Oh shoot, I guess the bottom LED can't ever turn on. It would have to be turned the other way around. I clearly didn't think that all the way through. 
Yup and if you turn it round then you have to have a cathode and anode connected to the same pin, along with a ground.
I just tried to breadboard it flipped around and could not get it, but that doesn't mean much. If you have a different schematic I'm all about it.
@christop @LarryD
I saw some sense in the idea but it doesn't work with a common pin.
It does work with two different LEDs.
While very cool, it doesn't quite fit the needs of this project.
@chrisknightley @alto777 is this 2 LED circuit in 64 safe to use? I can sketch it up if the video isn't clear enough.
Yes, if I read it correctly.
If you never can see the output pin sinking or sourcing current beyond spec, it don’t much matter what’s hooked up to it or where the s
current leads.
But please do draw the schematic so we be sure.
a7
I don't think it's a problem, but it may not work if the LED Vf is low.
Because there is a current path from VCC to GND through the two LEDs.
Circuit of my understanding by the simulator (resistance value is sloppy)
Agree @chrisknightley, lose that resistor, then the not operating LED is essential shorted out of the picture.
a7
When I originally ran it I used the pin and the 5V from the board, but the crossed LED stayed on a tiny bit, so I switched to the 3.3V for the constant power line.
I pulled R2 and it works fine of course. LED1 has twice the resistance of LED2 and it's still brighter. Looks like I need to step up R3 a bit more.
Right. Lose R2, put the 5 volts supply back in the picture if the output pin comes from a 5 volt powered chip.
Then tune brightness with the other resistors as you would any plain LED plus series resistor arrangement.
a7
1 Like
+1 to all things @alto777
I noticed that I don't need to post this. 
Just about perfectly balanced now.
This is kind of cool from a logic perspective, like a cheap DDR-like clock signal but not quite because it's not really on the edges.
As you may know, you can turn off both by set the digital-pin as an INPUT (a.k.a. Hi-Z).
Sensitive to LED Vf and VCC voltage, there can always be a very small leakage current, but it works well in most cases.
A little math to file away 
I suspected this but was hesitant to until the circuit was reviewed. I guess I could have figured it out if I factored everything else out and saw it as a 4.7K resistor and a diode between the 5V pin and an input which is a safe configuration.
OK, now I gots to ask: where you picking up those 2K37 resistors? 
White LEDs have a high enough Vf, shouldn't hear a peep outta them at hi-Z on the pin.
a7
Car audio resistors, I have a thing for non standard and vintage stuff. Saw the odd value and had to have them.
Turns out they are great for halving a 4.7k.
Got these on Amazon I think. I'll dig up the link.
Edit: I got them off ebay, there are loads of links now.
I feel that I can get it up to E96 series.
But I basically use it up to E24 series.
That did not work as expected:
Probably my fault.
Here's the code:
byte led_pin = 2;
void setup() {
}
void loop() {
pinMode(led_pin, OUTPUT);
digitalWrite(led_pin, HIGH);
delay(500);
digitalWrite(led_pin, LOW);
delay(500);
pinMode(led_pin, INPUT);
delay(500);
pinMode(led_pin, INPUT_PULLUP);
delay(500);
}
Forgive the delays, they never make it past this kind of test.
