Trimpot R21 broken? / LCD problem

I’m close to finishing the build of my first Struthi. Now it seems that there might be something wrong with the trimpot controlling the contrast of the LCD. Turning the screw, it never reaches “the end”. No clicks or anything. At first the screen was brightly lit, now turning the screw doesn’t seem to change anything. The screen is unreadable and dim. Any guesses? Do you think the trimpot is dodgy or should I check something else?

The trimpot has something close to 21 turns until it reaches its ‘end’, you should however only need to turn it 5-6 times in my personal experience…
What exactly do you mean by the screen being dim? Is the backlight not strong enough?

I don’t think we’ve had one case of a failing contrast trimpot here in the forum (and the end of the trimpot is really hard to feel or hear), have you really tried some 21 turns, both ways?

-I managed to break one of the traces on mine, which resulted in no characters on the screen
(except when flexing the board, which is a really, really stupid thing to do…)…
Still, I don’t understand the problem :confused:

  • The trimpot slightly “clicks” at the end of its course but continues rotating.
  • The trimpot does not adjust the brightness of the screen. This is set by the resistor on the other side of the board (Noted R LCD. 3.3 ohm with your kit probably).

It is not clear if you have a problem with the backlight being not bright enough, or the contrast being insufficient. Please post a photo!

OK here’s how it looks, sorry about creating confusion. Maybe the problem is that the characters are missing?

Anyway at first those squares on the screen were really bright and after turning the screw, it changed into this and doesn’t seem to change anymore.

Thanks for the help! Amazing sound, can’t wait until I can start making music with it.

What is the voltage on the middle pin of the trimmer?


Now the screen has gone all dark, so nothing happens when I power the synth.

About the lack of backlight -> what is the voltage on the +5V pin of the trimmer?

If you read 2.2V on the middle pin of the trimmer, it is not set correctly.

Sorry, I’m a complete beginner. Which one is the +5V pin?

Also, what voltage should I get from the middle pin? Does it change by turning the screw?

One end of the trimmer is at +5V, the other at ground, the middle varies between 0V and +5V by turning the screw. It should be between 0.5 to 1 to get good contrast on the screen.

But first you have to understand why no light is on when you power up the synth. This is a bit unexpected since your board seemed to work. Are you using the correct power supply; Are the two boards correctly connected?

Yeah, they are, and the leds light up. Only the screen is dark.

Something weird is going on, now I get no voltage on the middle pin of R21. Should I try to resolder it or what do you recommend?

Thanks for your help!

Yes, resolder all pads from the trimpot.

Ok. The one end gives me 0V, the middle one 0.629, and the last one .0L (not sure what that means)…

.0L means “out of caliber” - maybe you have set your meter to 2000mV caliber.

OK, I think I’ve got the settings right now. I get 0V and 5.02V on the ends. I tried turning the screw, the middle pin voltage went from 0.5V to 1V, but nothing on the screen. I see a minor change on the screen when I power off, it seems to switch off.

What should I do now?

Resolder all pins on the LCD

Cheers! It’s all working now. Thanks a lot.