634 neg. woes


New member
This might be a long post...

I recently purchased a Crystalfontz 4x20 negative backlight (green char. on black) serial LCD to go into my car mp3 custom-made plexiglass in-dash head unit (with power and volume knobs :D). From my plexiglass mp3 unit in the trunk I have a serial cable and 5V power lead going to the face.

Here's the problem: I just got the face sanded, painted black, and wired up with the volume control so I could install it all in the car. Despite the fact that I tested everything beforehand in my apartment, (and it was all working fine with my Winamp LCD and infrared remote plugins), when I put it in the car nothing happened. The unit worked fine, even with the remote, but the LCD displayed nothing. I pulled it all out again, checked the connections, and still nothing.

I pulled it all out and took it all to my apartment to check with my multimeter...+5V from the computer power supply, no problem. The problem lies with the power cable on the LCD. The ground cable will not work! I get a big fat "0.00" on my multimeter when I test the LCD cables! However, if I directly bridge the power supply ground to the negative solder point of the actual LED backlight connector, I get illumination. Any other way does not work. Also, now I have no characters displaying at all, no matter how I route the ground to make the backlight work.

Something to add: I have not mishandled the LCD in any way; in fact, since it cost my over $70 I've been treating it like gold!

Can anyone diagnose my problem? I am not new to soldering, wiring, or the concepts of a "good ground". This is why I am so perplexed by this problem...it worked fine, then suddenly nothing. Even with all the same wiring, etc. Also remember my LCD is serial, not parallel, so there is no wiring involved besides the power leads. I thank in adavance anyone who might have something to suggest!

(Attached is a pic of my MP3 unit for all who might be curious...)
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New member
It's not similar at all...I purchased the serial unit so I wouldn't have to bother with constructing those kind of cables. I am using a serial cable with a gender changer, and I'm just pulling +5V and the ground off of a molex connector with male and female barrel connectors sticking out of the side of my mp3 unit. I ran a twisted pair (literal, not UTP-type) of 16 gauge copper wire to the front of the car, with the same barrel connectors and wire colors so there was no chance of reversing the positive and negative connections. I looked over the UTP/5V combo cable beforehand and decided that I didn't want to go that route. The method that I was using worked great as it was.

Also, note that the LCD does not work directly connected to the unit (without the long power cable) but it did previously (about 1 week ago).

I think I have a theory, so tell me if I am completely off target here: is it possible that the actual "backlight" is a separate element from the negatively illuminated characters? I mean, could I make the backlight illuminate without making the characters illuminate? If that were true, my attempts to directly connect the ground to the LED backlight terminal would not be powering the characters, meaning I would see nothing but the backlight (which is what I see now).

Basically what I need to know is this: do I have a dead LCD, or do I have something that I can still modify to work now that it doesn't seem to want to operate normally?
USB LCD Displays - Graphic and Character LCDs with a Keypad

CF Tech

There are two sections powered seperately on the LCD.
  • The "LCD" section (microcontroller and LCD controller), which can be powered through the RS-232 port's or through the "+5v LCD" pin of J2.
  • The "LED" section, which must be powered through the "+5v (LED)" pin of J2
The backlight is controlled by the microcontroller, so to see the backlight, you must have power to both sections. (And the backlight must be turned on by the software.)

So in order to see any backlight, you will need to have the "LCD" section running and power to the LED section.

If all that is going and there is still no backlight, something might have gotten toasted.

If you shorted from the "K" backlight terminal to ground while power was connected to the +5v (LED) terminal, that could be bad news, since that would have bypassed the current limiting resistors.

Will the characters come on at all (even very dim, since this is a negative display) when you apply power to the LCD section (either through the +5v (LCD) oin og J2 or through the serial cable)? If so the backlight drive transistor may have gotten damaged.


New member
No, I get nothing at all when plugged into the serial port. No dim characters or anything else besides just the backlight. So it looks like I toasted my LCD eh....****. :(


New member
Attempt #2

Well, I've given up on my 20x4 ever working again...but I should have a 16x2 being delivered today via Fedex. I wanted another 20x4, but it's just a bit pricey compared to a 16x2. Besides, I'm really only displaying ID3 tag info so 16x2 is enough...although now I can't have the current time and other nifty stuff displaying simultaneously. ;)