5V power from pc

kostassite · Post by **kostassite** » August 28th, 2007, 10:29 am

Hi,
I have found a solution about the power.
First I have used a battery but sometimes I forget to turn off the power at the nght so at the morning the battery was dead.Then I try an AC adaptor 220v to 5v that was better but i had to put it off from the wall because the backlight remained on all the night.
So I solve my problem with this. I thought that the PS/2 port of the mouse (or keyboard) might have power so I search the pins and I find that pin 1(at the schematic looking the connector from the back) has +5v and the pin 2 is the ground.The lcd and the backlight(with a resistor) needs ~10mA so it is safe for te motherboard (You can take up to 100mA).NOW THE LCD SWITCHES ON AND OFF WITH THE COMPUTER.
http://kostassite.gr/forum/download.php?id=39 Here is the schematic.

fishthecat · Post by **fishthecat** » August 28th, 2007, 11:06 am

Well done!

I get my power from a USB port. It's 5V so I just butchered an old USB lead. I've not had any problems with it trying to draw too much power and like your solution, it turns on and off the LCD with the PC.

Post by **mattcro** » August 28th, 2007, 9:17 pm

10mA sounds like a rather low current for an LCD plus backlight (even for a 2x16 LCD), unless it's running very dim. 60-100mA is typical for a 16x2 LED backlight. My 4x20 LCD uses up to 200mA for the LED backlight.

PS/2 and USB ports are a handy way to get regulated voltage at low current. With an external supply (especially AC adapters) you need to be careful about the voltage regulation. Some provide poorly regulated outputs that are well above the stated voltage.

I took the power for my LCD from a spare disk drive power connector inside the case. If you do this, it's a good idea to use a fuse of a suitable rating in the +5V wire to the LCD. I used a poly-fuse (self resetting) for my external LCD. USB ports have current limiting built in, and I think PS/2 ports usually have a poly-fuse.

kostassite · Post by **kostassite** » August 29th, 2007, 11:18 am

YES I HAVE IT VERY DIM BECAUSE IN NIGHT ISN'T PRETTY THE VERY BRIGHT. BUT IN FULL POWER USES ~50 mA.
ME TOO AT MY DESKTOP PC I TAKE FROM THE DISK BUT THE OTHER IS LAPTOP AND I CANT. MOREOVER WITH THE USB OR PS/2 IS MORE PORTABLE THE WHOLE LCD.

Post by **mattcro** » August 29th, 2007, 1:55 pm

Ah, I see. I also have the backlight quite dim at night, and rarely have it above 1/2 brightness even during the day. Full brightness is too much.

_X7JAY7X_ · Post by **_X7JAY7X_** » August 29th, 2007, 7:16 pm

Why dont you put a photosensor on it then? Just a thought.

Post by **mattcro** » August 29th, 2007, 9:05 pm

Hmm, an interesting extension to the project. Ambient light compensation, like some monitors and phones have now.

I've got my backlight under PWM control from Smartie so it's easy to change if I need (which is basically never), but an automatic change would be cool.

A fixed-brightness backlight could be controlled with a photo-resistor and few resistors and transistors. There's probably even a schematic somewhere for it already...

kostassite · Post by **kostassite** » August 30th, 2007, 8:52 am

_X7JAY7X_ wrote:Why dont you put a photosensor on it then? Just a thought.

Because i have install buttons plugin if i have the backlight full consumes a lot and the buttons dont work. Moreover if the backlight is in full mode, sometiimes the lcd dont start.

_X7JAY7X_ · Post by **_X7JAY7X_** » August 30th, 2007, 1:25 pm

With a photosensor you dont have to go to both extremes (high and off). You can have the photosensor work within a range of half brightness to very very low brightness.

Mattcro, you could make smartie set the highest brightness and the lowest brightness through another command (say chr(254),chr(xx),chr(0-100),chr(0-100)). Then the micro could read the photosensor and vary the brightness between those two points.