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
Touch Screen Design
LEDs
Microcontroller
Bezel
ADC
USBPC
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 vs. Actual – Software
Bezel Prototype
Prototype Circuit
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
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 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