Backlight connected to instrument light?
 

Re: backlight brightness levels. Is it possible to change the backlight brightness with a potentiometer? The backlights in my instrument cluster are controlled that way, and it would be good if they could be controlled together.

I added this to it's own thread cuz it is a neat idea and not iduino specific.

This is entirely doable, but you have to tap into your instrument panel lights, then figure out how big a resistor you need to drop the voltage down to 4.3 volts max (I think) on the backlight. I would assume a system voltage of 14 volts. Use 9.7 volts divided by the max backlight current (from your LCD datasheet) in amps, to figure out the minimum sized resistor needed.

Then unhook the existing connection to LCD 15 and hook up your resistor instead which leads to the light circuit that is controlled by the dashboard potentiometer.

I also thought of using a photoresistor on an analog entry that will drive the brightness automatically. During the full day the LCD will be bright, during the night it will be darker, it would be linear and in "real-time" too.

Easy to do.

FYI on photoresistor:
Photoresistor - Wikipedia, the free encyclopedia

If you just tapped into the instrument panel lights for the backlight source, wouldn't the backlight not turn on in the daytime (whenever the dash lights are off)? Is the LCD readable without the backlight on?

McTimson, I would presume you are correct. What about making them dim when you turn on the lights. Just like most car stereos.

If it's LED, a simple transistor circuit can be used to reduce the current for dimming. If it's CCFL or OLED, you'll have to somehow reduce the duty cycle of the inverter.

I'm pretty sure they're LED, at least the one I have looks like it is.

It could easily be controlled with a separate potentiometer, if you were willing to do that. Connecting it to the same pot as the dimmer in your car would be a little more difficult.

But really, how often do you use the dimmer in your car? Maybe it's just me, but I've never had to dim my dash, I guess it just might not be that bright to begin with.

I think a separate potentiometer for the MPGuino would actually be better, because finding a sweet spot for the dashboard brightness and the backlight brightness would be hard with one pot controlling both, with 2 separate ones, you could fine tune each one to its' own brightness.