The link in Michael Kohne's comment is the answer:
The iDevice needs to see a resistance in the neighborhood of 5k
between the microphone conductor and ground. That tells it that a
microphone has been plugged in. If it is a direct short, it thinks a
headphone was plugged in. Open circuit means nothing was plugged in.
The exact value is not important, from experimenting I found that 4.7k
seems to work. This resistor is put in series with the isolation
transformer.
I found that by putting a 10k resistor across "computer tape in" and ground the iPhone switches to the external mic. The record level is a fraction high - I don't know if that's related to the resistor or simply due to the signal from the Microbee.
Disclaimer: I have no idea what I'm doing, use at own risk!
The nature of LCD is different from LED in that the first uses a white backlight and filters the light with liquid crystal pixels. The other is transmissive, and every pixel is its own source of light.
If you want to make an LCD look at though it is a lower-resolution LED display, you could simply design your graphics to appear as whatever type of LED display you are emulating, as you mentioned. The brightness and contrast, however are going to give away the true nature of the display to any careful observer. Even if you put the LCD behind a dark translucent acrylic and increase the brightness of the backlight to compensate, the fact that the "LEDs" are made up of small pixels will probably always be detectable. However, casual observers probably won't notice.
The effectiveness of mocking an LED display depends on your purpose: If you're doing this for a special effect prop for film or TV, it then depends on how prominent it will be. If you're doing a tight shot on the prop, shooting in HD, etc. - it might not look as convincing as it might in a wider shot. If you're trying to make something with a display that's visible in sunlight, an actual LED display will probably be much better than an LCD with a bright backlight.
What pixel pitch are you trying to emulate/achieve? What pixel width and height? For me, that would be critical to decide which display tech to use. If you're going finer than say a few mm, I recommend LCD or OLED. Above that, you can find or make an LED matrix.
The effort to back-light only specific parts of the LCD sounds as complicated, if not more, than driving a normal LED display to begin with.
(Disclaimer: I work for a company that produces LED displays down to 3mm pitch.)
Best Answer
The iPhone LCD pixel configuration (I think starting with iPhone 3G) is built normally black. If you just power the backlight (which you can do fairly easily) you will only get a slightly brighter black screen :-)
You somehow need to send data to the display controller to set the pixel voltages to their maximum value to get them to let the backlight through.
A quick search yielded this
It might be overkill for your application but there might be a simpler way to send white pixels. Maybe search for something like "MIPI DSI controller" since that is the data protocol the iPhone displays use.
On the other hand, I think some of the older iPods use normally white LCD displays. You may have better luck with those.
Finally, you can simply remove the backlight panel (plastic backing which is sometimes mirrored) from the back of the display which will reveal the LEDs and some diffuser films. These make cool lights.