# Why is one LED more luminous (brighter) than the other?

Given that there is a circuit with two pins (12 and 10) from an Uno providing power to LEDs on two separate branches, why is the yellow LED dimmer than the green LED? (Please see below image.) To my understanding, the branches attach to GND in parallel, so the LEDs should be equally bright.

If it helps, the two branches are programmed so one LED is off when the other is on and vice versa, with a 1s interval between changing pin state. This isn't a PWM thing as using pin 8 instead of 10 makes no difference.

• Different colour LEDs have different forward voltages and require different value resistors to get the same current through them. Commented Aug 14, 2016 at 20:16
• What do you mean by "forward voltages"? Does voltage not travel in only one direction? Commented Aug 14, 2016 at 20:19
• You will need to read some tutorial about LED functioning in order to properly handle your circuit. I highly suggest learn.adafruit.com/all-about-leds/overview in it all terms like forward voltage and forward current are explained, among others. Commented Aug 14, 2016 at 20:21

Just like men, all LEDs have been made different.

Practically, that means two different LEDs (one green and one yellow for instance) will have different brightnesses (more precisely, luminous intensities) when they are traversed by the same current (forward current).

LED luminous intensity is a function of the current that traverses it (among other factors).

If you want the same brightness for those 2 LEDs, then you'll have to:

1. Get both LEDs datasheets (should be a couple of A4 pages for each)
2. Find the luminous intensity/forward current function curve in each
3. Determine which brightness you want for both
4. Deduce the current for each
5. Calculate the resistor value to get that current for each

Then you will have to use two different resistors values in your wiring, based on calculations of step 5.

Simpler, you could just replace one of your 2 resistors with one small pot that you can use to tune brightness of the second LED until it fits brightness of the first one.

Be careful with pot manipulation though, as a 0 resistance would probably grill your LED (and potentially the Arduino pin it is wired to).

• Thanks for the warning about burning LEDs with pots; I already did that to one of them testing out how much luminous intensity I could get out of a red LED. :( Commented Aug 14, 2016 at 20:22
• Don't worry that can happen to anyone, even not beginners (I grilled a few LEDs myself, including 7-segment displays, and also a few transistors :-)) Commented Aug 14, 2016 at 20:23
• Add to that the human eye responds in different ways to different colours of light - green being the most sensitive... so you have visual intensity as well... Commented Aug 14, 2016 at 20:38
• @Majenko correct, but that is not shown in the datasheets. This is why in the end, the pot solution is probably the most relevant. Commented Aug 14, 2016 at 20:39
• `Just like men, all LEDs have been made different.` - and women. ;) Commented Aug 14, 2016 at 20:56