HD44780 controlled 16*2 display

Tokesnugerd · Post by **Tokesnugerd** » January 19th, 2008, 8:38 pm

I tried to connect and LCD to the printer port with this circuit:

But it doesn't seem to work..
The basics are the same as a circuit on the lcd smartie website (http://lcdsmartie.sourceforge.net/circuits.htm , 3rd image)

I've used a 10K resistor for the contrast and I got the 5V power from the usb port.

When I start LCDSm and set everything right.. the BG wouldn't even light up.
If I bypass the transistor to activate the bglight (on the picture the dotted line) the display does light up. But That's all.. no text displayed.

Is there anyone who can see what I'm doing wrong here ?
Thnkyou very much!

Post by **mattcro** » January 19th, 2008, 11:23 pm

You have the contrast resistor (10k) connected wrongly. Connect it between GND and Vo (pin 3). Otherwise, you won't see anything on the lcd because the contrast will be very low. You might even find you need a smaller value resistor (1k). Sometimes, you can get a good contrast by connecting pin 3 directly to GND. You should see some blocks on the LCD when it powers up.

Double check the port settings in Smartie. You can probably use the HD44780 driver with the port set to LPT1.

For the backlight to illuminate, you need to have the brightness slider all the way up (or maybe it's just more than halfway).

Tokesnugerd · Post by **Tokesnugerd** » January 20th, 2008, 10:14 pm

I think the first tip will work.. I'll try to fix that tomorow..

But when adjusting the slider, the light intensisty won't change a bit... what might causing it ? the transistor ? Might it be broken ?

Then I'll have to get another one

Thnx alot

Post by **mattcro** » January 20th, 2008, 11:04 pm

I have a feeling the HD444780 driver does not automatically turn on the backlight when it starts up (I could be wrong - other display drivers do turn on the backlight on startup).

If that's the case, you might need to add an action to turn on the backlight (if 1=1 then backlight(1) ). You can check if Smartie is driving the backlight control with a voltmeter on pin 17 of the DB25 connector. You might have the trasistor wired wrong or it may indeed be damaged.

Note that the backlight will either be on or off - there's no cool fader control for parallel port LCDs, unfortunately.