I am planning on making a 8x8 LED matrix using a MAX7219.
As I will be driving up to 8 white LEDs in a row, the 40ma output of the 7219 will not be enough (?) and I am therefore planning on using a MOSFET for each row with an external power supply.
My question is this: How do I go about controlling the voltage/current for the LED rows? Since there will be anywhere from 1 to 8 LEDs on at once, how would I limit the maximum current for the row if the number of LEDs that light up arent constant?
The rows will have different forward voltages since they will be different LEDs (some white, some not).
I have been reading this which seems to adress the issue:
...but I'm still not sure exactly how it works.
I am planning on building a Daft Punk Guy Manuel helmet, this applies to the LED matrix as seen in the picture below.
Explain what you want to achieve, what patterns you want to control the LEDs, whether you need individual or group control of the leds. Post links to the specs of the LEDs you want to use. We can then advise what chips/mosfets could be used to drive them. Max7219 may or may not be appropriate, it's not clear from what you said yet. Attaching mosfets to max7219 may not the the easiest/best way. Have you considered ws2812 or apa106 LEDs, for example? Why do you think 40mA will not be enough for your LEDs?
I want to have individual control of the LEDs. The LEDs must have the form factor of the ones linked above, since they have to fit within a very small space in the helmet.
That means that I would be powering up to 8 LEDs in a row.
Reading this, Arduino Playground - MAX72XXHardware , made me realise that I had misunderstood something previously and that the current draw probably wont be an issue.
If I've understood the text right, I shouldn't have to worry about different forward voltages for the different rows (except for selecting which Rset resistor to choose). Is this correct?
(Edit: I understand that the white lights will appear slighty more dim when settling for a higher Rset value to properly power the 2.0Vf rows)
You should also consider my ws2812 or apa106 suggestions. These will be much brighter because they are not multiplexed, can be individually controlled, you can control brightness and fine tune colours as you want them, no driver chip is required, saving space, and one wire controls them all, so wiring is much simpler.
Those seem interesting as well, but the problem is that I want the form factor to be rectangular for the bottom 6 rows of LEDs. Nice suggestion though!
