4 wire touch screens - what's the difference?

Forums › Gadgets, robotics, home automation, electronics (including wearables) › 4 wire touch screens - what's the difference?

brownbox

149 posts

Master Geek

#116462 29-Apr-2013 22:28

Evening all

I recently got given the fun task of repairing the broken screen on a car diagnostic scanner. Its a 7" colour LCD display with a 4 wire resistive touch screen digitizer overlay. I thought it would be fairly straight forward, which is why I opted to carry out this repair myself....sadly its proving to be a bit of a pain in the ass.

The display was easy to replace, I just plucked it out, got the numbers off the back and fitted an Ali-express replacement, all good. Unfortunately the touch digiziter was also damaged, it was not labelled so I matched a replacement unit (also from Aliexpress) from the dimensions and wire placement, but upon fitting I found that the X axis was inverted - press the top left corner, the top right is activated and vise-versa.

The device runs some form of Windows CE 5.0 and the simple calibration tool does not allow this fault to be fixed.

The question is, why has this happened? Is the touch screen faulty or is it the incorrect unit? Doing some reading, seems like its not uncommon for these units to have the inverted axis fault, a RMA replacement fixes the problem. More to the point, is there any way to distinguish between different screens? Is there even a difference or do they share a common "standard"?

Or is it possible to switch the polarity of the X axis wiring? I'd love to try it but the last thing we need at this stage is to let the smoke out of something.

A genuine replacement is going to be about $1,000 shipped so not really ideal. I'm nearly there with it, just need to get the touch sorted and I would've got the job done for less than 300 bucks.

Hopefully an expert can shed some light on my dilemma.

SepticSceptic

2190 posts

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#807885 30-Apr-2013 10:05

Having just replaced a touch screen panel myself last week, faced with exactly the same issue.

Running the touch panel alignment application sorted this out. You may need to test / align on a more fully featured OS platform ( mine was running Win 7).

Are you sure that the alignment tool you were using is suitable for the replacement digitizer ?

Niel

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#808250 30-Apr-2013 20:53

It is possible to swap the x-axis wiring. The link below explains how it works, it just forms a resistor divider on one axis and use the other axis terminals to sense, then forms a resistor divider with the other axis and one the first as a wiper. so if you have 2 lines swapped then you will mirror that axis with no effect to the other axis.

https://www.sparkfun.com/datasheets/LCD/HOW%20DOES%20IT%20WORK.pdf

You can never have enough Volvos!

brownbox

149 posts

Master Geek

#808273 30-Apr-2013 21:38

SepticSceptic: Having just replaced a touch screen panel myself last week, faced with exactly the same issue.

Running the touch panel alignment application sorted this out. You may need to test / align on a more fully featured OS platform ( mine was running Win 7).

Are you sure that the alignment tool you were using is suitable for the replacement digitizer ?

I was also hoping the calibration would be able to sort it, for some odd reason it would only accept touches on that half of the screen (e.g. the "X" was on the top right, it would only register a touch if it was somewhere on the left half of the screen). I'm guessing its just some kind of idiot proofing so people can't completely balls up the calibration?

Niel: It is possible to swap the x-axis wiring. The link below explains how it works, it just forms a resistor divider on one axis and use the other axis terminals to sense, then forms a resistor divider with the other axis and one the first as a wiper. so if you have 2 lines swapped then you will mirror that axis with no effect to the other axis.

https://www.sparkfun.com/datasheets/LCD/HOW%20DOES%20IT%20WORK.pdf

Brilliant, exactly what I was wanting to know. Now to conjure up enough willingness to attack that flat plastic backed ribbon cable

Cheers for the help guys, I'll let you know how I get on

Niel

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#809733 1-May-2013 17:52

Good luck with the soldering, probably better to get a new socket and solder wires from it to the PCB. The FPC (flexible printed circuit) will melt if you try to solder directly to it.

You can never have enough Volvos!

brownbox

149 posts

Master Geek

#809820 1-May-2013 22:22

Yeah its a prick of a job. My ham-fisted self managed to slice through the cable upon unpackaging the touchscreen, surprisingly I managed to sand the coating off using fine paper and the tracks tinned and re-joined fairly well with solder.

Ofcourse the longer you spend on it with the iron, the more of a mess it becomes. So naturally you try to fix it, it gets hotter, it gets even worse. Having already made a mess of it the first time (albeit a successful mess), don't really want to dive into it again.

Good idea on the socket though, any idea where I can find them?

Niel

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#809901 2-May-2013 04:44

Maybe Jaycar, certainly RS Components or Element14 or DigiKey. They are all mail order and expect a steep price (for fast service), but RS has a shop in Penrose Auckland if you are in the area. Let me know the pitch of the tracks and I should be able to get you a few connectors for a dollar or so. I happen to work at one of the largest electronic product manufacturers in the country... Don't have touch screens though.

You can never have enough Volvos!

brownbox

149 posts

Master Geek

#814822 9-May-2013 21:35

It worked!! I had a spare 4 wire extension cable, robbed the socket off that and used it, soldered in 20mm of IDE ribbon cable and re-arranged the X wiring as needed.

Thanks again Niel, you've saved me a major headache!

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Niel

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#814948 10-May-2013 06:12

No problem, been doing electronics since I was 11 (and this month I should start looking for a Mazda MX5 or Volvo C70 or something else to make me feel young again).

You can never have enough Volvos!

brownbox

149 posts

Master Geek

#814967 10-May-2013 07:59

Those older C70's are a solid machine, a lot of car for what you pay. What year are looking at getting? (off topic now

Niel

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#815354 10-May-2013 17:06

brownbox: Those older C70's are a solid machine, a lot of car for what you pay. What year are looking at getting? (off topic now

I'm just dreaming, turning 40 this month, but would look at 2002 as up to 2000 there were quality issues on some models and after about 2004 or so the ignition key is on the wrong side (in dash, setup for LHD). Being an electronics nut I'm not too bad at fixing the frequent electrical issues on European cars (see how I bring it kind of back on topic ;-).

You can never have enough Volvos!