Experiments with Touch – Bigscreen Android with 23 inch Resistive Touch Panel

Posted on October 5, 2011


Inspired by the allegedly Windows Android x86 demo with a 23 inch multi-touch screen by Martin Drashkov (see second video below), I decided to give my own version a spin.

[Edit: Oct 16, 2011] One of the readers of Hack a Day pointed out that a 23 inch Touch panel could not be found. I guess I made an err here.

My version of a 23 22 inch Android Touch “mega tablet”

It uses a single-touch resistive 23 22 inch Touch screen with an Difrnce DIT7000 Android tablet I bought for 135 euro on a heavy: “everything must go” sales discount.

I had hoped for higher resolutions of at least around 1200 x 800, but the HDMI output resolution of this tablet is probably also 800 x 480 pixels.

The video that triggered this experiment

The Megapad by Martin Drashkov.

As I already had done some experiments with resistive touch screens and seen the insides of another Resistive Touch tablet (see this post on taking the Archos 32 apart), I guessed it would not be that difficult to get stuff working

Also see

  1. The ScrabletThe concept, the execution. A 15 inch DIY mod, turning a Laptop screen into an external Touch panel
  2. Taking apart the Archos 32 – See this post About fixing the broken power switch, including a photo series of the insides ofd this tiny tablet
  3. Touch tablet concepts – Here. Once made for Wacom, for a possible gig that did not get through.
  4. “Minority Report interface” With FLAR Toolkit and FlashHere. Originally to test AR-toolkit markers for a table-setup, but done with “finger gloves” for fast prototyping
  5. HotForestGreen – an “Internet of things” framework here that will be used in future projects with this (and other) screens

Basic setup

Schema of basic setup - Android 23 inch Touch Screen

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.

4 wire resistive Touch Panel layout and wiring

A 4 wire Resistive Touch Panel has a relatively simple layout. See “A: Example layout of touch digitizer” in the image. I did some research online (see for instance this PDF document) and deducted the 4 wire connection schema by simply measuring the resistance between each wire pair, before flipping the wires.

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.

Photo: Getting the wiring right

Getting the wiring right: connecting A to C and C to A

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)

  1. 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
  2. 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.
  3. A LCD screen with HDMI input – I had a 150 euro, 23 inch Samsung from a previous experiment, bought in a local store
  4. Thin flexible wires – I harvested the insides of an USB cable from some device I killed
  5. Pins to connect your cable to the Touch digitizer – Bought at local electronics store
  6. A HDMI/Mini HDMI connector – To connect your tablet to the big screen, bought at a local store

Tools and other stuff

  1. Screwdriver, knife – To pry your tablet open
  2. Scissors – To cut the 4 wire flatcable from your tablet
  3. Sandpaper – To uncover the wires in the tablet touch screen flatcable
  4. Solder tin – To solder
  5. Soldering iron – To solder
  6. Super glue and paper – To make your connector hulls (see photos). I used Loctite flex gel (superglue) and the paper from an envelope
  7. 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

  1. 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.
  2. 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.
  3. 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

Trying it first – 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.

Frankensteining two tablets: linking one's touch screen to the other tablet

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

Going pro: 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:

  1. 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
  2. Disconnect the 4 wire Touch screen cable – This is a 6 mm wide flat cable with 4 wires
  3. 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
  4. Clear the end of your 1cm flatcable – Using sandpaper until you see the bright shiny surface of the copper ends
  5. Cut 4 lengths of at least 30 cm of thin wire – To allow some space to work between the tablet and your touch screen
  6. Solder the cleared ends of the flat cable – To create your connection points for the wires. Photo of the end result below
  7. Solder the wires to your flatcable/connector – Be careful and use a soldering iron with a thin tip
  8. 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
  9. Test – I found:
    1. 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.
    2. 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
    3. 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
  10. 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.
  11. Use – Your connector cable from your 4 wire Touch digitizer to your tablet is done.


The insides of the Difrnce DIT7000 - the disconnected 4 wire Touch cable

The make-shift Touch connector with cleaned wires

Make-shift connector with USB cable wires soldered to it - the white is paper superglued to the connector

The connector to the 22 inch Touch digitizer

Superglue and thick paper to make a solid wrapping/encasing for our connector

Almost done: finalizing the connector with extra support/sturdyness for the wrapping

The setup: screen and tablet, with Touch screen connector