[chbot] Blue HD44780 LCDs

[Helmut Walle]

On 09/08/15 14:10, Robin Gilks wrote:
> Greetings all
>
> I've only ever used green and yellow HD44780 style character mode LCDs and
> they have had a backlight (which draws a LOT of current) but been
> perfectly readable without the backlight being on.
>
> I'm now playing with a blue one and the backlight has to be on to read it
> - is this normal for blue LCDs ?
>
> I haven't checked the current yet (rebuilding alpha hardware into beta
> stage prototype) but I'm guessing its quite low since modern blue LEDs
> seem to be very efficient.
>
> Cheers

Differences in daylight readability between reflective and transmissive 
LCDs maybe? And backlights for LCDs have not always been LED... for 
example, if you are buying an "LED" TV today in many cases that means 
the backlight for the LCD is made up from LEDs, rather than the fluoro 
tubes that were standard for decades until recently. However, backlights 
for smaller LCDs often were incandescent bulbs back in the last 
millennium, and LEDs have been in use there for quite a while (much 
longer than for bigger LCDs).

How much current is "a LOT"? I would expect an LED backlight for a small 
module, say anything up to 4 x 16 character, to be around tens of 
milliamps at most. It it draws a LOT more I would suspect either a fault 
or a non-LED technology. You can find out the technology either by 
opening it up or by driving it with a rather low current - if it's an 
incandescent backlight it won't produce visible light at low current, 
while an LED will...

Kind regards,

Helmut.