laser modem

8/12/2019 Laser Modem

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ECE345 Senior Design Project

Group 8

Ta: Ajay Patel

Presentation : Mahlknecht Stefan

Michael Brown



Why build a Lasermodem?

• cheap point to point link

• connecting different buildings

• cable links not possible everywhere

• high data rate

• Internet access over network



Characteristics of our Lasermodem

• Speed

- from 2400 bps up to 115200 bps

• Distance

- Up to 400 m without focusing lens at the receiver

- Up to 3 km with 50mm lens and stable mount

• Sight contact between modems needed

• decrease in reliability under bad weather conditions



Block diagram :

Laser driving

circuitLaserdiode

Photodiode

Transceiver

receive/

transmit

Voltage

regulator

+Vcc

RTS

RXD

RS232

converter

TXD

DTR



Voltage Regulator :

• Provides stable 5V output from a range of input voltages



RS 232 converter :



Transceiver component:



Laser driving circuit :



Laserdiode :



Photodiode:



Optics :

Light (680 nm)

Photodetector

PIN - Diode

Convex lens (50 mm)

Light(680 nm)

Laserdiode

Collimator lens

Adjustable mount

Receiver optics and mount :

Sender optics and mount :



Software :

Open com port 9600

Set RTS on low = reset

Close port

Send initialization string

Set RTS back on high

Set DTR on low = control

Set DTR on high = data

Initialization procedure:

Set DTR on low = control



RS232 standard :

The standard RS232 waveform :

Corresponding logical TTL standard :



IRDA Modulation scheme :



LASERMODEM



Alignment & focus

Tradeoff between alignment and focus:

• Focused beam --> Much power at the receiver but

very precise alignment required

• Unfocused beam --> Only fraction of beam hits receiver but

alignment not so critical.

Combining the best of both:

Place a convex focusing lens at the receiver !Power received ~ Area of lens



Cost Overview

Student Labor Cost:

Machine Shop Cost:

Unit Cost Hours Multiplier Total

Unit Cost Hours Total

Cost for two modem

Volume cost

Parts $166.40Parts and Labor $11,616.40

Parts $79.80

$25 180 2.5 $11,250

$20 10 $200



Experience gained from the Lab:

• difficulty getting special parts

• long way from idea to a working device

•

teamwork requires a lot of communication--> good that we have Email nowadays

• handling laser diodes requires special care

• difficulty making printed circuit boards

• organization and time management very important



Conclusion :

The experience gained in this class will help

those of us starting in industry.

It was long hard work to get all the

components we needed for this project, but atthe end we were able to present a product that

would be ready to enter in the market.

laser modem

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