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?
@darshana
Please see the attached circuit diagram for the RGB backlight. You can supply 3.2V to the A and KG tabs to use the green backlight.
Hi terrydtech,
Thanks a lot for the quick reply.
Please confirm..supply 3.2V to just the A tab..right?
I think KG tab should not be supplied with 3.2V...right?
Sorry for the misunderstanding.
Thanks again.
Darshana
Corrected.
Hi Darshana,
You will need to supply 3.2V to the A tab. The KG tab (KR, KB) should be grounded.
Thanks,
Terry
Hi terrydtech,
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?
Darshana.
Hi Darshana,
Terry is out of the office, so I'll fill in. You should supply 0V (ground) on A and 3.2V on KG to turn the backlight on.
Trevor
Hi trevordtech,
Thank you for the reply.
I want to know one thing.
Does A mean Anode & K mean cathode?
If so, applying 0v to A & 3.2v to KG will reverse bias the leds...right?
If I am wrong, please let me know, if I need to put any resistor in series with the 3.2V to KG?
Thanks once again.
Darshana
Hi Darshana,
Oops sorry about that, yes 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.
Regards,
Trevor
