Character (HD44780 based) LCD backlight is always on

Question

I have a Solomon LM1112SYL 4x16 LCD. The display has 16 pins and the datasheet (http://www.allelectronics.com/mas_assets/spec/LCD-108.pdf) shows that pins 15 & 16 are for the backlight power supply. When I power up the LCD without having any connections made to pins 15 & 16, the backlight is on. I would like to have the ability to dim or power off the backlight.

NOTE: clabacchio has another post about this module at LCD Module Datasheet Or Other Info.

I see that on the back of the module it is has a block silk-screened on it which appears to be for the backlight.

    VDD/VSS JI,KI
    15K/16A JK
    15A/16K JA

I believe what this information is telling me is that JI is VDD and KI is VSS. If JK pads are shorted to the pads then pin 15 will be the cathode and pin 16 will be the anode to the backlight LEDs. If JA pads are shorted to the pads then pin 15 will be the anode and pin 16 will be the cathode to the backlight LEDs.

I have not followed the traces yet to confirm this. Does anyone know if my assumption is correct? I have not it tried yet as I don't want to ruin anything but I may give it a go once I trace the connections and make certain there will be no ill affects.

Update

Thanks Fido UK, for pointing out what should have been obvious but escaped me for a time. For some reason I was thinking that there was a via or something where those blobs of solder were. I took my solder wick out and unsoldered the blobs and found jumper pads underneath. Works great now.

Answer 1

The back light is being powered from the supply to the LCD controller - Pins 1 & 2, this is from the solder blobs marked JI & KI near R9.

To run the back light from pins 15 & 16 you need to unsolder the JI & KI blobs first, then solder the JA or JK links

If you want pin 15 to be +5v use both JA links (pin 16 will be ground) If you want pin 16 to be +5v use both JK links (pin 15 will be ground)

J stands for jumper - I means internal, K means pin 15 is the Cathode (I know but it is standard to use K as a short form of Cathode), A means pin 15 is the Anode...

Answer 2

I don't know this specific device, but every 16-pin character LCD module I've run across uses pin 15/16 to drive the backlight. Use an NPN transistor on the cathode to switch the backlight on and off.