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Low Cost Infrared Touch Screen Bezel for POS Systems
Rohan Verma, Jeremy Taylor, Freddie Dunn III
Georgia Institute of TechnologySchool of Electrical and Computer Engineering
December 4, 2009
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
Bezel Design
Red Line – Ground rail for deviceGreen Line – Power rail for deviceBlue Circle – PhototransistorsYellow Circle – Wide Angle IR LEDOrange 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 touchesthat 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
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 HardwareProduct Description Quantity Unit
Price
Price
Microchip PIC18F14K50 8-bit MCU 1 $2.40 $2.40
Infrared LEDs 9 $0.30 $2.70
Phototransistors 8 $0.30 $2.40
Bezel 1 $15 $15
Switching Transistor 1 $2.00 $2.00
Crystal Oscillator 1 $0.50 $0.50
Miscellaneous X X $1.50
Total Cost $36.75
Final Cost for DevelopmentProject Component Labor
Hours
Labor
Cost
Equip.
Costs
Total Component
Costs
Algorithm coding 150 $7,500 $7,500
Simulation 150 $7,500 $7,500
Code debugging 30 $1,500 $1,500
PCB Design and layout 30 $1,500 $100 $1,600
On board testing and debugging 30 $1,500 $1,500
Enclosure (bezel) design and
construction
40 $2,000 $75 $2,075
Demo preparations 40 $2,000 $2,000
TOTAL LABOR COSTS 470 $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