LEDs dim/flicker issue using single TLC5940NT

Other Hardware LEDs and Multiplexing
1

I've shifted to the TLC5940NT from using a TPIC6C595 after not getting ShiftPWM to work properly.
I am trying to push a series of 12v RGB LEDs using a TLC5940 but only get a bright signal when my Uno restarts. After the Uno initializes, I only get a faint, flickering output on each LED.

No matter which code I use, it is the same issue, so I have to assume it's a hardware issue.

One sketch I have tested is the BasicUse example as follows:

#include "Tlc5940.h"

void setup()
{
  Serial.begin(9600);
  Tlc.init();
}

void loop()
{
  int channel=0;
  for (channel = 0; channel < 16; channel++){
    Serial.print(channel);   
    Tlc.set(channel,4000);
    Tlc.update();
    delay(200);
    Tlc.set(channel,1000);
    Tlc.update();
    delay(200);
  }
channel=0;
}

Another is the "KnightRider" swipe:

#include "Tlc5940.h"

void setup()
{
  Tlc.init();
}
  
void loop()
{
  int direction = 1;
  for (int channel = 0; channel < NUM_TLCS * 16; channel += direction) {
    Tlc.clear();
    if (channel == 0) {
      direction = 1;
    } else {
      Tlc.set(channel - 1, 1000);
    }
    Tlc.set(channel, 4095);
    if (channel != NUM_TLCS * 16 - 1) {
      Tlc.set(channel + 1, 1000);
    } else {
      direction = -1;
    }    
    Tlc.update();
    delay(75);
  }
}

The circuit is wired as per the library example as follows:
+5V from Arduino -> TLC pin 21 and 19 (VCC and DCPRG)
GND from Arduino -> TLC pin 22 and 27 (GND and VPRG)
digital 3 -> TLC pin 18 (GSCLK)
digital 9 -> TLC pin 24 (XLAT)
digital 10 -> TLC pin 23 (BLANK)
digital 11 -> TLC pin 26 (SIN)
digital 13 -> TLC pin 25 (SCLK)
2K resistor between TLC pin 20 and GND

Do I need a decoupling capacitor?
Is the Uno too fast for the TLC?

2

Do I need a decoupling capacitor?

You always need a decoupling capacitor with every chip you add to the Arduino.

You might want to put a 10K pull up on pin 23 of the TLC.

I assume your common anodes are connected to 12V?

Can you power supply provide enough current?

3

The 12v power supply was pushing 70 RGB LEDs no problem. I can connect the Uno's VCC to the LEDs anode and they light up fine. I'll add the 10K pull up on 23 and report back.

4

Sorry about the delay....

Ok I added a 10K resistor between TLC5940's pin 26 and 5V. No change - strong signal right at arduino reboot, followed by dim flicker before going out completely. Verified all connections and still no dice. I only have three outputs of the TLC connected. Does it only work properly when all outputs have something connected to them?

5

oracledude:
The 12v power supply was pushing 70 RGB LEDs no problem. I can connect the Uno's VCC to the LEDs anode and they light up fine.

That does not obviate the need for decoupling.

http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

You need to post a schematic not a net list, and a photo ( not more than 1000 pixels wide ) showing your wiring would also help.