Traffic lights: gpios against 5V or ground

Question

I have 3 standard LEDs (red, yellow and green) connected in parallel to GPIO pins 10, 11 and 12. I also have one resistor, to which they are all three connected.

What's the difference between connecting the resistor to ground (so that the leds are high active) or to the 5V output pin of the arduino (so that the leds are low active)?

Moreover: how many ohms should the resistor have? My understanding is: each of the 3 standard LEDs needs 20mA, so, if all three are lit, there is 60mA on the circuit. The 3 parallel LEDs need 2 V. So, the resistor should be chosen such that it has a voltage drop of 3V at 60mA, hence 3V/60mA = 50 Ohm. If only one is lit, I need 3V/20mA = 150 Ohm.

But, if its possible that every possible subset of the LEDs is lit at once, wouldn't it be better, to place a resistor in front of every LED, so that I have 3 LED-resistor pairs in parallel? The value of each resistor is 50 Ohm then, allowing for a voltage drop of 3V in each of the 3 branches, so that each LED only gets the needed 2V?

Well, a lot of thoughts for 3 simple lights... Thanks for any help.

Answer 1

I have 3 standard LEDs (red, yellow and green) connected in parallel to GPIO pins 10, 11 and 12. I also have one resistor, to which they are all three connected.

BAD BAD BAD!!!

What's the difference between connecting the resistor to ground (so that the leds are high active) or to the 5V output pin of the arduino (so that the leds are low active)?

Nothing, other than the reversed logic. AVR GPIOs (and most GPIOs in modern MCUs) are balanced - i.e., they have the same current limit for both sink and source.

Moreover: how many ohms should the resistor have? My understanding is: each of the 3 standard LEDs needs 20mA, so, if all three are lit, there is 60mA on the circuit. The 3 parallel LEDs need 2 V. So, the resistor should be chosen such that it has a voltage drop of 3V at 60mA, hence 3V/60mA = 50 Ohm. If only one is lit, I need 3V/20mA = 150 Ohm.

But, if its possible that every possible subset of the LEDs is lit at once, wouldn't it be better, to place a resistor in front of every LED, so that I have 3 LED-resistor pairs in parallel? The value of each resistor is 50 Ohm then, allowing for a voltage drop of 3V in each of the 3 branches, so that each LED only gets the needed 2V?

Yes. That is how you should wire LEDs - and you have already identified why.

For each LED calculate the resistance as R=(5-V_f)/0.02. You need to find the correct V_f for each LED from the datasheet to get the best results - but if you can't then a rule of thumb is: Red, Green and Yellow are about 2V, Blue and White are about 3V.