Mike - thanks.
Okay.. Then I've figured it out.
You were right, the 9XR drives the LCD backlights at 11ma; but that's 11ma EACH and there's 5 of them in parallel for a total of 55ma, which is EXACTLY what I get on my 9XR from a full pack. On the Pro that's 11ma/5 = 2.2ma per LED.
For this panel at a 5V source, that's 5V - 3.4V VF = 1.6V/0.055A =29.1Ω. Closest standard resistor is gonna be 33Ω 1/8W.
The LCDs ARE different; the polarized filter on the one from the Pro is noticeably darker and as you can see the ribbon cable is different.
That said, the difference in Contrast with the Pro LCD in my 9XR is NOTICEABLE; enough so that I'm going to keep the 9XR LCD in my Pro, but probably not enough that I'd buy a new 9XR LCD and take the gamble that I might wind up with EXACTLY the same part as came in the Pro if the different LCD was just a mid-production substitution.
[EDIT]
It would appear this is a moot point, as HobbyKing no longer offers the replacement LCD for the 9XR. ![Sad :(](./images/smilies/icon_e_sad.gif)
[/EDIT]
We'll see once I have a chance to replace the ballast resistor on the Pro. Since we do have a transistor in the circuit, I can do that. I might even investigate driving these LEDs to 15ma ea; typically these little SMDs are good for 20-25ma max, so still quite a bit of leeway. That will probably lighten up the shade of blue quite a bit as well.
Mike - since this circuit is controlled via PWM... maybe a 4.7-10uf cap across the LEDs to smooth out the ripple? Most of these LEDs will actually drive a lot higher current in pulsed mode though... 5 of 'em can probably handle more than Q3 can put out.
Thoughts?
Is the PWM frequency range low enough that flicker is a concern?
mnem
*Lit*