I would like to drive 5x 12V RGB ledstrips (only 5x3 leds) on 2x 74HC595 shiftregisters (using
the ShiftPWM library). The ledstrips have an common anode. They should stay individual
addressable.
Do I have to use transitors for each R,G,B line of all the 5 ledstrips?
That would be 15 transistors on the pcb!? Is there a better solution?
Do I have to use transitors for each R,G,B line of all the 5 ledstrips?
If using a 74HC595 then yes.
That would be 15 transistors on the pcb!
Yes.
s there a better solution?
Depends on how you define better, lower cost or lower component count?
What current does each of these strips take? That will determine your other choices.
No that is a current sink so it assumes you have a common anode strip.
So that means that I can connect the outputs of the chip directly to the kathode of each R,G,B lines?
Sorry... to be honest in the ULN2803 schematics I believed that the chip has common GND and
all the outputs are positive. Do you think I can use that chip?
Are you sure?
Hmm... I thought fading through all colors would give an good average of the current. Its actually jumping between values and
I saw 30-40mA max.
At what color would be the max. current? All PWM ports on 255?
Sorry... to be honest in the ULN2803 schematics I believed that the chip has common GND and
all the outputs are positive.
No all the outputs are not positave, the outputs pull down to ground. That is the output switches between being connected to ground and being connected to nothing. There is no voltage output from this chip.
That current looks better.
There are lots of chips you can use at that sort of current. Iin addition to the one above you can also use a TLC5940.
What purpose has pin 10 (COM) of the ULN2803 if not VCC?
It is the common connection for the reverse bias diodes used when switching inductive loads. Normally you have no need to connect pin 10 unless you are driving things like a motor.
Should I put limiting resistors before the inputs of the ULN2803 or
could I directly route the traces from the 74HC595 to these inputs?
It depends on how you are wiring things and how you are driving the multiplexer. Generally the resistor goes in the line that will see only one LED on at any one time.