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Hi

We are trying to do the QlockTwo project by using a 11 per 10 LED Matrix by using a multiplexing scheme.
The issue is that the brightness provided by the LED is too low
•           When we use only one white LED with 5 V power  and a 100 ohm resistor directly linked the result is perfect (for what we are looking for).
•           When we use the same components (LED , power and resistor) but linked by 4 shift register (74HC595) to provide 11 lines (with resistor) and 10 row in order to monitor 110 LED, the brightness of the LED is too low ... even if we decide to highlight only one LED of the 110 LED.
 
We really don’t know the way to solve this issue, we are two inexperienced French guys and we are lost on electronic topics ... is somebody could help us to increase the brightness of our 110 LED ?
 
Many thanks for your help.

Francki

[Reply #1 on:]

Multiplexing will be much less bright - here your LEDs will be on for only 10% of the time. If you use more shift registers (say 4 shift registers giving 32 rows) then you'll need fewer columns to multiplex (4) and thus 25% of static brightness...

Also you could reduce the resistor values so that the LEDs are overdriven (at 10% duty cycle you may be able to overdrive somewhat - not by a factor of 10 though!)

Another approach is using direct drive from 16-bit LED driver chips - these are basically latched shift registers with constant-current drive outputs for LEDS - with 7 chips you can drive 112 channels at 100%...

[Reply #2 on:]

Hi MarkT
Thank you for your answer

Could you explain your proposal to use more shift registers to increase the brightness
I m using 4 shift registers, 2 for the 11 lines and 2 for the 10 rows in order to monitor easily my Clock 11per10 matrix.
How can I have fewer columns?

I tried to reduce the resistor values but the result is not good

Could you detail your last approach to use direct drive from 16-bit LED driver chips?

What are you thinking about component "ULN2003" or "using a "high-voltage" supply 12V, a small resistor to drop the rest of the voltage in series with the LED's, and a logic-level MOSFET (e.g., NDP6060L or similar) to turn the whole thing on". (Suggestion found into this topic "http://arduino.cc/forum/index.php/topic,55519.msg397941.html#msg397941")

Thank you for your answer.

Francki

[Reply #3 on:]

He was suggesting that you increase the number of LEDs in the matrix in one direction so that at any one time there are more LEDs on at the same time. That means you have to do less multiplexing, that is the on to off ratio is lower. This minimizes the reduction in LED brightness.

However unless you post the schematic of what you have in terms of the LED matrix (are they in an block or are they made of discrete components), it is hard to be specific.

[Reply #4 on:]

Hi Grumpy_Mike

Thank you for your answer and sorry for the delay

You will find below the schematic (without software, we are very beginner) It summarize our brightness issue with a 5 v output
Have you got an idea?

Thank you

Francki

[Reply #5 on:]

You really  need parts that can handle more current. 74HC595 is only rated for 8mA, you want more like 20mA for much higher brightness. Part such as 74AC299P for sourcing current, and ULN2803 for sinking current on the output of the cathode driver shift register.
In your example, 24mA drivers would source current into the red columns, 1 ULN2803 pin would go low to turn LEDs for that row, then off.  Red data would change, next blue row would turn on, off.
Red data would change, blue row 3 would turn on/off.  Continue for all 10 rows, repeat!

[Reply #6 on:]

Hi CrossRoads
Thank you for your answer, I will try to investigate on this two parts during the WE, I will come back later.
Does  anybody have others good suggestions like that ?
Many thanks
Francky