LCD again

Sorry that I have to ask a LCD related question again. I read the FAQ and the forum, but somehow it doesn’t work anyways.
The LEDs are on correctly as said in the manual (LED 3 and 7). I even connected a keyboard and an amp and playing around with the potis I was happy to hear some interesting sounds. So far everything is working right.

But the display remains blank. The two rows are slightly visible but turning the screw of the R21 trimmer 23 times in either clockwise or counter clockwise direction doesn’t do anything.
I tested the pins of the trimmer with a multimeter and get values with each combination.

Any suggestions what to do/test next? Thanks!

Resolder all pins to the LCD. Whats the Voltage at Pin 3 (thats the Contrast Voltage Pin on the LCD). It should be the same as on the middle Pin of the Trimmer.

The voltage on the middle pin of the trimmer should be in the 0.5V - 1V range. If not, try adjusting the trimmer.

are you building a kit or did you buy bare boards and parts from a third party? If you bought the parts from a third party then you can get this behavior if you didn’t burn the boot loader or it didn’t burn correctly. I had a similar problem. I wasn’t watching the output of AVRdude when programming the ATmega and the boot loader hex file was corrupted.

Thank you for your answers! I tested the trimmer. One sensor to the upper pin and one on the middle pin shows me 0.6 V. One Sensor on the upper pin and one on the third pin of the lcd shows the same but negative voltage (-0.6V). If I put one sensor to the middle pin of the trimmer and one to the third of the lcd it shows 0.0V. One to the GND and the other one either to the middle trimmer pin or the third lcd pin shows +1.2V. There seems to be nothing wrong. The LC display is illuminated, but I can’t see any symbols.

And I use the kit and no parts from third parties.
I did not resolder the pins yet.

I am sorry but I need more help…:-/

Voltage measurements are done by putting the black probe of the meter on a grounded point, and the red probe on the point to test (here the middle pin of the trimmer). From what you describe, it looks like this pin is at 1.2V, so you’ll have to turn it until it goes in the 0.5V - 1V range.

I forgot to say that when I turned the screw counterclockwise six times the voltages changes to 0.2V/-0.2V. The trimmer seems to work…

0.2V and -0.2V are meaningless numbers unless the voltage is measured with reference to ground.

Thank you for your answer. I did what you said and screwed all the way from 1.2V to 0V with the black probe on the GND and the red one on the third pin…nothing happens.

Can you post a photo of the display ; with the trimmer set such that the middle pin of the trimmer reads at 1V?


From the picture it looks like there’s no backlight on the LCD. Am I correct?

No, the LCD is illuminated when power is on. It lightens up and you can see a bright spot on the very right side where the clear plastic is.

But on the other hand I don’t know what kind of brightness to expect. It’s hard to tell if it’s on when connected to the power source in daylight. But there sure is a difference…

Can you please measure the Voltage (in respect to GND) on Pin 3 of the LCD (3rd from the left) - it should be exactly the same as in your pic above. For me it looks like there are some bad solder joints so i suggested resoldering all Pins…

Here is a picture of a Shruthi with the side of the LCD illuminated:

I believe there’s something wrong with the backlight. Have you checked that it’s a 68R resistor (and not another value) on the other side of the board? What are the 2 voltages at its terminals?

It doesn’t look on to me either. The backlight is very bright.

Thanks for all your help!

  1. The voltage on the 3rd pin is exactly the same as on the middle one of the trimmer.
  2. The resistor is blue grey black gold as it should be.
  3. There is definately something going on with the display when power is on, but it really is not that bright.
  4. Which are the pins to measure the voltages at its terminal? Sorry I am not that experienced with soldering…

Resoldering the lcd seems to be a good idea, but do i have to remove the whole lcd first before resoldering it again?

Pin1 (leftmost) on the Display should be GND, Pin 2 +5V, Pin 3 somewhere between ,5V and 1,5V

For resoldering just reheat every joint and maybe add a pinch of fresh solder.

PIN Voltage
1 0
2 -2,8
3 0
4,5,6 0
7 -0,5
8 -0,5
9 0
10 0,5
11 -2,8
12,13 0
14 0
15 -2,7 (display goes off when I measure here)
16 0