Hi all,
im building at LED matrix to display scrolling text. I have constructed as simulation in Multisim and I have discovered that i am unable to provide enough current to source the LED anodes. Would any body be able to suggest a device i could use to provide enough current to source a row of 95 LEDs. For an idea of what i am building have a look at this link.
thanks
On a sunny day (12 Nov 2006 12:29:49 -0800) it happened "hybrid_snyper" wrote in :
There are a few problems here, effectively the LEDs are in parallel when a row is on, so they will probably never have the same brightness.
If you put resistors between the transistor collectors and the LEDs, and drive those LEDs via some transistor that can handle the current for
8 LEDs (not via a 74HC04, then all LEDs will have about the same current, and enough current will be available.On a sunny day (Sun, 12 Nov 2006 20:56:47 GMT) it happened Jan Panteltje wrote in :
Eh, excuse error, the more economic way would be: + + | | row 1 |/ \\| -----| PNP (low drop) or NPN emittor follower |---- row 2 drive |\\ /| drive |----------------- ------------| | | | | --- --- --- --- \\ / / LED \\ / / LED \\ / / \\ / --- --- --- --- | |-------------------| | |---- to B | B _| [ ] R [ ] R | | |/ |/ ----| ----| |\\ / |\\ / | | /// ///
I see what your saying, i was looking for some sort of IC solution. This is my circuit so far, its just a test to try and get a feel of what components i need. the word generator and the function generator simulate the operation of a MCU. You can see from the yellow bubble where the problem is.
Yours
HS
Oh yeah this would help
hybrid_snyper wrote:
You could use a power darlington like a TIP120. On a nice heatsink, it can provide up to 5 amps, and your 95 LEDs at
20 mA each need under 2 amps total. You'll need to limit the current through each of the LEDs so they don't burn out.Ed
Ed,
2 amps is all LEDs on at one time.The link provided shows a multiplexed system. So, one row of X LEDs x 20ma + 10% overhead is the total current required.
hybrid_snyper,
What is the configuration are you looking at.
5 columns x 95 rows = 95 LEDs, or do you have another configuration in mind. Only 5 LEDs would be on at a time. This would require 95 low side transistors and 5 high side buffers.Good Luck
donald
this is the setup i have so far.
On 14/11/2006 the venerable hybrid_snyper etched in runes:
Sorry, I never download Word documents.
Then you haven't read the front page of the data sheet which tells you that the drive capability of the 2981 is 500mA.
If you are multiplexing on a row-by-row basis then the sink transistors will only ever have to drive one LED load.
Why aren't you using a MAX7219, which is designed for this function?
He wants 95 leds in a row. "Would any body be able to suggest a device i could use to provide enough current to source a row of 95 LEDs." I must be missing something. His link shows a total of 32 LEDs - 4 rows of 8. Maybe he wants to source a *matrix* of 95 LEDs, not a *row* of
95?Ed
In spite of my typo [19->95], the OP did say "im building at LED matrix".
Is hybrid_snyper still out there.
donald
Yes, sorry i have not been feeling to good these past few days.
I'm building a LED matrix the DOC i posted is just test circuit to which I am going to build-up with more 5x7 LED arrays. The LED matrix is made of 95 columns and 7 rows. I'm not multiplexing the rows put rather feeding data into rows and scanning the columns at a high frequency with a decade counter made up from 4017s. The matrix will be controlled from a PIC. Columns will be scanned in such a way that the data will change with each clock of the counter. So data1 displays in column 1; CLK; data2 displayed in column 2. Obviously this syncing will be software controlled in the MCU.
With the direction of John B, i have looked at the MX6952
Thank you all for your input on this, im learning something new with every post, even that I have bad grammar, seem to type faster than my train of thought ;o)
One last note.
Is this a one off (2 or 3) or will you building many units.
Cost of each unit could be reduced by using more software and less hardware.
Good Luck donald
PS: Please post pics of what you end up with.
I understand that it is a matrix.
The matrix part is irrelevant. He is going to have
95 LEDs in each row. The fact that he has multiple rows doesn't change that. His spec was enough current to source a row of 95 LEDs. He did not provide a spec on the maximum number of LEDs per row that can be lit at any given time. Who knows - for testing, he may want the ability to light an entire row, or maybe the entire matrix, simultaneously. We can't assume that he will light only X LEDs per row at any given time - he will have to tell us that.Ed
On Nov 16, 12:16=A0am, "hybrid_snyper" > > >>>> im
Hi, this is a bad plan, you should arrange your matrix to be more like a square otherwise your leds have a very poor duty cycle requiring a large current.
My spec is to be able to display a line of scrolling text, to have a square would give me multiple lines and a whole load of LEDs i wont be using. Each 5x7 array will show a single ASCII character.
As for the question if i would ever have all the LEDs on at once, i could do this, maybe as a sort of test mode to show that all LEDs are working and that the scan frequency is high enough to reduce flicker.
The project is a one off.
Would using the MX6952 be the best thing to use?
Here is a link to a gif of my circuit to date
He means electrically more like a square. Actually, a rectangle. Imagine a matrix of, say, 19 x 35 LEDs. Now, break it into 5 rows of 19 x 7, but keep the wires connected; use infinitely stretchy wire and just put the second row alongside the first instead of under it, and so on, kind of zigzagging the column lines.
Figuring out which character in your string goes to which rows & columns could be as simple as a look-up table: code it once, and you're done. :-)
Good Luck! Rich
Think im going to go with the MAX6952 option it seems the simpler but at the moment is the most expensive. On the maxim website is quoted at $15 does anyone else know anywhere else i could pick up these chips for a lower price. Ive tried all the obvious places RS, maplin & Farnell.
On 16/11/2006 the venerable hybrid_snyper etched in runes:
Max7219 is cheaper, will drive 64 LED's and has software controllable intensity. Also available from RS & Farnell.
Yeah i see, the only problem with this IC is that its mores suited to an 8x8 array or almost two 5x7 arrays. The 6952 would drive four 5x7 arrays. So for setup of twenty 5x7 arrays i would only need five max6952 where i would need more ICs with the max7219
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