This has happened twice now on the same 20x4 sparkfun serial LCD.
I had been using the LCD fine with the backlight and then I looked back at it after it having been plugged in for 2 hours ish and the backlight had gone.
Tried to turn it back on in code, decided that somehow the backlight had broken. About a week ago, I plugged the LCD back in (after not using it since - about a month or more) and the backlight was working again. No problems.
Then after using an external voltage regulator to power the LCD, the backlight disappeared again, not sure when it happened, I just noticed that it wasn't working.
I am hoping that it will come back again soon but I was wondering if anyone has any idea as to why it has happened? I have not over-voltaged it or anything...
All then connections are fine, I have checked them...
I suggest 10R between the back light + and the PWM transistor's emitter. Ot to +5 if you are not controlling the brightness.
Also check that the PWM pin is still active when you have a no back light condition. It could be that this is being held off.
I will check if the backlight pin is active but it is all integrated, there is no need for an external resistor, it is all built into the serial backpack board...
Interesting.. the few LCD's I've purchased did have a resistor for the LED backlight.
Have you tried code to turn the backlight on and off? I think for MatrixOrbital LCD's it's like, uhh, 69 for backlight on, and 70 for backlight off.
But that's assuming you didn't accidently turn it down internally, which is command 153. You can try something like this, which should set the backlight to max.
mySerial.print(254); // matrix orbital command byte
mySerial.print(153); // command for setting backlight brightness
mySerial.print(255);// set the brightness to full
mySerial.print(254); // another command byte
mySerial.print(69); // backlight on command
This of course, all depends on if you haven't burned out your backlight, and assuming it's a Matrix Orbital.
Did you take the time to look at the datasheet for the device he is using? Following his link it took me all of two minutes to find this: "Pulse width modulation of backlight allows direct control of backlight brightness and current consumption".
Therefore his setup does not need a serial resistor and that is why there is no place to put one.
Just a guess... check the solder joints on the backlight connections between the two boards. This would be pins 15 and 16 which are probably the two closest to the center of the display. Look for 'cold' solder joints which usually look dull and grainy as opposed to bright and shiny. You could also try a little torque between the two boards.