LCD backlight control - HC12

Started by Lewis, Bob February 11, 2004
Hi Everyone

Can anyone suggest a best practice for backlight control for an LCD. We have
spare pins on Ports for control. I have looked at digital pots from Maxim
with up/down control but is there any other approach someone can suggest?
The LCD is used mostly in direct sunlight and possibly at night so a lot of
graduations are not really essential. The main reason for controlling the
backlight at all is device life. A full brightness, the life is reduced by
50%.

Thank you in advance for the help..

Robert Lewis



Hi Bob,
for LED back light I use a simple N-FET and a PWM pin. Not sure
about EL backlights.
PL
At 03:46 PM 11/02/2004 -0800, you wrote:
>Hi Everyone
>
>Can anyone suggest a best practice for backlight control for an LCD. We have
>spare pins on Ports for control. I have looked at digital pots from Maxim
>with up/down control but is there any other approach someone can suggest?
>The LCD is used mostly in direct sunlight and possibly at night so a lot of
>graduations are not really essential. The main reason for controlling the
>backlight at all is device life. A full brightness, the life is reduced by
>50%.
>
>Thank you in advance for the help..
>
>Robert Lewis >
>
>--------------------To learn more
>about Motorola Microcontrollers, please visit
>http://www.motorola.com/mcu
>o learn more about Motorola Microcontrollers, please visit
>http://www.motorola.com/mcu
>
>Yahoo! Groups Links >
>




Robert,

What type of backlight are you using?

I suggest you look into LED Array (lots of LED's, but lots of current & heat too) or Fiber Optic (1 or 2 LED's good lighting, low power 20-40mA, manf. by Lumitex) for a backlight and simply leave them on. LED's don't fade (at least for a very long time 4-5 years continuous use) like EL panels and EL power supplies can be noise generators in your circuit, can overheat and unfortunately can burn up.

JB

-----Original Message-----
From: Lewis, Bob [mailto:]
Sent: Wednesday, February 11, 2004 5:47 PM
To:
Subject: [68HC12] LCD backlight control - HC12 Hi Everyone

Can anyone suggest a best practice for backlight control for an LCD. We have spare pins on Ports for control. I have looked at digital pots from Maxim with up/down control but is there any other approach someone can suggest? The LCD is used mostly in direct sunlight and possibly at night so a lot of graduations are not really essential. The main reason for controlling the backlight at all is device life. A full brightness, the life is reduced by 50%.

Thank you in advance for the help..

Robert Lewis
------------------------ Yahoo! Groups Sponsor ---------------------~--> Send the freshest Valentine's flowers with a FREE vase from only $29.99! Shipped direct from the grower with a 7 day freshness guarantee and prices so low you save 30-55% off retail! http://us.click.yahoo.com/_iAw9B/xdlHAA/3jkFAA/dN_tlB/TM
---------------------------------~->

--------------------To learn more about Motorola Microcontrollers, please visit http://www.motorola.com/mcu o learn more about Motorola Microcontrollers, please visit http://www.motorola.com/mcu

Yahoo! Groups Links


At 08:41 PM 2/11/04 -0600, you wrote:
>Robert,
>
>Lumitex) for a backlight and simply leave them on. LED's don't fade (at
>least for a very long time 4-5 years continuous use) like EL panels and EL

Not true. White LEDs are created with blue LEDs and glowing phosphors, and
they suffer noticeable degradation after a couple thousand hours, depending
on conditions.

Gary Olmstead
Toucan Technology
Ventura CA





Hi Robert,

I'm using a couple of PWM ports, to a fet for the LCD back
light and the output enable for the panel leds. This works
well, and the user can set the brightness for both, I also
use a 3rd PWM to generate the contrast voltage for the LCD.
I have a R-C filter for that to change the mark space ratio
to a voltage.

So all three cost about 20 cents plus the PWM ports on the
micro. The fet the main cost.

Regards,
Darren > -----Original Message-----
> From: Lewis, Bob [mailto:] > Hi Everyone
>
> Can anyone suggest a best practice for backlight control for
> an LCD. We have
> spare pins on Ports for control. I have looked at digital
> pots from Maxim
> with up/down control but is there any other approach someone
> can suggest?
> The LCD is used mostly in direct sunlight and possibly at
> night so a lot of
> graduations are not really essential. The main reason for
> controlling the
> backlight at all is device life. A full brightness, the life
> is reduced by
> 50%.
>
> Thank you in advance for the help..
>
> Robert Lewis
>




I drive mine using a Maxim multi-chemistry, battery charger chip.

It is the kind that regulates charge current up to the point that battery
voltage exceeds a threshold and limits the voltage thereafter.

Set the charge current to your desired backlight level. The voltage limit
keeps the regulator in control if the LCD is removed/switched off.

There are many power supply chips that can swith from voltage to current
limit as load conditions change.

Back lights tend to consume a fair bit of power so used a switch mode
regulator if power efficiency is of any concern to you.

Best wishes, Bob Smith --- Avoid computer viruses, Practice safe hex ---

-- Specializing in small, cost effective
embedded control systems --

http://www.smithmachineworks.com/embedprod.html Robert L. (Bob) Smith
Smith Machine Works, Inc.
9900 Lumlay Road
Richmond, VA 23236 804/745-1065
----- Original Message -----
From: "Lewis, Bob" <>
To: <>
Sent: Wednesday, February 11, 2004 6:46 PM
Subject: [68HC12] LCD backlight control - HC12 > Hi Everyone
>
> Can anyone suggest a best practice for backlight control for an LCD. We
have
> spare pins on Ports for control. I have looked at digital pots from Maxim
> with up/down control but is there any other approach someone can suggest?
> The LCD is used mostly in direct sunlight and possibly at night so a lot
of
> graduations are not really essential. The main reason for controlling the
> backlight at all is device life. A full brightness, the life is reduced by
> 50%.
>
> Thank you in advance for the help..
>
> Robert Lewis >
>
> --------------------To learn more
about Motorola Microcontrollers, please visit
> http://www.motorola.com/mcu
> o learn more about Motorola Microcontrollers, please visit
> http://www.motorola.com/mcu
>
> Yahoo! Groups Links >