Losing SerialLCD backlight

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...

Mowcius

You do have a series resistor fitted?

Is the connector soldered or is it through some sort of contact / plug socket arrangement.

The serial backpack is directly soldered onto the LCD, as I bought it...

The power is in through the screw terminals that I have checked (the LCD works fine still, just no backlight)

Mowcius

What resistor value are you using? Maybe it is too low.

What resistor? It doesn't need a resistor...

It just takes 5v, Gnd, and Rx :o

Mowcius

Problem solved.

huh? how? :o

Well you need one don't you.

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.

huh? how?

I think he's implying that you need a resistor? :-/

Well you need one don't you.

umm, no...

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...

Mowcius

there is no need for an external resistor, it is all built into the serial backpack board

Not according to the schematics.

Unless they have come up with an LED that defies the laws of physics that is.

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. :slight_smile:

Not according to the schematics.

Unless they have come up with an LED that defies the laws of physics that is.

Well there isn't anywhere to put it as the backpack is soldered onto the lcd and requires 5v, gnd and rx:

Mowcius

Have you tried code to turn the backlight on and off?

Tried to turn it back on in code

Yep...

Well there isn't anywhere to put it

So that means you don't need one does it?

Grumpy:

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.

Don

Hmm, anyway, I am pretty sure that a resistor is not needed anywhere...

Any other ideas?

Mowcius

Any other ideas?

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.

Don

He's right you know Mike, I have exactly the same display and no resistor is required to be added.

He's right you know Mike, I have exactly the same display and no resistor is required to be added.

Yeah I didn't think I was going mad! :wink:

I will leave it a bit and see if it wants to start working again. I don't need if for at least a week or so...

Mowcius