldering iron and a good pair of tweezers, and maybe a good magnifying glass ( depending on your age ).

The big advantage is that they have such a low profile and you can get them close to the diffuser. I use them on normal 0.1" strip board soldering them between the tracks. Or I use a scalpel to give me a finer pitch when needed.

Grumpy_Mike:
Those chips will only sink current, that is switch an output to ground, they will not source current, that is supply a voltage.

Ah, okay. So it's not like the 74HC595 which can provide a current? I have seen a tutorial where someone does an 4 by 4 grid of LEDs using just one 74HC595. It seemed like they were using would set one pin high, and one pin low, to drive a row and a column.

It seems like the max chips you recommended could work if I used two of them. It seems though that if I were to do that, and then buy the LEDs required, I am still about 40 bucks in it. I figure maybe it's just worth it go go for 117 WS2812's, since they all have control logic built into them. Would give me more flexibility with colours and such. I see I can get them for 46 dollars on sparkfun.

Once I have a working circuit, I want to build maybe 100 of these, if possible. So I want to keep production cost as low as possible. If I was just building a few, I wouldn't be so stressed about the price of the parts (within reason of course). Maybe this changes the nature of the advice?

Grumpy_Mike:
Surface mount is nothing to be scared of, as long as you have a fine tipped soldering iron and a good pair of tweezers, and maybe a good magnifying glass ( depending on your age ).

The big advantage is that they have such a low profile and you can get them close to the diffuser. I use them on normal 0.1" strip board soldering them between the tracks. Or I use a scalpel to give me a finer pitch when needed.

i just seem to have trouble picking one. I want one that rivals the average brightness of a typical 5mm led, and is diffuse or cool white. There is just so many to pick from, and since I am trying to design something in eagle cad I am just getting a bit overwhelmed/intimidated.

The problem with any sort of matrix is the duty cycle. As the LEDs spend most of their time off it will not look as bright as an LED that is on all the time. That is why a simple shift register is probably the best way to go like that original project. That is where the power shift register comes in to its own by powering the LEDs directly.

Yes there is a lot of choice with LEDs, even professionals find that. It is best to get just one or two to test first before committing to any one sort. Don't make a PCB the first time you see the circuit, prototype at least parts of it first on strip board.

Hi, how physically big, in cm or inches, are the digits going to be?

Tom....... :slight_smile: