Backlight Help

M4TVW · Post by **M4TVW** » January 26th, 2007, 11:16 pm

Hello all.

I have just wired up my HD44780 and all is good...for now...

One problem being is the backlight.

I have put in a 10k pot in the 5v line to the backlight and i get just about 2 clicks out of the pot before it turns off the backlight. So really, One touch of the pot and its on or off.

What's up with this as everyone says to use a 10k pot. I think its too big.

I'm thinking I need one that goes from 1-5v...is this possible?

Also when its got nothing in there and the 5V is directly fed the LCD gets very hot....Is this right???

Many thanks
Mat

Post by **caesar** » January 27th, 2007, 12:28 am

You should get a 250 ohm pot (a bigger dimension one is better as it should withstand ~100-400mA if the backlight is brighter).
A 10k pot is ok for contrast setting but not for backlight.

It really gets hot when connected directly to 5V because the LEDs are overvolted and the current is not limited as necessary and if you keep it this way it will burn them!
Read your display's datasheet to get the minimum resistor value you need and after that insert a 100 ohm pot between the resistor and +5v or GND, depending on your wiring method.

You should generally keep the display at the lowest convenient light intensity possible to maximize its life.

M4TVW · Post by **M4TVW** » January 27th, 2007, 12:38 pm

Many thanks for your help

Ok so i need a 250ohm for the backlight. This will turn its from off to on in a full turn of the pot? Betting brighter on the way of course...

Is it ok to leave the contrast on full 5v as i dont really need to limit them. Full is how i like it...Will 5v direct hurt them too?

On my datasheet the person i bought it off has writen...

Pin 3: Supply voltage for Logic (+5v): "Connect to Gnd, Note: Workes better with negative volts IE: -1V"

Im not understanding this....

Also

Pin 15: LED power (+5V): "Use 100R or more pulled to 5V"

Dont really understand this too...

All i want is the contrast at full 5v so as dark as it can be. And the backlight on a pot.

Many thanks
Mat

Post by **mattcro** » January 27th, 2007, 7:34 pm

It's a very good idea to use a potentiometer (variable resistor) to control the contrast (how dark/black the characters are) since you'll probably need to adjust it to suit the viewing angle and the particular LCD.

Pin 3 controls the contrast (your supplier has called it "supply for logic" which is not correct) and should be connected as per the circuit diagram given on the Smartie site. You can connect it directly to 0v (ground) but the contrast will probably be too dark (display all black).

Some LCDs have built-in resistors for the backlight so you can connect the LED terminals directly to the 5v supply - my 20x4 LCD has a 5 ohm resistor to allow direct connection to 5V supply. For your LCD the supplier has recommended 100 ohms minimum external resistor so try this to start with (100 ohms between LED+ terminal and +5V supply) and see what the brightness is like. You may even have to reduce the resistance to get the required brightness. You can then add a 250 ohm or so variable resistor in series with the 100 ohm resistor (ie 100 to 350 ohms total) and see if that gives you enough variation in backlight brightness. You need to use a higher-power potentiometer (ie not a tiny trimmer type) that can handle the rated LED current (up to 200mA for larger displays).

Note that this won't allow you to turn the backlight off completely by winding the potentiometer right round. It'll just be dim. You can add a switch to turn the backlight off completely. If you're feeling adventurous you can try the backlight control circuit shown on the Smartie circuits page. This uses a transistor that allows Smartie to turn the backlight on and off under software control.

M4TVW · Post by **M4TVW** » January 28th, 2007, 12:23 am

The contrast being at its darkest is fine for me. It looks better being dark.

Ok so out of these pots then..what best for me..

The following values are available with a linear track:
4k7, 1k, 10k, 22k, 47k, 100k, 220k, 470k, 1M

Im not sure whats the difference between ohms and K..if any...

Also the resistors range from 0.1R and then go up to about 100R+

What is R?

Sorry about this. Im stuck with all this electrical speak!

Many thanks
Mat

_X7JAY7X_ · Post by **_X7JAY7X_** » January 28th, 2007, 1:26 am

K is kilo or thousands of ohms. For example a 4k7 is 4.7kohms or 4700ohms. The best one for you from the list you made would be the 1k.

Some LCDs require a negative voltage on the contrast pin. Most, but not all, low temp LCDs use negative voltage contrast. Some LCDs have built in negative voltage generator that works on a 0-5v positive scale. I hope I didnt lose you.

In short it is a good idea to place a pot on both the contrast and backlight. Make sure for the backlight you use a regular resistor in series with the pot so when the pot is all the way turned down you still have some resistance on the backlight.

Post by **caesar** » January 28th, 2007, 11:49 am

I suggest to have a look together at the datasheet. Upload it somewhere and post the link.
Then we could draw you a wiring schematic.

M4TVW · Post by **M4TVW** » January 28th, 2007, 12:46 pm

Wow. That would be great!

Here is my datasheet. This is all i have as i bought the LCD about a year agao.

http://gti-online.pwp.blueyonder.co.uk/ ... CF0268.JPG

WARNING: Very big picture!

Also this is my current wiring. The yellow cable tie

Is holding all the grounds together at the moment.
The red one not connected to anything is the backlight and i have just put the contrast and 5v into a switch for the time being.

http://gti-online.pwp.blueyonder.co.uk/ ... CF0270.JPG

All looks ok to me!

Notice the connector too. Means i can change LCD's easy!

Many thanks for your help guys.

Mat

Post by **caesar** » January 28th, 2007, 6:50 pm

Ok, this what we use for our lcd's.

All resistor values are adapted for your specs.

This one uses a potentiometer for backlight control.

This one's backlight is controlled by smartie.

Have fun!

M4TVW · Post by **M4TVW** » January 28th, 2007, 7:09 pm

Wow thanks alot!

Ok im understanding this as..

Pin 1 to ground.
Pin 2 to 5V and then to 10K pot is that? Then to ground?

Pin 3 to Pot?

And
Pin 16 to 100 (1k pot?) then to 250ohm resistor and gnd?

Sorry. Im not good at reading diagrams...

Im not sure of the values...

Shouldnt pin 15 be to pot for the backlight?

Many thanks
Mat

Post by **mattcro** » January 29th, 2007, 12:40 pm

It looks like you have the contrast pin (3) connected to ground. If that gives you good contrast, stick with it and you can forget the 10k ohm pot - makes life simpler!

For the backlight, you can use a 1k (1000 ohm, or use 250 ohms if you can get it) pot with switch (same style as you have for the 10k pot I think) to allow brightness control and be able to switch it off completely, all with one control. Note that the brightness control won't be very linear - most of the useful control range will be at one end of the pot rotation range. Some clever circuitry is needed to make a really smooth brightness control.

You can put the brightness pot on the ground side or the +5V side of the backlight LEDs - it doesn't make any difference to the performance. Most of the diagrams I've seen (including the LCD Smartie site) have the pot on the ground side so I'll stick with this method.

To wire your LCD as per caesar's diagram (like standard Smartie circuit):
Pin 1 to ground.
Pin 2 to +5V
Pin 3 to ground (if the contrast is OK like this)
Pins 4-14 to appropriate port pins
Pin 15 to +5V
Pin 16 to one end of 100R (100 ohm) ordinary fixed-value resistor
Other end of 100R resistor to middle pin of pot.
Pot's clockwise-end pin to ground.

What I mean by "clockwise end" is the pot terminal that has zero resistance to the middle terminal when the pot is rotated fully clockwise. That will give max backlight brightness when the pot is clockwise, and min brightness when counterclockwise (like a volume control). You can wire the pot's switch (if it has one) between the clockwise-end pin and ground to allow you to turn the backlight completely off.

Right, I hope I've explained that properly and that it makes some kind of sense!

PS: For the resistor values, "R" means ohms (eg 470R means 470 ohms, 2R2 means 2.2 ohms), "k" means kilo or 1000 (3k3 means 3300 ohms, 10k means 10000 ohms), "M" means mega or 1000000 (1M5 means 1500000 ohms). A similar system is used for capacitor and inductor values, but these are usually very small numbers with values in the nano ("n" = 0.000000001), micro ("u" = 0.000001) and milli ("m" = 0.001 - note lower case) range (eg "4u7" on a capacitor means 4.7 microfarads). Like any technical subject, electronics has heaps of jargon!!

Post by **caesar** » January 29th, 2007, 3:40 pm

Thank you mattcro, very well explained.

I think this post could become a sticky one, it explains a lot of wiring questions.

Post by **limbo** » January 29th, 2007, 6:10 pm

caesar wrote: I think this post could become a sticky one, it explains a lot of wiring questions.

Done.

The "Team" is rocking

M4TVW · Post by **M4TVW** » January 30th, 2007, 10:25 pm

That is great!

Very many thanks to you all!

I have just ordered a couple of different 1k pots as 250R

seems to be hard to get hold of!

Also a few 100R resistors and some other bits like LED's just to jazz up the look of it...Oh and an enclosure too!

I will keep you updated when the parts come in hopefully this week.
Will take a few pics and report back

Thanks again!
My first sticky here

Mat

M4TVW · Post by **M4TVW** » February 1st, 2007, 9:46 pm

Hi Guys, Me again!

I got my parts today.

Have connected it the way you have suggested and its nearly there!

I am using a 1k pot and 100R resistor. The backlight at full is still very dimm... If i take the resistor out its full 5v and too bright!

Not sure if i have done something wrong or got the wrong resistor?

Many thanks
Mat

_X7JAY7X_ · Post by **_X7JAY7X_** » February 2nd, 2007, 6:16 am

Do you have a full datasheet of your LCD? At 100 ohms your backlight current is 50mA (I = V/R). This seems a bit low. You could put two 100ohms in parallel, this will give you 50ohms. At 50 ohms your backlight current will be 100mA.

Be careful though, its easy to burn these backlights out with too much current.

Post by **mattcro** » February 2nd, 2007, 11:27 am

OK on the dim backlight. That means 100 ohms is too much resistance. We were just going by what the seller said, but it's likely you will have to use a much smaller resistance. What size is the LCD? 4 lines of 20 characters?

If you have a multimeter that can measure current (preferably a 200mA range) you can check what size resistor to use. Try a few different values starting at 100 ohms and working down, to find a value that gives good brightness without the current going too high. Max current for 4x20 backlights is around 200mA, and for 2x16 backlights about 120mA. If you have the full datasheet for your LCD, you can look up the correct details. You may find that something like 10 ohms works well for good brightness. In that case, you will need a lower resistance pot to give a reasonable brightness range...

It could well be that the LCD includes a suitable resistor on the circuit board. My 4-20 LCD has a 5 ohm resistor built in which sets the max current at about 200mA at 5V supply (2 sets of LEDs in series drop about 4V, 5-4=1, I=V/R=1/5=0.2A), which is pretty bright.

Ah, Jay has beaten me to it...

M4TVW · Post by **M4TVW** » February 2nd, 2007, 1:40 pm

Thanks guys. Will see if it has a built in resistor tonight as im at work at the moment.
Where would it be on the board becuse i dont remember seeing one sticking out atall...

I dont have the data sheet either. Although i might ask the ebay seller for it as i bought it a year agao...

Could i possibly leave the resistor out and just make sure i dont turn it up too high?

Or is it still drawing 5V from that?

I will try 2 100R's aswell. Might be worth a try!

Thanks again.

Mat

Post by **caesar** » February 2nd, 2007, 2:56 pm

Please, never again try to connect it without a resistor! It can easily burn.

You should try with two 100 ohm resitors in parallel first then if it's still too dim use 3 resistors in parallel or go even up to 10.

If you say that it gets hot, then your display surely doesn't have an onboard resistor.

Post by **mattcro** » February 2nd, 2007, 6:00 pm

If there is a resistor built-in, it will be a small surface-mount type (or several in series/parallel). You might be able to identify it/them by tracing the connections from the LED pins.

It's safest to assume there isn't a resistor built in, and always use an external resistor. As caesar and Jay have said, the LEDs can easily be damaged if you apply too much current. Only a small change in resistor value or supply voltage can cause too much current to flow.

I only have my backlight at about half brightness and that's usually plenty unless there is strong daylight shining on the LCD, so don't assume you need it to go really bright.

Are there any manufacturer part numbers printed on the LCD board (not the chips themselves). That might give a clue where to search for the datasheet if you can't get one from the seller.

M4TVW · Post by **M4TVW** » February 2nd, 2007, 8:37 pm

Ok so I just added more 100R resistors in parallel and its getting better. I think about 5 would do it!

I don't know how much that resistance is stopping though.

And as you have said. Its very "on and off" again with the control. Not very smooth through the control range.

The screen is a 4 x 20 and the only numbers on it are on a sticker

"AMC2004A-B"
"-Y6NFDY-SP"
"Lot#00543"

Mean anything to you?

M4TVW · Post by **M4TVW** » February 2nd, 2007, 9:59 pm

I think this could be it.

Can you work off this?

http://character-lcd-lcds.shopeio.com/i ... C2004A.pdf

Post by **caesar** » February 3rd, 2007, 9:48 am

Nice finding!

So off that datasheet:

If you have a yellow/green backlight [180mA max for the LEDs] you would need for the brightest setting a 5 ohm 1/2 watt resistor (connected to +5V or ground; doesn't matter).

If you have a white backlight on blue display [30mA max for the LEDs] then you would need for the brightest setting a 33 ohm 1/8 watt resistor.

In both cases adding an additional potentiometer of 100 ohms will do the trick to vary the brightness.

M4TVW · Post by **M4TVW** » February 3rd, 2007, 1:19 pm

I have a green backlight..well...yellow/green

So im guessing I need a resistor to the value of 5R at 1/2 watt

And a pot of 100R

Connected the same way as described in page 1 or this thread?

And this will give me good backlight control and not max voltage to the led's?

Many thanks
Mat

Edit- A 5 ohm resistor at 1/2 watt and a 100R pot with a switch seems to be impossible to find

Post by **mattcro** » February 3rd, 2007, 6:23 pm

As an alternative to the pot and switch arrangement, you could use a multi-position switch (rotary sort) and several resistors to allow perhaps 4 brightness "preset" levels plus off. It's not as neat and cool as a variable brightness, but at least you can have several sensible levels between "max" and "off" using one control. This is the kind of switch I'm talking about: http://www.maplin.co.uk/Module.aspx?Mod ... 1&source=1

I had a quick scan of the datasheet you found (looks like the right one!) and it seems there is NO resistor built in after all. Caesar's calculation of 5 ohms minimum resistance (to give max allowable brightness) sounds about right.

For say 5 brightness levels plus off, you could use resistances of 5, 10, 20, 40 and 80 ohms (the nearest standard values are 5.6, 10, 18, 39 and 82 ohms) which I think would give you roughly equal brightness steps. That would need a 6-position switch.

5 resistors of 100 ohms connected in parallel is 20 ohms, so you can figure the appropriate steps based on that. You said 5 resistors of 100 ohms together looks about right for brightness, so you could have one or two steps above that and 2-3 below.