Public Lab Wiki documentation

I need help with some transistor math for a 74hc595

1 | 1 | | #14507


im trying to dimm the ouput channels of a 74hc595 by using a transistor between pin 13 and Ground.As the 74hc595 datesheet here.

Im currently have a 100 Ohm resistor between the transistor and the PWM output pin of the Aruino, but no matter what state i set the output pin, the LEDs always have the same brightness.

The IC is controlling 40 LEDs (each with its own transistors) with 20mA when everything is on at maximum and im using an all purpose SMD transistor as well as an SMD 74hc595.

I would greatly appreciate any input :)