Low Cost Infrared Touch Screen Bezel for POS Systems Rohan Verma, Jeremy Taylor, Freddie Dunn III Georgia Institute of Technology School of Electrical and Computer Engineering December 4, 2009

Touch Screen Design LEDs Microcontroller Bezel ADC USB PC Phototransistor

Project Overview Infrared LED bezel that attaches to a computer monitor converting it into a touch screen Lower LED/Phototransistor count when compared to the current market products to make for a more cost-efficient and affordable device Development costs and a production run of 5000 units totals $458,000 with a single unit cost of approximately $91.60

Proposed vs. Actual – Bezel Proposed # of phototransistors = 4 10 LED’s evenly spaced with 4 phototransistors located in the corners of the bezel Actual # of phototransistors = 8 9 LEDs evenly spaced with 8 phototransistors located in the corners as well as on the side at half the length

Proposed vs. Actual – Software Proposed Accurately identify distinct touch points Ability to identify single clicks, double clicks, and drag Actual Can identify a grid of 9 distinct points Identify single clicks and drags. Double clicks are intermittent

Bezel Prototype

Prototype Circuit

Bezel Design Red Line – Ground rail for device Green Line – Power rail for device Blue Circle – Phototransistors Yellow Circle – Wide Angle IR LED Orange Circle – Acute Angle IR LED The current orientation of the LED’s/Phototransistors produced the best results for touch recognition By placing an acute angle LED (~20 degrees) we are able to more accurately detect touches that occur in the middle of the screen

MCU firmware nuances 10-bit Analog to digital conversion, scaled down to 8-bit, of the phototransistors converts their voltage into a digital reading on a scale from 0 to 255 By polling the sensors with the LED’s on and then off, and computing the difference of the two, we are able to eliminate ambient IR levels Take a pseudo running average of current light levels, valuing the most recent reading as only 1/10 of the current reading to minimize random fluctuations of readings (Must fill pipeline) Take 200 A2D readings at boot to fill the averaging pipe and determine the unobstructed IR levels Touch is determined by creating a touch threshold of 90% and 110% of the unobstructed levels and comparing to the current readings

MCU Pin out

Testing Procedures Implemented a serial debug interface for viewing raw ADC values from the phototransistors and later to detect touches – MCUs UART (9600 baud), Viewed over HyperTerminal Methodology – By choosing a series of random points (~20 points) we were able to test various spots in our design for “blind spots” and by altering the orientation of the LEDs and phototransistors, we were able to achieve a much better touch detection sensitivity and accuracy USB interface was written separately and tested by having the mouse move to the center of the screen and continually right- click (via code)

Did it meet requirements? Proposed: A device that can accurately identify a set of distinct points on the screen and is able to move the mouse to that spot – Result: Achieved through the testing of a series of points that clearly are defined and allows the mouse to be placed at 9 unique locations Proposed: The ability to achieve single-click, double click, and drag – Result: Achieved through the USB interface which allows, through code, that the mouse will “click down” as long as a touch is detected and will be “released up” once a touch is no longer detected. – This allows for drag to be implemented as long as the “mouse down” action is in effect

Problems/Issues Encountered many “dark spots” with original configuration – Resolution: Improved upon by altering orientation Too sensitive when objects are in close proximity (~3 inches outside the bezel) Getting the optimum number of phototransistors

Final Costs for Hardware Product DescriptionQuantity Unit Price Price Microchip PIC18F14K50 8-bit MCU1$2.40 Infrared LEDs9$0.30$2.70 Phototransistors8$0.30$2.40 Bezel1$15 Switching Transistor1$2.00 Crystal Oscillator1$0.50 MiscellaneousXX$1.50 Total Cost$36.75

Final Cost for Development Project Component Labor Hours Labor Cost Equip. Costs Total Component Costs Algorithm coding150$7,500 Simulation150$7,500 Code debugging30$1,500 PCB Design and layout30$1,500$100$1,600 On board testing and debugging30$1,500 Enclosure (bezel) design and construction 40$2,000$75$2,075 Demo preparations40$2,000 TOTAL LABOR COSTS470$23,500 TOTAL PARTS COST$36.75 Total Project Cost$23,536.75

Recommended Future Work Continue to increase the efficiency/accuracy of the touch screen bezel by altering/adding the LEDs and Phototransistors. Find a functional relationship between light levels and finger placement to allow for interpolation and determination of touch location Find new LEDs/phototransistors that increase sensitivity for more touch points