LED dot matrix brightness!!

[mindstorm88]

Hi guys , I need small help , i did a small message board of 8 row by 80 column , using code that was produce by Mark few years ago , everything works well , but brightness is low , i know that it is due to the fact that scanning is fast and pulse to the dot are not long , but is there a way to increase that brightness ?? circuit ??

btw it is only for hobby , no commercial future

Right now i'm using 74hc595 for column and uln2803 for row , the 8 row are scanned within the column pulse to insure all dot have the same brightness !!!, So all dot are ON one at a time !!!

Thanks for your help

Bruno

[kevcon]

With out seeing your circuit it's hard to say, but two questions I would ask would be.

How much current are you driving the leds with?
typical ciruits I've seen drive them at about 100ma

Are you using one resistor for 8 leds or one resistor per led?
One resistor per 8 leds will cause a large voltage drop do to the current draw for 8 leds.

[mindstorm88]

I'm using a 56 Ohms res per column at the 595 output , my Rows are scanned too so 1 pixel is On at a time !!! i'm probably driving the 595 outputs to the max , I'm looking at using mosfet for column drive !!!

[rnielsen]

You can over-drive the LED's fairly safely if the ON time is simply pulsed. Usually, the MAX current that's listed in the spec. sheets is for when it's on constantly. This is to prevent the device from over-heating. This current can be raised significantly if it's simply pulsed and the ON cycle time is low.

Also, I would not have one resistor limiting the current for multiple LED's unless the LED's are in series. If you are trying to put LED's in parallel you could have a problem with some of them not conducting since their threshold voltage might not be reached due to an adjacent LED holding the voltage too low.

Ronald

[kevcon]

You need more drive current, the 74hc595 only can drive 25ma

[mindstorm88]

Yep , i think i'm gonna try TPIC6C596 for column and fdn358pct to drive column !!!
i let you know guys how it goes !!!

[mindstorm88]

ok , i did my board using tpic6c596 for column and mosfet to drive row !!, each column as a 59 ohms resistor , vcc for the leds is 14v . With that the brightness is ok but not to the max possibility of the leds !!!

i guess it will probably be impossible to get max brightness due to the fact that scanning 512 leds at 100 hz means that each leds are on only about 2ms per second !!!

any idea for improvement ???

[kevcon]

Can you post your circuit?

[SherpaDoug]

Can you scan by row instead of by LED. With 8 rows you should be able to have each LED on 1/8 duty cycle.
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[mindstorm88]

Hi Kev , i don't know how to add a drawing!!!, but it is very simple , all column goes to an output of the shift register through a resistor 59 ohms , each row get the vccled via a mosfet , so 8 mosfet driven by the controller

to answer Sherpadoug , the reason i scan by led instead of row is the fact that a row with few led is brighter than a row with all led , because there is no current regulation !!!!

oups as i'm writing , i just realised that if i remove all resistor from the column and put resistor only on rows , i could scan by rows !!! should i though of it @#$@#$

[mindstorm88]

Latest results , i did remove all column resistors and put the 59 ohms resitors on each row , meaning that 8 resistors are replacing the 64 fisrt one , should i though of it before !!! things like that happens when you start from someone else design !!

now i can scan by colunm instead of dots increasing to "on" time off each dot from 2 ms to 16 ms per second , now i'm getting very good brightness at 8v on led power !!! driving about 100mA per dot !!!

[Matrix4U]

Hello mindstorm88

Please can u help me to build the same as u did LED moving message 8x80 i am begineer in embeded new to ccs c i am trying but till now no results i did in 74hc154 8x32 now i want to do using 74hc595 for learning purpose so 1 of the gorup member suggest first do led movin message then u will understand all shift register and the c programming

please can u tell me the shcematic and little code i will make and try

i be so thankful to u

thanks