# 4 wire resisitive touch screen not working

I am having a problem with making 4 wire resisitive touch screen work. : This is the touchscreen I am using. I also have 100 Ohm resistors in series with all the pins. If I understood correctly: Y+ $\rightarrow$ +5V, X- $\rightarrow$ GND, X+ $\rightarrow$ Voltmeter. Which should show me diffrent values, depending on where I press on the screen but it shows me 5V no matter where I press it on. I tried out 3 same touchscreens. On one of them, I even tried out every possible combination of +5V, GND, Voltmeter, but always the same result.

Any help is greately appreciated!

• Do you have any more information on the touchscreen? Like the page you bought it from? The pdf does not give very much information. – Nick Williams Jun 5 '14 at 13:52
• I couldn't find another data sheet with any extra information than you provided, however this link to a similar one may help – Gurn64 Jun 5 '14 at 14:02
• Have a look at this awesome TI's application note: resistive touch panel – Vladimir Cravero Jun 5 '14 at 15:15

What information that I did find for the LGM12864B touchscreen does not seem to match the one you have because the datasheets explanation for how the screen works is through a protocol and doesn't use a resistive response.

So I'll answer for how the resistive touchscreen works in your case. When you press on a resistive touchscreen, the pressure will introduce a dynamic voltage divider between the two X nodes, and the two Y nodes. This image may be useful for understanding:

You can see on page 3 of the document that Gurn64 provided that you can read the touchscreen in 2 steps:

First, to determine the horizontal position, set Y+ to be 5V and Y- to be ground. X- is essentially floating, and X+ is now your output. Measure X+ and the ratio of X+ to 5V is the ratio of the distance from the bottom of the screen to the length of the height of the screen.

The vertical sensing is similar. Y+ and Y- are no longer 5V and ground respectively. X+ and X- are 5V and ground respectively, and Y+ becomes your output like X+ was before. The explanation is the above picture is pretty clear.

Also, another note is to remove the 100Ohm resistors that you currently have hooked up, they don't aid in the procedure of determining touch positions.

• If you're clever, you read both open ends of the "X" horizontal resistor, and average them for X measurements, and do the same for Y. – Connor Wolf Jun 6 '14 at 1:52

You can't measure X and Y positions at the same time with this kind of touch screen. You have to measure one and then the other. To measure X, apply a voltage between x+ and x-. For example connect 5v to x+ and 0v to x-. Then measure the voltage at y- or y- (it should be the same) to get your X position.

Then to measure Y you must apply a voltage between Y+ and Y- and then measure from X+ or X-.

A 4-wire touch screen like this is made of 2 squares of resistive material. So when you apply voltage between X+ and X- the voltage will gradually drop across one of the squares. When you touch the screen you connect the two squares together at the point where your finger is. Because you are only sensing a very small current from the Y+/Y- sqaure there is no voltage drop there so all of the Y square will be at the same voltage as the point on X where the two are touching.

You can test touchscreen with with only ohmmeter.

First you should check resistance between X+ and X− and also Y+ and Y−. Mine (AMT 98245) have about 500 ohm at X± and 400 ohm at Y± (i name it Rx anr Rx for later use). When you touch the screen these values can lower a bit (less than 5%) but not much.

All other combinations (between any X and any Y) should give ∞ if screen is not touched.

Then connect ohmmeter between one X and one Y and touch the screen. You should get values somewhere between Rz (interlayer resistance) and Rx + Ry + Rz. I got values from 350 ohm to 1 kohm with mine. Y'll get low value when touch near electrodes you mesure -- e.g. if you measure between X+ and Y+ then lowest value you'll get at upper right corner, and highest value at lower left.

When you ensure youe touchscreen is ok, you should follow other's recomendations here to measure touch position correctly. :)