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					SAFE STREET CROSSING SYSTEM FOR VISION IMPAIRED PEOPLE


                            By



                     Fatih Degirmenci
                      Morad Oumina
                    Abdessettar Ibourki




            ECE 445, SENIOR DESIGN PROJECT

                      SPRING 2006




                    TA: Alex Spektor
                     01 May 2006
                     Project No. 47
                                               ABSTRACT

The goal of our project was to build a reliable system for vision-impaired people to cross safely streets at
crosswalks where “walk/don’t walk” or the walking man and the flashing hand signals are implemented.

Using low cost receiver, transmitters and other electrical parts, such as photocells, flop-flops, counters,
and a pager motor we build a communicating wireless system consisting of:
   - A dependant receiving unit operating with four AA batteries which will tell the user which way it
       is safe to cross after turned on by the user and set to the specific corner (NE, NW, SE, or SW)
       where he or she is standing.
   - A transmitting system which is activated only when a “Walk” signal is on.
We also built a working solar-cell empowering unit but its cost was beyond our expectations.

Our entire design worked on a full scale but there are still other improvements that can be added. Such
improvements include a sound system that accompanies our vibrating system. The number of transmitter
may be reduced to four or even two instead of eight. The solar component may also be redesigned to be
more cost efficient.

The final cost of the design implementation was higher than we expected because of the high price of
the solar cells we used. However, compared to most products available so far in the market, this design
is very cost efficient if we draw the power for the transmitters directly from the city pre-build system
instead of having them empowered by our solar-cells component.




                                                     ii
                                                         TABLE OF CONTENTS

1.   INTRODUCTION ....................................................................................................................1
     1.1 Purpose ...............................................................................................................................1
     1.2 Specifications......................................................................................................................1
     1.3 Subprojects .........................................................................................................................1
         1.3.1 Power Supply Module ...............................................................................................1
               1.3.1.1 Battery Recharging Module ..........................................................................1
               1.3.1.2 Recharging Control Module ..........................................................................2
         1.3.2 Transmitting Unit ......................................................................................................2
         1.3.3 Receiving Unit...........................................................................................................2

2.   DESIGN PROCEDURE...........................................................................................................3
     2.1 Power Supply Design .........................................................................................................3
         2.1.1 Recharging circuit Design ...........................................................................................
         2.1.2 Recharging Control circuit Design..............................................................................
     2.2 Transmitting and Receiving Units Design..........................................................................3

3.   DESIGN DETAILS ..................................................................................................................5
     3.1 Transmitters Power Supply.................................................................................................5
         3.1.1 Recharging Circuit Unit ............................................................................................5
         3.1.2 Recharging Control Circuit Unit ...............................................................................6
     3.2 Receiving Unit ....................................................................................................................6
         3.2.1 Data Storage Circuit ..................................................................................................6
         3.2.2 Vibration Circuit........................................................................................................7
     3.3 Transmitting Unit................................................................................................................7
         3.3.1 Transmitters...............................................................................................................7
         3.3.2 Switching Component ...............................................................................................7
         3.3.3 Photocells ..................................................................................................................8
         3.3.4 Antennas ....................................................................................................................8

4.   DESIGN VERIFICATION.......................................................................................................9
     4.1 Current Generated from the Solar Cell ...............................................................................9
         4.1.1 Recharging Control Circuit .......................................................................................9
         4.1.2 Recharging Circuit ....................................................................................................9
     4.2 Receiving Unit....................................................................................................................9
     4.3 Transmitting Unit .............................................................................................................10
         4.3.1 The Rang of the Wireless Communication .............................................................10
         4.3.2 Light Emitted by the “Walk” Signal .......................................................................10

5.   COST ......................................................................................................................................11
     5.1 Parts ..................................................................................................................................11
     5.2 Labor.................................................................................................................................11

6.   CONCLUSIONS ........................................................................................................................

     REFERENCES .......................................................................................................................13




                                                                            iii
APPENDIX A – BLOCK DIAGRAMS...............................................................................A.1

APPENDIX B – SCHEMATICS..........................................................................................A.2

APPENDIX C – TEST DATA .............................................................................................A.7

APPENDIX D – PICTURES..............................................................................................A.11




                                                            iv
                                          1. INTRODUCTION

   We designed and created a device that will allow people with vision disabilities to cross the street
   safely. Since the number of this group is small, the big companies don’t focus mainly on investing in
   its needs. The system will consist of three main elements:
         Transmitting Unit: this part of the system is to be mounted on every traffic light system. The
           goal of this element is to send signal to the opposite side of the street whenever the “WALK”
           sign is ON. When this sign is OFF no signal is sent. For the sensing of this part, we are using
           photocells.
         Receiving Unit: This is a device that the user will carry. The goal of this element is to catch
           the emitted signal. Once the signal is received the device should vibrate so that the user
           knows which way it is permissible to cross the street. This element consists of a receiving
           and vibration circuits.
         Transmitting Unit Power Source: constituted of solar cells, so that we will not use the city
           crossing light system to empower our device.

1.1 Purpose

The purpose of this project was to use our knowledge of communications, signal-processing, and digital
system design, to engineer a product that would people with vision disabilities to cross the street safely.

1.2 Specifications

For an optimal design of our device, the following considerations to be counted:

          Signal range is less than 40 meters,
          There are 3 vibration modes corresponding to 3 different states in which it is safe to cross.
          The vibration option is a good way to let the user know that he can cross the street safely in
           case of hearing disability or external noise of traffic or a machine operating on the site.
          The Cost of the final product may be less because the system will be less expensive to
           implement, easy to install, easy to access, and easy to maintain.
          The system is Self Powered by the use of photocells that “absorb” the emitted light from the
           “Walk, Don’t Walk sign.” This will help in saving the cost of the power.

1.3 Subprojects

The design was broken into three units, which each perform specific tasks (see figures A1, A2 and A3).
These units are the Transmitters Power Supply Unit, the Transmitter Unit and the Receiver Unit.

1.3.1 Transmitter Power Supply Unit

The Transmitter Power Supply Unit is constituted of two main modules. Recharging Module and
Recharging Control Module.

1.3.1.1 Battery Recharging Module

This module is composed of a Solar Cell, as a power source, Connected to a Circuitry that adjusts both
the current and voltage. These voltage and current are used to Recharge the Batteries as well as
empower the Transmitters’ Circuit.


                                                     1
The Solar Cell provides 12V*200mA. The Circuitry reduces the input voltage of 12V to 5V, and
regulates the input current of 200mA to an output current of 227mA. This output current feed 47mA to
the Transmitters circuit and delivers 180mA to recharge four 1600mAH Batteries.

1.3.1.2 Recharging Control Module

The Circuit in this Module is for a temperature termination of a constant current battery charging. It
works with the principle that the rechargeable batteries show an increase in temperature when they
become fully charged. Overcharging is one of the main causes of short cell life. Hot cells pop their
internal seals and vent out electrolyte. As cells dry out, they lose capacity.

1.3.2 Transmitting Unit

Eight transmitters are placed at the four corners of the cross section (two for each corner fed by the same
power supply). Each transmitter is connected to its corresponding “Walk” signal via a photocell that
senses the light coming from the signal and triggers the transmitter to send a digital data that it is safe to
cross in that specific direction. This data is translated by the receiving unit accordingly (see figure C1).

1.3.3 Receiving Unit

The user inputs the corner where he is standing by two switches and turns on the receiver unit. Then the
receiver unit starts to check for the signals coming from the relevant transmitters. According to the data
sent by the transmitters, the receiver vibrates to tell the user which way/s is/are safe to cross. If only the
right or the left street is safe to cross, the receiver will vibrate in short or long pulse mode respectively.
If both ways are safe, the receiver will vibrate in continuous mode.




                                                      2
                                      2. DESIGN PROCEDURE


2.1 Power Supply Design

As mentioned before, the Transmitters power source is mainly formed of a solar cell, and it is composed
of two units that are the Recharging Unit and the Recharging Control Unit. However there are certain
considerations that we are to take while we are designing the circuit:

    Transmitters need about (5V*47mA) power.
    Recharging Batteries daytime. During nighttime these batteries provide current to the transmitter.
    For winter (daytime about 8 H and night about 16 H).
    Average Rechargeable Battery efficiency is 66%.
    By the nighttime the batteries should provide 90%C of the available current (worst case, in 16
     Hours during the winter.)
    Charging the batteries should take 8 hours

2.1.1 Recharging circuit Design

For the solar cell, we used the PowerFilm MPT6-150 (12Vx200mA). To adjust these current and voltage
values, we used MAX639EPA to drop the voltage to 5V and adjust the current to 250mA to provide
(5Vx47mA) needed for functioning of the transmitters circuit, and (5Vx180mA) to recharge four
(1.2Vx1600mAH) batteries.

2.1.2 Recharging Control circuit Design

The circuit is meant to stop the recharging process when the batteries are fully charged. This circuit is
based on the Temperature termination of recharging. For this circuit, the main elements are temperature
sensors (one for the ambient and the other is for the batteries temperature), differential voltage Op-Amp
and a VMOS FEF.

The differential temperature sensors present two voltages to the differential voltage Op-Amp. The Op-
Amp output switches ON or OFF depending on which input is a higher voltage than the other. As the
temperature sensors warm up, their resistance drops, lowering the associated comparator input. Since the
ambient temperature can vary, the circuit will only react to the difference in temperature between the
sensors.

2.2 Transmitting and Receiving Unit Design

Initially we planned to use the polarization of RF signals to recognize the signals sent by the
transmitters. However, after our discussion with Professor Cangellaris, we realized that this idea was not
a good one to pursue for the environment where the project is supposed to work. There would be many
reflections of the RF signals in a crowded cross section therefore, it would be possible for a vertical
polarization to change into a horizontal one sending a wrong message to the user. Professor Cangellaris
advised us to use a frequency selection approach. After our discussion with our TA, Alex Spektor, we
learned that we can use 900 MHz Linx HP-3 series transmitters and receivers for wireless
communication through eight different frequencies. We decided to fix a different channel for each
transmitter and transmit the signal with the corresponding frequency.

Since the channels of the transmitters were fixed, the receiver had to know where it is standing to check
for the signals from the relevant transmitters. The next question was how to let the receiver know its
                                                     3
location. The easiest way was to make the user enter his/her location; therefore, we chose that approach.
Then the receiver will check for the signals sent from the WALK signs to the right and left of the corner
where the user is located. The data out of the receiver is all time HIGH when there is no data sent to the
receiver. If the WALK sign turns ON, the transmitters will be sending LOW signals to the receiver
indicating that the street is safe to cross. The values coming from the transmitters are stored in D flip-
flops.

According to the value in the D flip-flops the receiver vibrates in three different values. If both values
are 0, the receiver vibrates continuously. If only right or left flip-flop is 0, receiver vibrates in short or
long pulses respectively. When both of the values are 1, there is no vibration.




                                                       4
                                          3. DESIGN DETAILS

3.1 Transmitter Power Supply

The transmitter Power Supply Module was designed so that it provides (5Vx47mA) to the transmitter
circuit and (5Vx180mA) to charge four (1.2Vx1600mAH) batteries.

3.1.1 Recharging Circuit Unit

To decide the Solar Cell we are to use we started with the Charging rate:

          Rate_Chrg = (90%C)/ (8 hours) (where C is the Battery Capacity => Unit: mAH)
          C = 1600mAH => Current provided is: Ib=11.25%x1600= 180mA
                        => Total Current the Solar Cell needs to provide is Is
                        => Is = Ib+Ic (Ic is the transmitters current Ic = 47mA)
                        => Is = 180mA + 47mA = 227mA.

To decide the voltage needed to provide this current (Is = 227mA), we connected the circuit to a Power
Source in the Lab. We changed the input voltage coming from the Power Source and record the output
current. Table C3 shows the results of this experiment.

We Can Clearly notice that the output voltage is stable about 5.0V and the output current Is is 227mA
corresponds to a voltage input of 11.3V.

Therefore the solar cell needed should provide (11.3Vx227mA). To produce these values we used four
MPT6-150 PowerFilms (each provides 6Vx115mA), 2 in series in parallel with the other 2 that are in
series as well.

The Recharging Circuit module uses MAX639EPA as its primary component. This is a step down
switching regulator that presets the output voltage to 5V, and delivers up to 250mA current

For the circuit in figure B1, there are 2 diodes in this circuit. D1 protects against a current dump from
the batteries set to the MAX639EPA and D2 to drop the Solar Cell generated voltage to about 11.3V as
well as protects the Solar Cell from any dumped current from the rest of the circuit. For an optimal
output current the resistances R3 and R4 where 1.2M Ohm and 75K Ohm respectively.

3.1.2 Recharging Control Circuit Unit

In the circuit of figure B2, the current needed to charge the battery comes to its positive pole through the
Recharging Circuit. The 500 Ohm across the VMOS FET (IRFZ34N) sets the trickle charge current
which flows through the batteries set after the bulk charging is finished.

The differential temperature sensors present two voltages to the differential voltage Op-Amp. The Op-
Amp output switches ON or OFF depending on which input is a higher voltage than the other. As the
temperature sensors warm up, their resistance drops, lowering the associated comparator input. Since the
ambient temperature can vary, the circuit will only react to the difference in temperature between the
sensors. In this case we set the Variable Resistance so the temperature difference is set to 15 deg C.




                                                     5
3.2 Receiving Unit

There were three main considerations taken to be account of. The first one was how to get the location
of the user to the receiving unit. The second one was how to store the data coming from the two
transmitters relevant to the corner that the user is standing. And the last one was how to implement
different vibration modes for the motor.

We decided to use two switches one standing for East(1)/West(0) and the other standing for
North(1)/South(0). Therefore the four corners SW, SE, NW, NE are coded as 00, 10, 01, 11. The
transmitters to the right and left of a specific corner, xy, will be fixed to send signals in the channels xy0
& xy1 respectively. The outputs of E/W & N/S switches are fed to the two most significant bits of the
channel selection, CS2 and CS1, and the least significant bit, CS0, will alternate between logic 0 and 1.
This way, both of the channels relevant to the corner where the user is standing will be checked.

3.2.1 Data Storage Circuit

In order to produce an oscillating signal, we used a 555 timer. The configuration ,in figure C3, of the
555 timer can be used to produce oscillating signals of certain rising and falling times. The formulas for
the rising and falling times are as follows:

   Trising=0.693*(R1+R2)*C1
   Tfalling=0.693*R2*C
   Ttotal=0.693*(R1+2*R2)*C1

We decided that a period in the order of a millisecond will be fast enough to update the signals sent by
the transmitters. Therefore, we used R1= 5 kΩ, R2= 33 kΩ, C1= 100 nF in order to get a period of 4.92
ms.

   Trising=0.693*(5kΩ+33kΩ)*100nF = 2.63 ms
   Tfalling=0.693*33*100nF = 2.29 ms
   Ttotal=0.693*(5kΩ+2*33kΩ)*100nF = 4.92 ms

The next thing is to store the data from the transmitters to D flip-flops. First we tried to use the
oscillating signal as the enable bit of D-latches but this method didn’t work due to timing issues. We
needed a more controlled storing of the data therefore; we decided to use D flip-flops and raise the
enable signals E0 and E1 when the channel bit CS0 is stabilized to 0 and 1 respectively. We wanted the
enable signals to look as in figure C4.

In order to get these waveforms we decided to use our oscillating signal as the clock of a 7493 counter in
order to get 4 different waveforms with periods of 10, 20, 40 & 80 ms. Next, we decided to use the
output Q1 of the counter as the alternating signal for CS0. E0 and E1 should have looked like the figure
C5 with respect to Q1 and Q0.

Based on figure C5, E0= Q1’.Q0 and E1=Q1.Q0. Therefore, when Q1 is low, CS0 goes low and the
receiver starts to receive signal from the transmitter to the right of the current corner. Then E0 goes high
and the output of the receiver is stored to flip-flop D0. Similarly when Q1 is high, CS0 goes high and the
receiver starts to receive signal from the transmitter to the left of the current corner. Then E1 goes high

                                                      6
and the output of the receiver is stored to flip-flop D1. Essentially, the data of the right transmitter is
stored to D0 as Data0 and the data of the left transmitter is stored to D1 as Data1.

3.2.2 Vibration Circuit

Next, we needed to decide how to implement different vibration patterns for the motor. We needed
different vibration patterns for the motor according to the safety of the cross section. If only the street to
the right was safe, the motor would vibrate in short pulses and if only the street on the left was safe the
motor would vibrate in longer pulses. If both of the streets were safe then the vibration would be
continuous.

In order to get different pulse lengths we used a similar idea to the previous circuit. We thought that
pulses with periods of 1 & 2 s would be appropriate for the vibrations relevant to the safety of right and
left streets respectively. A 555 timer was used to produce a 0.5 s pulse and the output signal was fed to a
7493 counter to get 4 different pulses with periods of 1, 2, 4 & 8 s. In order to get a signal with a period
of 0.5 s as the output of the 555 timer, we used the values R1= 5 kΩ, R2= 33 kΩ, C1= 10 µF this time.

   T1=0.693*(5 kΩ+33 kΩ)*10 µF= 0.263 s
   T2=0.693*33 kΩ*10 µF= 0.229
   Ttotal=0.693*(5 kΩ+2*33 kΩ)*10 µF=0.492

The output Q0 and Q1 of counter 7493 had periods of 1 s and 2 s respectively. The vibration mode was
selected through a 74LS153 multiplexer according to Data1 (Right Transmitter) and Data0 (Left
Transmitter). Table C6 shows the truth table for 74LS153:

In order to get the truth table C6 we input values 1, Q1, Q0 & 0 to the input bits, I0, I1, I2 & I3 of the
multiplexer. Since the output current of the multiplexer wasn’t high enough to drive the motor, we used
a current amplification through a BJT by feeding the output current to the base pin. We used a pager
motor for the vibration which was perfect for our design due to its size and low power consumption.

3.3 Transmitting Unit Components

Our transmitting unit module consists of four simple components. (See figure A2)

3.3.1   Transmitters

The transmitters we used were 900 MHz HP-3 Series Linx transmitters. These transmitters are pre-build
to communicate with corresponding receivers through eight different channels. Each channel is specified
by a single frequency ranging from 903.37 MHz to 921.37 MHz (See table C7). We used eight
transmitters; one transmitter for each “Walk” sign. The input of the transmitters is fed by the output
Vout of the photocells components.

3.3.2   Switching Component

Each single transmitter uses one switching component which simply sets the transmitter to the channels
desired at each corner of the cross section. The user input of his/her location tells us how to set CS-2 and
CS-1 (Channel Select 2 and 1) of the transmitters which correspond to the most significant and the
middle bit in our design. For the CS-0 (Channel Select 0) to be consistent in our design we assigned
logic “0” to the transmitter on the right of the user and logic “1” to the transmitter on the his/her left.
(See figure C1)

                                                      7
3.3.3   Photocells

Knowing the kind and intensity of light used in the “Walk” signal, we used photocells and
corresponding resistors to sense the light coming from the “Walk” signal. These photocells trigger the
corresponding transmitters to send the message saying in what direction it is safe to cross. In our case,
when the photocells passes the current its output Vout drops to zero a logic to be detected by the
receiving unit (see figure C8).

3.3.4   Antennas

For a better wireless RF communication we used eight 80 mm Linx Antennas that corresponds to the
frequencies in hand.




                                                    8
                                     4. DESIGN VERIFICATION

Besides testing how well all the modules work together, we wanted to make sure that all of our modules
were independently operating to our satisfaction. The testing of each module is described in their
respective locations below.

4.1 Current Generated from the Solar Cell

After connecting the Solar Cell to the Transmitters circuit and after exposing it to the sun we checked
both the output voltage and current with a portable Ammeter. We were so pleased to find that the result
we got were almost identical to the expected values (Vout = 11.76V and Is = 232mA).

4.1.1 Recharging Control Circuit

To check this circuit I’ve gotten some help from a friend of mine working for his PHD in Material
Science (his name is Taner Ozel). In his lab there are hot plates for which we can control the
temperature. We tapped the temperature sensor in the middle of the hot plate; we kept changing the plate
temperature and check the output voltage of the Voltage Differential Op-Amp (LM2904N). Table C9
shows the results we got.

We can clearly notice that a temperature between the ambient (22degC) and (35degC) the output is logic
zero. However after a temperature of (40degC) the output is a logic one. Our expectation was to have a
logic one when the temperature is (37degC). We couldn’t check the output at this value because the
plate allows an increase of temperature of a multiple of (5degC). But so far we were pleased with these
results.

4.1.2 Recharging Circuit

The main task of the Recharging Circuit is to provide a total current that will allow the functioning of
the transmitters’ circuit as well as a current to recharge batteries. We were please by accomplishing this
result by getting the current at the pin 5 of MAX639EPA (Iout = Ib + Ic = 226mA). However, at the end,
we found out that whenever the batteries have some charges in them, they will provide current to the
circuit.

We were not pleased at all with this result, because we were expecting that the batteries will not deliver
any current as long as they’ re not fully charged. This can be solved by implementing a sub-circuitry that
will allow the batteries to provide current to the transmitters’ circuit only during the night time. During
the daytime the circuit current will come only from the Solar Cell.

4.2 Receiving Unit

The main concern regarding the receiving unit was the battery life. We included an On/Off power switch
for our design in order to prevent battery consumption when the device is not in use. We measured the
current used by the receiving unit by using the power generator in the lab for a fixed voltage level of 6V.
When the motor is in operation the circuit uses 245 mA, and if not it uses 140 mA. After we got these
values, we needed to make some assumptions for the operation time of the receiving unit.

On average we estimated the waiting period for the WALK sign, and WALK sign duration to be 30s and
10s respectively. In addition we assumed the number of crossings a day to be 25 on average. In the
worst case scenario, our estimations for those values were, 60s, 20s and 60 crossings respectively.

                                                    9
Then we calculated the battery power corresponding to these scenarios, the values turned out to be:
Average = (30s*140 mA+10s*245 mA)*25
           = 46.18 mAH
Worst-case = (60s*140 mA+20s*245 mA)*60
               = 221.66 mAH
We recommend our users to use 4 regular 1.5 V, 2000 mAH rechargeable batteries when they need a
replacement. In this case the battery life will be:

Average lifetime: 2000/46.18 = 43.3 days

This is a reasonable value for most electronic portable device applications. Even in the worst case
scenario the battery life turns out to be:

Worst case scenario: 2000/221.66 = 9 days

This value is still reasonable. The user will be able to use her receiver unit more than a week even in her
busiest schedule.

4.3 Transmitting Unit

We verified to aspects of our transmitting unit.

4.3.1 The Rang

We used the whole receiving unit (pager included) and a transmitter to test the range for our system
We selected the average range to be 50 (+/-20) feet. We started moving the receiver farther than 60 ft
from the transmitter and adjust the power fed to the transmitter so that the receiver should not receive
the signal after a distance of 70 feet. The best fit was 4.8 V and 15 mA an equivalent of 72mW which is
4 AA batteries in practice.

4.3.2 The light emitted by the “Walk” signal

After a visit to Champaign City Traffic Signal Department, we learned that the city uses two different
kinds of “Walk” signal. The first kind and the most used one is the sign where regular tungsten
incandescent bulbs are used behind the “WALK” sign. The second kind, which is less used, is the
walking man which uses “White” LEDs (Light Emitting Diodes). We tried both kinds of light using two
different brands of flash lights. In some we used regular tungsten bulbs and in others we used white
LEDs. (See figure D1)




                                                    10
                                                 5. COST

                PART                     PART#                # NEEDED               COST
              Solar Cell               MTP6-150                    16         16*$32 = $512.00
              Receiver             RXM-900-HP3-PPS                  1               $46.03
             Transmitter           TXM-900-HP3-PPS                  8         8*$30.33=$242.64
               Antenna            ANT-916-CW-QW-ND                  8            8*$7.35=$58.8
        Rechargeable Battery        1.2V-1600mAH                   16               $60.00
        Rechargeable Battery        1.5V-2000mAH                    4               $25.00
        Step Down Regulator          MAX639EPA                      4           4*$6.50=$26.00
        Diff Voltage Op-Amp            LM2904N                      4           4*$7.99=$31.96
             VMOS FET                  IRFZ34N                      4           4*$2.99=$11.96
            Temp. Sensor               LM335AZ                      8          8*$1.39 = $11.12
             Pager motor             7mm Namiki                     1                $1.50
             Photo sensor               MD 300                      8           8*$1.50=$12.00
       IC-cap-res-wire-switch        miscellaneous                  -               $10.00
               TOTAL                    -----------            -----------        $1049.01

5.1 Parts

The above table shows the cost of the main parts used to implement our device. It can be clearly seen
that most of the project money was invested in buying the solar cell set, it represents 50% of the over all
cost of the project. An alternative to improve this is to empower our device from the same source used to
empower the crossing light signals, but this need the approval of the city.

5.2 Labor

We assume a future salary of $60 per hour. We estimate the amount of time spent for our project to be
150 hours on average. Therefore the total labor cost will be:

       Labor Cost = $60 x 150 x 3 = $27,000

One receiver unit costs $49 and we plan to sell it to our customers for $100. On the other hand, if we
assume that we use the city electricity, a cross section setup will cost $488 and we plan to sell it for
$1000. Our major customers therefore, will be the cities rather than the vision impaired people because
there are many cross sections in a city and the profit of a cross section setup is 10 times larger than
receiver units. We can recover our labor cost just by selling 53 cross sections or, 52 cross sections along
with 8 receiver units.




                                                    11
                                               6. Conclusions

        We’re pleased with our design that we were able to demo the different vibrations corresponding
to different safety conditions of the cross section. However, we think that we can improve our design by
redesigning the empowering unit to be more cost efficient, incorporating sound warning in addition to
vibration, and preventing interference from other devices.

        Our main displeasure about our final design is that the cost of the project turned out to be more
than we estimated. That is mainly because of the fact that, the solar panels used for the empowering of
the transmitters were too expensive. We believe that we can improve our charging circuit in order to
reduce the number of solar panels thereby reducing the overall cost of the system. We can also try using
the city electricity instead of using solar power. This way the overall cost of the project will be reduced
to the half of the current value.

        We may incorporate sound warning system along with the vibration in order to solidify the
safety of the vision impaired person. This way, the security level of the vision impaired user will be
doubled. If one of the systems fails, the other system will still continue to warn the user. The sound
feature will also allow two users to use the same device. In addition to the sound, we can add a ‘failure
mode’ in order to warn the user. If there is a possibility of failure due to inconsistent signals, the device
will get into a “failure state” and flag a signal. Therefore, the user will know that it’s a good idea not to
rely on the device for this time and use other physical means to cross the street.

       The last issue is the interference. We did not take into account interference from other devices.
We are assuming that we will be given permission from the city to operate in a frequency range
forbidden to outside users. We can perfect our design by using microcontrollers for the receiving and
transmitting units to encode and decode our signals in order to prevent the effects of interference.




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                                           REFERENCES

[1]   MAXIM Technologies, Datasheet MAX639EPA, [Online Document], [cited 04, July 2005],
      Available HTTP: http://pdfserv.maxim-ic.com/en/ds/MAX639-MAX653.pdf.

[2]   National Semiconductor, LM335AZ Datasheet, [Online Document], [cited Feb 1995], Available
      HTTP: http://cache.national.com/ds/LM/LM135.pdf.

[3]   Not Available, “Intelligent NiCd/ NiMH Battery Charger –Construction Project,” [Online
      Document], [cited Not Provided], Available HTTP: http
      http://www.angelfire.com/electronic/hayles/charge1.html.

[4]   T. van Roon, “Timer 555/Oscillator Tutorial,” [Online Document], 4 Sept 2003, [cited 17 Oct
      2003], Available HTTP: http://www.uoguelph.ca/~antoon/gadgets/555/555.html

[5]   Philips, “Data Sheet AD725AR: Triple 3-input NAND gate”, [Online Document], [cited January
      1995], Available HTTP: http://www.priory.bromley.sch.uk/students/electronics/pdf/hef4023b.pdf




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