Download ppt - School of Electrical and Computer Engineering at Georgia Tech

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
October 19-23, 2009
Project Overview
• Optically driven bezel that affixes to a computer
display to provide touch screen capabilities.
• Existing products are enhanced by retrofitting the
bezel to displays.
• Product targets service industries whom seek to
increase efficiency.
• Development costs and a production run of 5000
units totals $458,000 with a single unit cost of
$91.60
Design Objectives
• Design a precision touch device with as few
sensory components as possible
• Interprets single clicks, double clicks, and mouse
drags
• USB is used as the communication mechanism
between the MCU and PC and also serves to
power the device
– Plug and Play
• Bezel attaches to a 4:3 19” monitor
Building Blocks
Phototransistor
LEDs
ADC
Bezel
PC
USB
Microcontroller
LED Circuit Design
• Used to create a
plane of light
• Current divider to
determine resistor
values
• Account for voltage
drop across the
LEDs
• Incorporate an
on/off transistor to
allow for measures
to be taken with in
two states
Two sided LED configuration
Problem areas /
dark spots
LED Output Grid
15.2”
2.85”
11.4”
4.5”
6.2”
Phototransistor Circuit Design
• Used to detect light levels
• Voltage across ADC input
terminal and ground
proportional to infrared
light levels
• Voltage reference used to
specify maximum voltage
at the ADC input
Microcontroller Program Flow
PC/MCU Interaction
• USB 2.0 Interface
• 5V supply
• 500mA current limit
– No external supply
necessary
• Simplifies design
– Plug-n-Play: OS supplied
HID drivers
Evaluating Touches
• Store ‘base’ light intensity to determine if further
processing required
• Strobe the LEDs to remove noise
 Difference of the two readings for a given sensor forms the
current light intensity at that sensor
• Hardcoded table of phototransistor values
 Used to find relative finger placement
 Extrapolate intermediate touch points from this table
Problems/Issues
• The original LEDs selected had too small of a
sensitivity angle to be effective in this use
• Initial LED/Phototransistor orientation left
“dead zones” on the screen
• Complexity of USB development
Demonstration Plan
Bezel
attached
• Bezel overlaid
on LCD
• Bezel connected
to PC through
USB
• Power supplied
through USB
OS
recognizes
touch screen
• OS automatically
recognizes the
touch screen
device
• OS should
detect touch
screen device as
HID mouse –
interpret touch
as mouse click
Pointer
controlled
by touch
• Slide finger 
mouse pointer
should follow
• Mouse clicks are
detected at
correct locations
• Distinguish
between single
and double click
Project Schedule
Test and
build
prototype
• Test under
different
transceiver
configurations
• Test under
different
ambient
environments
• In progress
• Finish by 10/23
Design PCB
and MCU
• Layout PCB
• Develop and
optimize touch
detection
algorithm
• In progress
• Finish by 11/6
Interface
with OS
• Utilize USB
stack to enable
MCU to talk to
OS
• Debug any
issues that may
arise
• Finish by 11/20
Current Status
• Created/Programmed simple PIC
microcontroller firmware
– Working on ADC polling implementation
• LED/Phototransistor circuits fabricated and
tests performed on them
– Waiting on new LEDs/Phototransistors