LED cube PWM TLC 5940 with 7404 not gate

I make a 444 RGB led cube,but I use the wrong LEDs,it should be use common anode but I use common cathode…
I want to ask
if I can use 7404 not gate to invert the LED PWM to let TLC5940 drive the common cathode LEDs?
The 7404 can supply about 16-20mA for output(from datasheet),but I wandering can 7404 fast enought to invert the PWM signal from TLC5940? :confused:

if I can use 7404 not gate to invert the LED PWM to let TLC5940 drive the common cathode LEDs?

You loose the current control the TLC5940 gives you and you can never turn the LEDs fully off.

So in my book that would be a no. But yes they would be fast enough.

Here's an idea to rescue the cube. Remove some of the horizontal wires in the cube, so that you are left with 4 separate towers, each of size 2x2 base x 4 high. This gives you access to 16 groups of 4 common cathodes. Then common up the anodes into 4 red groups plus 4 green groups plus 4 blue groups such that when each group of anodes is energised, current flows through only one of the 4 LEDs in each group of common cathodes. This means only one tlc chip will be needed, and you will have 12 anode groups instead of 4. The duty cycle with be 1 in 12 instead of 1 in 4, but you may be able to boost the led current to partly compensate for that, depending on the specs of your LEDs.

Thanks for your replies

I did an experiment yesterday,it works well.
And the TLC5940 provide 1kHz PWM signal :slight_smile:

What experiment?

I connect tlc5940 into 7404 not gate input ,and connect output to common cathode LEDs.
Also connect it to oscilloscope,And it work

It only appeared to work because an unconnected TTL gate floats high, at the least you need pull up resistors on the input of each TTL gate and then a seriese resistor in line with each LED to prevent premature failure of buffer and / or LED. It won’t instantly go fizz but you need to keep the current in the LED below 20 mA.