Problem with LCD?

Hello everyone,

Just getting to grips with the arduino LCD, and found that I had no pots available for contrast...

I maybe stupid but, I can see the display at an angle however looking straight down it seems like white boxes is this due to not having a pot? I will now post pics :)

elliottr: I can see the display at an angle however looking straight down it seems like white boxes is this due to not having a pot?

Sure is! Try just a 470 ohm resistor from pin 3 to ground.

And if 470 ohm does not provide enough contrast lower the resistance. The closer to ground the pin 3 is the more contrast.

You might just as well connect LCD pin 3 to GND rather than use a resistor, but to provide the contrast voltage correctly you need a voltage divider, ie two resistors.

A resistance near 1K ohms between GND and LCD pin 3 and about 10K ohms between +5V and LCD pin 3 will typically be a good starting point.

Don

groundfungus:
And if 470 ohm does not provide enough contrast lower the resistance. The closer to ground the pin 3 is the more contrast.

Hmm, don’t know whether you have played with your LCD modules recently, but the condition demonstrated in the pictures is too much contrast, caused in fact by connecting Vo to ground (Vss). The display I have sitting next to me is optimally adjusted at 4V contrast (which is to say, the voltage measured from Vcc to Vo). 470 ohms will drop only 200 mV (given that the divider chain on the LCD itself totals 11k) and if you are powering this from 5V exactly, that means the contrast voltage will still be 4.8V; it is hardly likely to be too low!

floresta:
You might just as well connect LCD pin 3 to GND rather than use a resistor

That is precisely what he has done, and what is the cause of his problem!

floresta:
but to provide the contrast voltage correctly you need a voltage divider, ie two resistors.

But actually, one will suffice in general as the internal divider chain already totals 11k (if you care to examine it - five “222” or 2.2k resistors).

floresta:
A resistance near 1K ohms between GND and LCD pin 3 and about 10K ohms between +5V and LCD pin 3 will typically be a good starting point.

That will drop about 0.68V since your 10k is in parallel with the internal 11k, giving a contrast voltage of about 4.3V. Even this may still be too much contrast; and you may need a higher resistance. My 470 ohms is just a “ballpark” figure and if it only partially fixes the problem, may need to be increased.

Too little resistance, too much contrast; white blocks. Too much resistance, too little contrast, no “blocks” and text fades out.

Note that because what is being altered is the contrast voltage, by tapping off less than the 5V Vcc, reducing Vcc itself will also quite effectively reduce the contrast. Something I notice as I load my USB port and the voltage “sags”.

I trust we all realise that there will be variability from LCD to LCD as to the optimum contrast voltage for the same appearance.

But actually, one will suffice in general as the internal divider chain already totals 11k ...

Where have you been able to find information about the internal workings of the controller? I have been looking for this.

Don

floresta: Where have you been able to find information about the internal workings of the controller? I have been looking for this.

I confess - I had to use a magnifying glass (actually, head-mounted magnifier). Otherwise simple inspection.

My memory serves that some (ten?) years ago I studied the HD44780 datasheet which explains the six-level multiplexing voltages of the LCD display. These voltages are set by a resistor chain on the board comprising R1 to R5.