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[on:]

Hi there,

the old thread is kinda bugged (the site dont loads and is white :/ ) so i deleted it.

i am planning a 7x20 rgb led matrix.
i want to get the leds off a rgb led stripe like this: http://www.ebay.de/itm/LED-Strip-SMD-Streifen-Band-RGB-300-5M-Leiste-5050-12V-LD62-/250970230849 because i have much time but limited cash . these leds have 3 anodes and 3 kathodes, but i would combine the anodes to a common anode to ease the wire-ing.
thinking that i would have to handle a lot of leds (140 ) i read through a few topics and decided i would multiplex the leds 1:7 to keep the leds bright.
the leds im propably going to use need (according to a datasheed i found) between 2.5 and 3.5 V depending on the color channel. they are usually supplied with 20mA per color, but can handle up to 80mA pulsecurrent.
 --> 20 leds * 3 colors * 80 mA pulsecurrent = 4,8 A !
i thought of using some transistors or mosfets to supply the current, but i am not sure how to do it exactly.
should i multiplex 1:20 to keep the current low? but will the leds be bright enough and be flicker free?
how would i control the matrix best?
i came up with 8x 74HC595 shift registers to conroll the R, G and B cathodes of the leds and one binary to decimal converter to control the anodes.
these ic's would be controled by an arduino uno with the help of the ShiftPWM library (http://www.elcojacobs.com/shiftpwm/), which can controll the shift registers.

sorry if i express things a little bit complicated

do you have any suggestions about powering the leds or any general improvements?
i want to try to spend little cash as possible

thanks for your help

tom

[Reply #1 on:]

74HC595 cannot sink current for 7 LEDs.
Use part like TPIC6B595 instead.
What is IC2? Those kinds of parts generally have 1 output low based on the ABC pins, so also not a good choice.
As wired, you need something that can source 60mA to the common anode, and something that can sink 140mA from the common cathodes.
Maybe this would work using UDN2981 as buffer for IC1 output for the source, and TPIC6B595 for the current sink.
However, you need an anode high for any color to turn on. You could not get only Red on LED1 and only Blue on LED2.
Is your intention to have only 1 color on at a time in any column?

[Reply #2 on:]

i think i got cathode and anode wrong.. edited the first post :/

anode = +, cathode = - right?

ic2 is a 74HC238 which has 1 line high at a time.. i want to control each color of each led individually from the RGB color scheme.
i thought i would use the 74HC238 to controll 7 mosfets which are powering the cathodes.
because i want to multiplex the leds i would need 4.8 A output of such a mosfet. (80mA per color * 3 colors per led * 20 leds a row = 4.8 A)
each cathode would be at 80mA because i have 1 cathode per color. the 74HC595 can do both: source and sink power. the question is if it can sink enough (100mA) for my project..
correct me if im wrong ^^

[Reply #3 on:]

Okay, so you will enable one row across the screen at a time - then you need a 520mA source for each row.
74HC595 cannot sink enough current, see prior message.

[Reply #4 on:]

yes i want to enable one row at a time. one row are 20 leds á 3 colors á 80mA. this would be 4.8 A if you look at my schematic it should get clear. every led in one row is parallel. so i need ~3V per row. the currentsink chips have to handle 80mA per pin.

would these 80mA be a constant, or a pulse current? because since i want to multiplex i am constantly pulsing with the shift registers, it could be both.. i need to know because in the datasheets some shift registers are values for pulse and constant current

would it be ok to use a ULN2003 and a 74HC595 to sink the current.. ? (seen it on other projects.. )
how could i get 4.8A at 3V ?

again sorry if i write to complex

[Reply #5 on:]

What LEDs are you using that need 80mA?

How about this - only turn a small group of cathodes at a time so you don't such big current (and hefty wiring)?
Treat is as dual  multiplexing - across each row, and then down the multiple rows.

[Reply #6 on:]

i just want the pulsecurrent to be 80mA so the leds are bright. the normal current they need is 20mA, but they are only on 1/7 of the time.. so i thought it would be good to go up to 80mA, the maximum the leds can handle.
you are suggesting that i only power 1 led at a time? wont it flicker if i do so?