Help required with LED backlight dimmer - Electronic Circuits Projects Diagrams Free

pizzzzed · 9th January 2009, 05:22 PM

Hi, I'm new to the forum and have an application that I need help with. I hope someone out there may be able to help me out!

I am trying to replace the faulty fluorescent backlight in a vehicle touch screen control panel with LED's.

I have suceeded using 12v but this it at one brightness level and is a little overpowering at night time. The touch screen has a dimmer control which gives an output between 3-5vdc, I have another LED backlight that works perfectly on the bench when the voltage is adjusted between this range. When hooked upto the original control the 3-5v being a signal voltage is dragged down to 3v so the backlight is at the dimmest level.

What I need is a variable regulator so that I can use a fused 12v supply as the input voltage and vary the output between 3-5v using the 3-5v signal from the controller, this way there would be enough power to drive the LED's using the original touch screen control. My electronics is basic (too many years passed since I did my HNC!) and I'm sure I could get it to work using an additional knob on the dash but really want to use the original control for obvious reasons. The LED backlight is far superior to the original as it gives a very even display wheras the fluorescent lamps give brighter spots in certain areas of the screen, but the original is 19 years old!

Hopefully I have given enough detail and someone will be able to help in my plight, thanks in antisipation.......

ericgibbs · 9th January 2009, 05:57 PM

hi,
Look at the LM317 voltage regulator, the datasheet has circuits.

Datasheet Archive - Free Datasheet Search Engine :: PDF Datasheets :: Data Sheet :: Datasheet :: Application Note : ??? : ??? : ???? : ????? : ??????? : Fiche technique : Datenblatt : ????????

crutschow · 9th January 2009, 07:42 PM

How much current does the LED backlight draw?

Edit:
You could use a low-offset dual transistor emitter follower driven by the control voltage such as shown below. The value of R1 is determined by the gain of Q1 and the amount of backlight current. It would be R1 = (12-5) ÷ (Iled/β) where Iled is the maximum backlight current and β is the minimum current gain of Q1.

Help required with LED backlight dimmer-low-offset-follower.gif

.

crutschow · 9th January 2009, 11:15 PM

Quote:

Originally Posted by ericgibbs

hi,
Look at the LM317 voltage regulator, the datasheet has circuits.

More specifically, if you connect the LM317 sense terminal to the dimmer control output, the 317 output will be about 1.2V higher than the control voltage, giving an output of approximately 4.2V to 6.2V for a 3 to 5V control voltage. You could put a resistor in series with the backlight to reduce the backlight current, if needed.

pizzzzed · 12th January 2009, 09:08 PM

Eric & Crutscrow,

Wow thanks for the help, the LED backlight power consumption is 180mA, if I were to use a LM317 what value resistor (R1) would I require?

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9th January 2009, 05:22 PM	#1
pizzzzed	Help required with LED backlight dimmer Hi, I'm new to the forum and have an application that I need help with. I hope someone out there may be able to help me out! I am trying to replace the faulty fluorescent backlight in a vehicle touch screen control panel with LED's. I have suceeded using 12v but this it at one brightness level and is a little overpowering at night time. The touch screen has a dimmer control which gives an output between 3-5vdc, I have another LED backlight that works perfectly on the bench when the voltage is adjusted between this range. When hooked upto the original control the 3-5v being a signal voltage is dragged down to 3v so the backlight is at the dimmest level. What I need is a variable regulator so that I can use a fused 12v supply as the input voltage and vary the output between 3-5v using the 3-5v signal from the controller, this way there would be enough power to drive the LED's using the original touch screen control. My electronics is basic (too many years passed since I did my HNC!) and I'm sure I could get it to work using an additional knob on the dash but really want to use the original control for obvious reasons. The LED backlight is far superior to the original as it gives a very even display wheras the fluorescent lamps give brighter spots in certain areas of the screen, but the original is 19 years old! Hopefully I have given enough detail and someone will be able to help in my plight, thanks in antisipation.......

9th January 2009, 05:57 PM	#2
ericgibbs	hi, Look at the LM317 voltage regulator, the datasheet has circuits. Datasheet Archive - Free Datasheet Search Engine :: PDF Datasheets :: Data Sheet :: Datasheet :: Application Note : ??? : ??? : ???? : ????? : ??????? : Fiche technique : Datenblatt : ???????? __________________ Eric " Good enough is Perfect " I will NOT answer PM's requesting technical help, please use the Forum PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

9th January 2009, 07:42 PM	#3
crutschow	How much current does the LED backlight draw? Edit: You could use a low-offset dual transistor emitter follower driven by the control voltage such as shown below. The value of R1 is determined by the gain of Q1 and the amount of backlight current. It would be R1 = (12-5) ÷ (Iled/β) where Iled is the maximum backlight current and β is the minimum current gain of Q1. . __________________ Carl Curmudgeon Elektroniker Last edited by crutschow; 9th January 2009 at 10:42 PM.

12th January 2009, 09:08 PM	#5
pizzzzed	Eric & Crutscrow, Wow thanks for the help, the LED backlight power consumption is 180mA, if I were to use a LM317 what value resistor (R1) would I require?