Author Topic: resistors and led with a pro micro  (Read 875 times)

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Offline godzillq25

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  • Posts: 3
  • Location: australia
resistors and led with a pro micro
« on: Wed, 13 May 2020, 21:33:55 »
hi there first post here, read a lot of pages haven't come across what i looking after.
i posted this on reddit and got hit with the ohms law, yes i get ohms law and how to use it one one led 2 led how do i implement it on 68 leds

i got this pcb printed from
mcu is just a chinese pro micro, it has rated 500mA usable but after searching around that only 300mA

the leds im using are :
the leds in question warm white
working current: 20(mA)
working voltage: 3-3.2v
wavelength: 2800-3200

also there is something interesting that i don't understand this part on the pcb
are these resistors for to have the pcb running or for leds

Offline vvp

  • Posts: 804
Re: resistors and led with a pro micro
« Reply #1 on: Thu, 14 May 2020, 02:35:22 »
Pictures of PCBs are useless, especially when traces are hidden by solder resist paint. Don't bother posting them. Post a schematic.

Notice that the maximum current through one ATmega IO pin is only 40 mA. That means: if you want to switch on/off the LEDs from firmware then you may need to use a transistor to boost maximum output pin current.

You have more options:
* connect all LEDs in parallel (each LED will have its own resistor) - that means one led can use at most 300/68 ≅4 mA
* try to connect two LEDs in series and check how much current they take (if it is less than 8 mA and they are still bright enough) then you can connect blocks of LEDs in parallel; each block  will have two LEDs in series (without any resistor), this allows you not to waste energy in resistors but it will be more sensitive to slight differences in LEDs
* use a LED driver IC, you need this option if you want to control brightness of each LED separately; there are hundreds of LED driver chips. select one and read its speciffication
« Last Edit: Thu, 14 May 2020, 03:22:20 by vvp »