Problem with 16x2 display

I have a small problem with my 16x2 lcd display (HD44870 compatible). I connected it to my Arduino like described here:
And it works, except for the fact that the characters are barely visible. V0 is connected to a 10k trimmer, when even when this is set to 0V the contrast is really low. If I reset the arduino and/or reconnect the display I sometimes get random charactes with decent contrast.
Any ideas?

How often are you updating the display?
Are you clearing the display? If done rapidly, that can cause flicker which can make
the pixels to appear to be dim.

--- bill

How often are you updating the display?

Along these same lines...
Since you have not posted your code we have to guess. My guess is that you are writing all of this stuff to the LCD in loop(). You should rework your code so that the information that does not change, the top line and the " value is :", is sent to the display in setup(). Only the changing information should be written in loop(). You will have to look into the setcursor() command and you will have to deal with values that change from three digits to two digits (such as 100 --> 99). Try it, you will see what I mean.


Edit: That is an interesting display, with duplicate terminals at the top left and bottom right. Could you identify it and/or point me to a data sheet?

This is the ebay auction I got the display from: As you can see it looks different in the item description - that's China for you I guess :slight_smile:
Unfortunately it didn't come with a data sheet...

I reworked the code, included a delay of 0.5 s, but no change - but the problem is something else entirely as I just found out:
It's the power supply. I bought a 5V/1A power supply, which I used when I took the photo. Just now I connected the arduino to my computer to upload a sketch - and now it works perfectly!

I checked the voltage on the 5V Terminal of the arduino: When I use the external power supply I only get 3. something Volts there, which explains why the contrast of the display was so low. But the power supply does output 5V when nothing is connected, and when I power the display separately with the power supply it works as well (but then I don't have a common ground, plus I don't want to use two power supplies).
How can this be, the external power supply has more juice than a usb port, right? Is this a problem with my arduino?

Edit: Nevermind, should've read the data sheet of the Arduino more carefully: 7-12V for the power supply , but 5V operating voltage :zipper_mouth_face:

How can this be, the external power supply has more juice than a usb port, right?

Yes, if it actually can deliver what it says it should. Did it come from China as well?


No, I got it from a German store.

You haven't said which Arduino board you have.
Measure the voltage on Vin.
I bet you see +5V.

My guess is that the voltage regulator isn't working with 5v input
because the Arduino uses a very simple power regulation circuit
on the power input plug. Because of that, the input voltage must
be higher than 5v or the regulation circuit won't be able to
reliably supply 5v.

--- bill

Sorry for not mentioning it, I've got the Uno. Thanks for the tip with the Vin - when I plug the display in the Vin it works (makes sense :))
Is this ok for prolonged usage, or should I switch to a 7-12V power supply?

Actually, what I meant was plug the power into the normal plug and wire everything
up as you had originally but measure the voltage on Vin.
That will measure the powersupply voltage before the regulator circuitry.
I’m guessing you see 5v on Vin but something else on Vcc.

At below the dropout voltage, the voltage regulator will not work properly
properly so I wouldn’t want to depend on it to supply power to anything including
the AVR.
To use the power input plug you need a higher input voltage.

As long as the power supply you have is a 5V regulated supply,
then you could make/buy an adapter to allow plugging it into the USB connector.
Power into the USB connector does not go through the voltage regulator.
On the plus side using the USB connector for power is more efficient since
the higher voltage doesn’t have to be regulated down.
On the minus side if the wiring is backwards or the power supply supplies more
than about 6 volts, the 5v circuitry can be damaged.

— bill

I'm guessing you see 5v on Vin but something else on Vcc.

Yes, that's right.
Then I'll look for another power supply. Thank you very much for your insights, appreciate it!