kernelpicnic: Bigscreen Android with 23 inch Resistive Touch Panel

Basic setup

The Android tablet is connected to the screen via a standard issue HDMI cable (with Mini HDMI connector for the Android tablet).

We connect the 4 wire Resistive Touch Panel (or Touch Digitizer) with a home brew cable to the Android tablet, replacing the built in Touch screen of the Tablet itself.

To make sure we can run this setup until kingdom come (or one of the components fails) we connect the tablet to a power supply.

Wiring – and flipping the pair

Jumping ahead in the story: this is the (assumed) setup of the Touch Digitizer wiring and the expected and implemented layout of the home brew cable to connect the external (23 inch) Resistive Touch Digitizer to the tablet.

A 4 wire Resistive Touch Panel has a relatively simple layout.

In most cases, the wiring would be straight forward (see “B: Expected layout”): A to A, B to B, C to C and D to D. For some reason, the Android tablet I used considered the Touch film to be implemented “upside down” where: “up” become: “down” and vice versa (see first part of my video).

This was corrected by twisting the pair connecting to the Y-axis digitizer (see “C: Implemented layout”). The layout became A to C, B to B, C to A and D to D. (In the image I show the position of the Touch Panel wires: C, B, A, D.) The Touch screen input then worked as expected.

As I only did this experiment with one Tablet, I do not know if this is the exception or the rule.

Getting the wiring right

In the picture above you see how I solved the issue on the wrong wiring: by using 4 clamps and switching “A” to “C” and “C” to “A”. If you look closely you see that the yellow (going from “A” to “C”) and green (going from “C” to “A”) clamps are the ones I switched.

Ingredients (and where to get or buy)

A resistive touch Android tablet – To run Android and allow you to attach a 4 wire touch screen. I used the 135 (199 without discount) euro Diffrnce DIT7000. It runs Android 2.2 and has mini HDMI out. I bought it at a local store that had a big sale
A Do It Yourself resistive 4 wire Touch digitizer – I bought mine on eBay: a flexible touch film for a 23 inch screen. Cost around 90 euro. There are two sellers I trust: Visual touch (for flexible film Touch panels) and NJYTouch (Touch panels on glass surface). Both delivered the ordered product as promised. Additional info: NjyTouch also came up to their promise to replace a broken Touch screen when the first arrived in that state.
A LCD screen with HDMI input – I had a 150 euro, 23 inch Samsung from a previous experiment, bought in a local store
Thin flexible wires – I harvested the insides of an USB cable from some device I killed
Pins to connect your cable to the Touch digitizer – Bought at local electronics store
A HDMI/Mini HDMI connector – To connect your tablet to the big screen, bought at a local store

Tools and other stuff

Screwdriver, knife – To pry your tablet open
Scissors – To cut the 4 wire flatcable from your tablet
Sandpaper – To uncover the wires in the tablet touch screen flatcable
Solder tin – To solder
Soldering iron – To solder
Super glue and paper – To make your connector hulls (see photos). I used Loctite flex gel (superglue) and the paper from an envelope
Clamps – As uses in the photo above. If you do not have them, buy them. They proved out to be very handy

Estimated required build time

4 hours to modify Android tablet/Touch cable and make it work – It took me about 4 hours to do all the handy work and get the first proof of concept running. As I already had the setup with the LCD screen and the Touch Panel, that time is not included.
6 to 8 hours for all – I think a first time implementation of the full setup (including the LCD screen and Touch Panel) can be done in 6 to 8 hours time.
2 to 3 hours when repeated – Naturally, the first time implementation includes learning time. I made several small mistakes I had to correct, and there were some things (like the wiring of the Resistive touch Panel) I had to figure out. When repeated, most of that learning time can be skipped

Frankensteining two tablets

As I did not have the touch screen at hand at first, I used another resistive tablet for reference. The main question to be answered was: are 4 wire touch screens interchangeable?

So I opened both low cost tablets (110 euro 8 inch Yarvik Tab310 and the 135 euro Difrnce DIT7000) I bought for tinkering purposes. I then did a Frankenstein with the touch screens. Photo and video below.

The tablet on the bottom is the Difrnce borrowing the Yarviks Touch screen.

Connecting it to the 23 22 inch Touch screen

Once I had access to the Touch screen and monitor I started to do the required modding. Below are the steps:

Open the resistive screeen tablet – Depending on the manufacturer this might be quite the ordeal. The Difrnce DIT7000 was somewhere in the middle. Not easy, but simpler than – for instance – the Yarvik
Disconnect the 4 wire Touch screen cable – This is a 6 mm wide flat cable with 4 wires
Cut a 1 cm long piece from the flat cable – Just long enough to use it as your connector and short enough to reconnect the remaining length of the original cable to restore the tablet in its old function
Clear the end of your 1cm flatcable – Using sandpaper until you see the bright shiny surface of the copper ends
Cut 4 lengths of at least 30 cm of thin wire – To allow some space to work between the tablet and your touch screen
Solder the cleared ends of the flat cable – To create your connection points for the wires. Photo of the end result below
Solder the wires to your flatcable/connector – Be careful and use a soldering iron with a thin tip
Solder the wires to your connector – Using tin. As i used cheap quality — the hull melted a bit. Prepare the pins with tin beforehand to make soldering easier and the reuslt more reliable
Test – I found:

Not all the same wiring: down is up, up is down – That the Difrnce tablet crosses the wires for the Y-axis readout of the 4 wire Touch digitizer. So “up” is “down” and “down” is up. See my movie and the layout of wires above.
Direction matters – There is a clear “pin 1, 2, 3, 4″ situation. If you connect “pin 4″ to “pin 1″ the tablet will not respond to touch
Try the standard setup first – Connect the pins straight forward first and make it work before you start solving possible issues like #1: down=up, up=down

Make the hulls for both connectors using paper and super glue – To avoid your cable/construction to be too vulnerable. The wires you use have a 3 or 5 wire core of very thin copper wires. They break easily. Photos of the end result below.
Use – Your connector cable from your 4 wire Touch digitizer to your tablet is done.