Trouble with backlight on my HD44780 4x20

Hey guys.

I'm kind of new to electronics in general and got my Arduino Uno about a week ago. I've purchased a 4x20 HD44780 compatible LCD screen that is supposedly a blue background with white characters. I've connected this with a 10k pot according(hopefully) to the example guide on the official site.

I do get the "Hello World" message and counter, but it's really dark and hard to read. When changing the potentiometer it goes from black with no text -> black with hardly visible text -> all blocks lightly lit with hardly visible text. I'm just assuming it should be a whole lot brighter than this. The LCD says "2004D v2.0" on the back which, but from the googled results I found I don't see that this would be differing from the described setup in the official guide.

Does anyone have any ideas what I could be doing wrong? Could this be an issue with the LCD unit? I'll get you a photo of my messy setup tomorrow when I have some better natural lighting.

Thanks.

Measured with multimeter both on the pot and on the LCD connections. I get the same variable voltage on both ends.

So I finally figured it out.

There two last pins are apparently for LED+ and LED-

I had just followed the official guide which didn’t state that you needed to connect the last two pins. However it did mention that these pins are LED+ and LED-. At the time when reading this I did not reflect upon this. Now that I figured it out it makes sense I suppose. Since the guide has a LED with no back lighting?

You really should check the datasheet for your LCD module to see if the backlight requires an external current limiting resistor. Some (most) do and some don't.

Don

floresta: You really should check the datasheet for your LCD module to see if the backlight requires an external current limiting resistor. Some (most) do and some don't.

Don

There was no included datasheet and it didn't seem clear cut as to which specific manufacturer this was, other than it was using a 2004D v2.0 module? Winstar module? This could probably be read from the datasheets I did find however. I'm still learning. But I did solve the issue after all :)

But I did solve the issue after all

What did you come up with?

Don

I kind of already answered your last question and this question in one of my posts further up. I didn't realize that I had to hook up the LED+ and LED- at first.

I think you missed the point about the current limiting resistor.

How did you power the LED+ and LED- pins?

According to the data sheet I found, the backlight LEDs require a 4.2V power source. If you want to use the +5V power supply typically used for the LCD module (pins 1 and 2) then you need a series dropping resistor.

Unfortunately the same data sheet does not supply the information one needs to calculate that resistance so I was wondering how you dealt with that problem.

Don

Well... I connected 5V on pin 15 and ground on pin 16 and it lights up beautifully. Are you saying it should work at all with 5V? or not just optimally?

Well... I connected 5V on pin 15 and ground on pin 16 and it lights up beautifully.

A 12 volt bulb lights up really well when you apply 15 volts. An LED rated at 20 mA lights up really well when there is 30 mA flowing through it. Get the idea?

Are you saying it should work at all with 5V? or not just optimally?

It will work but you are most likely exceeding the forward current rating of the LEDs. As I said in an earlier post, the LEDs in the backlight require a series current limiting resistor. That resistor is sometimes provided on the module but usually it is not.

The best solution is to determine exactly which LCD module you have and then find a data sheet for it. Post the link to the data sheet and we can work out what to do.

Don

Oh I see what you are saying.

Well the closest I could find was this.

So LCD at 4.5V
(5V - 4.5V) / 0.240A = 2.08ohm

and LEDs at 4.2V
(5V - 4.2V) / 0.240A = 3.33ohm

Am I correct or are my calculations off?

Thanks for pointing this out.

That looks good for the LED. The LCD typically is run at 5V. Remember to check for the power dissipation in the resistors. I get about 0.2W which will get a 1/4 watt resistor pretty hot. You might want to start out with two 10 ohm resistors in parallel and see if the LED is bright enough. Add a third if it isn't.

Don

I am always against using an LCD as a light bulb ]:D ]:D ]:D What you calculated is too bright. Don't use the back light as a light source. You can easily see the text with even 150 ohm resistor. Try it, turn off all light and try some resistance values.

Thanks for the advice guys.

What would the benefits of not using too strong back light be besides the subjective matter of prefered light amount? Longer life span?

I tried 2 x 6 ohm resistors which is what I'm running at the moment. Also tried a 150 ohm resistor which would dim quite a lot more, still highly readable though.

I have a noob question. Where do I measure with my multimeter to see the achieved voltage after connecting these resistors? I tried measuring at the resistor and at ground, which always gave me 5V no matter what resistor I would use. Shouldn't I be getting something like

Yes, less current means longer life for the back light, also less power consumption if you operate on a battery. If you also run other stuff on arduino, arduino's voltage regulator can only supply a fixed maximal power so by cutting down power of LCD you can save it for other devices like a small motor or else.

To get voltage ACROSS a device, use the voltmeter and measure across it. So use the two leads on each end of the resistor will give you voltage across it.

liudr: Yes, less current means longer life for the back light, also less power consumption if you operate on a battery. If you also run other stuff on arduino, arduino's voltage regulator can only supply a fixed maximal power so by cutting down power of LCD you can save it for other devices like a small motor or else.

To get voltage ACROSS a device, use the voltmeter and measure across it. So use the two leads on each end of the resistor will give you voltage across it.

Thanks for the info.

I tried measuring voltage (multimeter on 20V mark) on both ends of a 150ohm resistor, I got a reading of 2.17(?) and with a 6ohm resistor I got a reading of 0.73(?). What do these values represent? The volate consumed by the resistor or what? So the 150ohm consumed 2.17V out of the 5V supplied, leaving 3.87V? The 6ohm consumed 0.73V out of the 5V supplied, leaving 4.27V. Is this correct?

2.17 and 0.73

Almost except the 150 ohm resistor leaves 2.87V, not 3.87V to the LED. The rest can be explained by ohm's law on the resistor:

The 150ohm resistor I=V/R=2.17V/150ohm=14.5mA So the LED consumes P=2.83*0.0145=0.04W of power

The 6 ohm resistor I=V/R=0.73/6=122mA So the LED consumes P=4.27*0.122=0.52W of power, 13 times the power.

What you are doing is applying Ohm's law to the resistor (which is the only part of the circuit where that law is valid) to calculate it's current. By virtue of the fact that the LED is in series with the resistor you know that the LED current is the same.

You are applying Kirchoff's voltage law in order to determine the LED voltage, but you could have measured it between pins 15 and 16 of the LCD module.

You are also demonstrating that the human eye is extremely non-linear when it comes to brightness. When the LED current went from 14.5mA with the 150 ohm resistor to 122mA with the 6 ohm resistor that was an increase of more than 7x. I don't think the LED looked 7x brighter.

So it looks like the 6 ohm resistor was just a bit too small (resistance is too low) to meet the ratings of the LED. If the 150 ohm resistor gives adequate visibility I would go with that. Otherwise use a smaller resistor but don't go below 10 ohms or so.

Don

Oh of course. Sloppy math on my part.

Definitely not seemingly 7x brighter.

Thanks a lot for all the info guys.

Hard to tell brightness with your eyes. I don't know about light sensing on the retina but in front of it is an adjustable pupil so if it's too bright, the pupil closes down. :)

Just go with what is sufficient to read in complete darkness, which is not much current. 20mA on most back lights does just fine with my eyes.