Sensor Report: Capacitive vs. Resistive Touch Screens

Introduction

Touchscreens are everywhere and fully integrated in our everyday lives. From our smartphones, tablets, and personal computers, to game consoles, ATMs, and POS systems, it’s almost counterintuitive and discombobulating now when we encounter a screen that doesn’t possess touch-enabled functionality. There are many touchscreen technologies with different methods of sensing touch, but the most common of them are either resistive or capacitive. 

How Resistive Touch Screens Work

Resistive touchscreens work on the basis of pressure applied to the screen. They are made up of layers separated by thin air gaps. The top layer is typically a thin, flexible plastic and the lower layer is a firmer plastic or glass. Both layers are coated with electrically conductive and resistive layers and separated by spacer dots. Electric current moves in between the layers. When a finger or stylus applies pressure to the top layer of the screen and the layers are pressed together, it causes a change in current and the touch is registered. The change is then interpreted by a processor that determines the coordinates of where the screen was touched and the assigned function is carried out.

How Capacitive Touch Screens Work

There are three main components to a capacitive touch screen — the touch sensor, the controller, and the software driver. 

The touch screen sensor is a sheet of glass designed with a grid of hair-thin lines of conductive metal such as ITO (indium tin oxide). The grid lines in one direction are called driving lines, which provide a constant electric current. The lines in the other direction are called sensing lines, which detect electric current.

At every point where the sensing lines and driving lines intersect, there is a specific electrostatic field, which is registered as neutral by the controller in the device.

The user is part of the equation that makes the electronics work. The human body has a natural capacitance and can conduct electric current and store electric charge. When the finger touches the screen, the charge in the screen is drawn around that point, distorting the electrostatic field. The electrostatic field feels the effects of the user’s electric charge and redistributes itself accordingly. The controller transmits this information to the software driver, which maps certain patterns such as taps, slides, and multi-touch gestures to perform different functions.

Useful Applications

Besides being a display, touchscreens allow the user to interact and have tangible control over what is displayed. We practically have capacitive touchscreens glued to the palms of our hands in the form of smartphones and tablets. Resistive touchscreens are more commonly used in POS devices and self-checkout kiosks, interactive display systems such as shopping mall information screens, ATM and public transport ticket issuing machines, and gaming devices including the Nintendo DS and 3DS family.

Data Sheets

To compare the two, I referenced the capacitive touchscreen I interact with on a daily basis — my Samsung Galaxy smartphone, as well as the 2.8″ TFT LCD with Touchscreen Breakout Board with MicroSD Socket – IL19341 I purchased from Adafruit for $29.95. Adafruit also sells a 2.8″ TFT LCD with Cap Touch Breakout Board with MicroSD Socket for $39.95.

The technical details for both are as follows:

  • 8 bit digital interface, plus 4 or 5 control lines (12 pins minimum) or SPI mode with 4 or 5 SPI data/control lines (4 pins minimum) – not including the touch screen.
  • 5V compatible! Use with 3.3V or 5V logic such as an Arduino
  • Onboard 3.3V @ 150mA LDO regulator
  • 4 white LED backlight, transistor connected so you can PWM dim the backlight

Resistive Touch Screen | Data Sheet

  • 4 pins are required for the touch screen (2 digital, 2 analog)

Capacitive Touch Screen | Data Sheet

  • 2 I2C pins are required for the touch screen controller

Wiring for Resistive Touchscreen Controller with Arduino 

Data lines: D0 and D1 go to digital #8 and #9, then D2-D7 connect to #2 thru #7.

Control lines: Connect the third pin CS (Chip Select) to Analog 3, fourth pin C/D (Command/Data) to Analog 2, fifth pin WR (Write) to Analog 1, Connect the sixth pin RD (Read) to Analog 0. Optional: Connect the seventh pin to RST to reset the panel.

Code

Download Adafruit’s Touchscreen Library, TFTLCD Library, and GFX Library

*Note: I did not test Adafruit’s capacitive touchscreen, but the wiring and example code can be found here

Application Notes

I plan to use touchscreen as my tangible interface for the DMX lighting controller assignment.

Resistive Strengths & Weaknesses 

Pros: Lower cost, does not require a stylus or object with a capacitive tip to operate, can operate with gloved hands, higher sensor resolution for applications that require registering finer points of contact (ie. small buttons, handwriting), and does not pick up on accidental touches.

Cons: Low sensitivity/requires applied pressure, does not support multi-touch.

Capacitive Strengths & Weaknesses 

Pros: Multi-touch and gesture support, incredibly touch sensitive and responsive, higher image quality.

Cons: Higher cost, gloved hands will interfere with input, and sensitive to inadvertent touches and moisture.

Citations and References

www.slideshare.net/lavinkatiyar/capacitive-touch-screen
www.goodgearguide.com.au/article/355922/capacitive_vs_resistive_touchscreens
www.electronicdesign.com/displays/what-s-difference-between-resistive-and-capacitive-touchscreens
www.embeddedarm.com/blog/reach-out-and-touch-something-capacitive-vs-resistive-touch-screens
www.youtube.com/watch?v=wKuqNuzM1oM

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