Max Current for LCD in K221 kit

[MearCat]

I have a Oatley Electronics K221 Serial LCD kit (http://www.oatleyelectronics.com/kits/k221.html). Phanderson also use this in the #108 kit. I have a large character backlit LCD and am using the K221 kit to interface to it. Because my LCD uses a HD44780 interface, everything is fine when writing to the LCD.

My question is : The K221 kit documentation (first page - http://www.oatleyelectronics.com/kits/Notes/K221.pdf) says that the "current is controlled by a transistor which is included with a 4R7 resistor". The backlight on my 16x2 large char LCD is quoted as a rather chunky 300mA at 4.2V I have replaced the 4R7 resistor with a 2R7(0.8V drop / 0.3A = 2.666Ohms) resistor to limit the current to 300mA.

The "?B" command (see above K221 doc) allows you to change the backlight intensity but what does it actually do? Does it actually change the current that is supplied to the LED backlight? If I run the command "?B00" will it essentially turn off the backlight or just be really dull? If I run the command "?B7F" will it limit the current to only 150mA to the backlight? Will the backlight only light up if I set the backlight intensity with "?BFF" ?

MearCat

Edited by - MearCat on 11/23/2005 7:40:49 AM

 

[pha555]

You have it correct.

The ?B command changes the duty cycle of the PWM output on the LCD #108. Thus, a value of 7F or 80 will cause the output to be high 128 / 256 of the time. The average current will then be half.

BTW, I made an error which was only caught recently. If ?B7F doesn't work, try ?B7f. That is, I overlooked the fact that with hexadecimal, the alphabetical character can be upper or lower case.

Best wishes.

Peter Anderson, http://www.phanderson.com/picaxe/picaxe.html

 

[MearCat]

So what happens if teh backlight contrast is set to 7F (average current of half = 150mA in this case) will the bakclight on the LCD which draws 300mA, still work but be dull? Or just not work at all?

 

[pha555]

At 50 percent duty, the intensity will be somewhat dimmer than at full. But, you should certainly see some backlight.

P H Anderson
http://www.phanderson.com/picaxe/