Boarduino project, need to supply 3v3 to display. *SOLVED*

Hi all. I'm not amazingly hot on electronics, so forgive me if there are any significantly stupid questions in this post; I'm still finding my way.

Background information: I'm trying to build a new digital dash for my motorbike. A bit like one of these ones, but more awesome, and modifiable. I've already gotten a simple version starting to work using a 128x64 GLCD panel, but I need something brighter, with better contrast, and fewer pins.

I'm hoping to run an ST7565 LCD display through an HEF4050B, which is fine in of itself, but on a Boarduino (which is where my troubles start). From what I can see, the Boarduino will only supply 5v, as opposed to the 3v3 that the display requires.

Can I use a resistive voltage divider for the supply here?

Assuming that this is OK, what's the best way of controlling the anodes for the red, green and blue elements of the display backlight? Can I just put them into digital pins on the boarduino, and then set them to LOW, according to whatever colour I want the screen to be? My concern with this is, again, that the Boarduino pins supply 5v, and I don't really want to knacker the display, if I can avoid doing so - largely because I got stung by customs charges when I ordered it from Adafruit.

Final question (at least in this post, anyway) - assuming that the previous two problems have been worked around, if I wanted to be able to dynamically change the colour of the backlight, would using the PWM pins on the Boarduino be the way to do this?

Thanks in advance.

nowhere-elysium:
I’m hoping to run an ST7565 LCD display through an HEF4050B, which is fine in of itself, but on a Boarduino (which is where my troubles start). From what I can see, the Boarduino will only supply 5v, as opposed to the 3v3 that the display requires.

Can I use a resistive voltage divider for the supply here?

No. Use a small 3.3v regulator.

nowhere-elysium:
What’s the best way of controlling the anodes for the red, green and blue elements of the display backlight?

Final question (at least in this post, anyway) - assuming that the previous two problems have been worked around, if I wanted to be able to dynamically change the colour of the backlight, would using the PWM pins on the Boarduino be the way to do this?

Depends on what the cathodes are connected to. If the cathodes are connected to the Arduino ground and the LED’s need less than about 30mA you would drive the anodes like any other LED: current limiting resistor to PWM pin. Then you can cross-fade between any of the 17 million colors.

johnwasser: No. Use a small 3.3v regulator.

OK, thanks. Is an LM3940 appropriate?

johnwasser: Depends on what the cathodes are connected to. If the cathodes are connected to the Arduino ground and the LED's need less than about 30mA you would drive the anodes like any other LED: current limiting resistor to PWM pin. Then you can cross-fade between any of the 17 million colors.

Um, I think I may have gotten the terminology wrong here. The R, G, and B pins on the display are negative, and they use a shared positive. Sorry for the confusion. On that basis, can I still achieve the same result?

nowhere-elysium:
I think I may have gotten the terminology wrong here. The R, G, and B pins on the display are negative, and they use a shared positive. Sorry for the confusion. On that basis, can I still achieve the same result?

Yes. It get’s only a little stranger. You have to hook the common anode to Arduino’s +5V line and hook the three cathodes through current limiting resistors to PWM pins. Now when the pin is HIGH no current flows because both ends of the LED are at +5V. When the pin is LOW you get current and the LED turns on. This reverses the sense of the PWM value: 255 means OFF and 0 means ON.

johnwasser:
Yes. It get’s only a little stranger. You have to hook the common anode to Arduino’s +5V line and hook the three cathodes through current limiting resistors to PWM pins. Now when the pin is HIGH no current flows because both ends of the LED are at +5V. When the pin is LOW you get current and the LED turns on. This reverses the sense of the PWM value: 255 means OFF and 0 means ON.

Heh. Loving the inherent perversity of this. OK, so would I be right in assuming that the same rating of resistor that makes using the +5v safe (i.e. a 270 ohm) would be the right one to use on the cathodes, too?
Sorry for rehashing what must be pretty basic stuff; I’ve got minimal background in electronics from a design/make point of view.

:edit: - just had a crack at it, and 270ohm was fine.

Mr. Wasser, I thank you for your time and patience, I’ve got to go order a voltage regulator now, and start crunching some code.