Questions? Call us: +1 760-918-6722

S64128L LCD Backlight not functioning

I am working on S64128L LCD. It displays data well but the back-light doesn't turn On.
I am driving the green backlight KG.

My connections are:
Microprocessor port pin ->resistor(1K ohms)->NPN transistor base
NPN transistor emitter->GND (0v)
NPN transistor collector->resistor(20 ohms)->KG.
NPN transistor-MMBT2222A

When I make the port pin high(logic 1) transistor turns ON & Vbase=0.691V but Vcollector & VKG remain 0.

I am not using other back-lights (Blue & Red) so KB= KR should be 0 ..right?
How do I get to start the backlight ?Any other way?


dtechsupport's picture
August 11, 2010

Here is the circuit diagram for the RGB backlight:

You will need to supply 3.2V to the A tab. The KG tab (KR, KB) should be grounded.

Guest's picture
August 12, 2010

Thanks again for your continuing & quick support.
I have another LCD with working backlight.
I also observed that after Green back-light in ON on this lcd:
KG= 0.564V KB=1.572V, KR=2.113V A=3.2V
So I gave same voltages externally to KG & A for my lcd with non-working back-light (with resistors in series or current protection.). I confirmed the voltages at KG & A pins to be as above.
(I did not give any voltages externally to KB & KR & they remain 0v)
But still my backlight is OFF.
Any idea, what's significance of KB & KR greater than 0V while using green back-light?

dtechsupport's picture
August 12, 2010

You need to supply 3.2V to Anode and 0V to Cathode.
In order to turn on and off one color, lets say green, you apply 3.2V to anode, then the green can be turned on by connecting KG to GND and turned off by connecting the KG to 3.2V or disconnecting it.
As an example if you wanted both red and blue turned on, connect KR and KB to GND. Anode must be connected to 3.2V, also KG should be connected to 3.2V or opened.
Hope this clears it up.