Demo based

Digital Optical Fiber Communication

using IC UM9121 B in Transmitter

& CM 8870 in Receiver

OPTICAL FIBRE communication is the unique name in the field of

communication. In this communication we transmit the data through the

OPTICAL FIBRE we need not to use the carrier frequency for this project.

This model is very useful to line to line communication. Data, signal is

transferred from one end to other end with the help of optical fiber.

1. One is Recorded signal

2. Signal from mike.

Here we transmit two signals one is recorded and second signal from mike.

We use one change over switch to select to signals. We feed data from mike,

then mike convert the audio signal into electrical signal and this electrical

signal is coupled to the pin no 2 at I.C 741. Here we use I.C 741 as a mike

amplifier. Pin no 7 is connected to the voltage at pin no. 4 is connected to

the -ve voltage at pin no6 is output pin. Amplification factor of this I.C. is

depend on the feedback between pin no. 6 and input pin no. 2. Amplified

output is available on pin no. 6. For further Amplification we use transistor

circuit. Output of Transistor circuit is connected to the laser light. Actually

second signal from transistor circuit is converted into light. We use Zener

diode with laser light to protect them from high voltage. Data is now super

impose on the laser light and we focus this laser light on the receiver. In

receiver section we use one photo sensor as a Receiver part. When laser light

focus on the photo sensor the photo sensor convert this light into voltage,

this Small voltage in the form at signals is again fed to the pin no. 2 at

I.C. 741. Again Amplified output available on pin no. 6 at I.C. 741 and

further Amplified by a transistor circuit. Here we explain the complete

working at op-Amp, because these details are easily available in the books.

It is new project so, no detail is available in the book, that‘s why I use very

simple circuit to explain this concept. At present we use this project far up to

the range of 50 feet. If we want to increase the range them we use powerful

laser for this purpose . Here we use very simple laser for small range.

OPERATIONAL AMPLIFIER

Operational amplifier, commonly called op-amps, because they are of great

utility, and most are of low cost. Op amps are used in all the low cost

scientific projects and instrument. Incidentally the term OPAMS was

originally coined because the device was designed to perform mathematical

operations for ANALOGUE computers. However, op amps have proved so

useful for instrument design that the term instrumentation amplifier is likely

to replace the older name. An op-amps is another example of an integrated

Circuit and consists of many components which like transistor‘s diodes,

resistors mounted on a single silicon chip. The chip is encapsulated inside a

plastic casing, from which several leads emerge the IC has well defined

input-output behavior according to the configuration of the external circuit

of which it forms part. There are many type of op amps available in the

market, but here we discuss about the IC 741 is a commonly available in the

market, it is 8 pin Dual in line package.

FUNCTION OF PINS

An op-amp is not a passive device like a resistor. It is active and requires a

dual polarity power supply (in many cases). An active device, such as a

transistor or op-amp, can respond to a very low power input signal and by

drawing power from the supply produce an output of increased power. This

is not possible in passive circuits elements such as resistors and capacitors.

The 741, like many other op-amps is very tolerant power supply voltage and

will operate the dual supplies giving any thing between 5 volt to 15 volt. The

positive supply is connected to pin no. 7, and negative supply to pin no. 4.

The quiescent supply circuit is about 2mA. The maximum power dissipation

is 50mw and the maximum output current about 30mA. Pin no 1&5 are

offset. These are used to arrange that when the input is zero, the output will

also be zero. Pin no 1 & 5 are usually connected through a potentiometer to

pin no 4, and the pot is used to achieve the zero in, zero output out

condition. For some applications this offset correction is not required.

The input signals to the 741 are fed to the pin no 2 & 3 and the output comes

from pin no 6. It should be noted that all voltages are measured with respect

to a common ground line. This is usually, the center tap of power supply.

Pin no 8 is not connected in the 741 op-amp. Its presence merely serves to

provide a symmetrical physical appearance, and perhaps assists secure

location of the IC in a socket.

Digital transmission.

In this digital transmission we transfer the data in digital form. In digital

form we use one numeric keyboard and at the receiver section we use one

numeric display circuit.

In transmitter section we use 9 switches for the nine decimal number’s and

the output circuit we use one seven segment display for the numeric display.

In transmitter section we use one DTMF signal generator for generating a

signal in the form of PWM (pulse width modulation.) for this purpose we

use IC UM 91513 as a transmitter. This UM 91513 is a commonly DTMF

signal generator IC. All the nine switches are connected to the input of this

IC . All the switches are connected together as a matrix network. All the

switches are divided in four row and three column. We indicate the row as a

R-1, R-2, R-3, R-4 and column as a C-1, C-2, C-3 . When we press the

switch no 1 then Pin no 15 and pin no 12 is activate because pin no 15 is

connected to the R-1 and pin no 12 is connected to the C-1. Pin no 6 of this

IC is connected to the positive supply. This IC is very sensitive to voltage

and require only 3.3 voltage. For this purpose we use one zener diode

regulated power supply with this project. Resistance 150 ohm limit the

current and 3.3 volt zener regulate the input supply voltage. One crystal is

connected to pin no 3. 579545 MHz is connected to the pin no. 3 and 4. This

frequency is a carrier frequency of this input pins. Output signal is available

on the pin no. 7 of this IC. This output signal is directly connected to the

operational amplifier circuit. In this operational amplifier we use IC 741 as a

amplifier circuit. Input signal from the pin no 7 of this IC is connected to the

pin no 2 of the IC 741. Pin no 3 act as a comparator. Pin no 6 is output pin.

Pin no 7 is connected to the positive supply and pin no 4 is connected to the

negative supply. One resistor is connected between pin no 6 and pin no 2 to

give a positive feedback. Output from the operational amplifier is further

amplified by the two transistor. We use two transistor’s are here one is NPN

and second is PNP. collector of the npn is connected to the positive supply

and collector of the pnp is connected to the negative supply. Output is

available from the emitter of the both transistor. Output from the transistor is

further connected to the laser beam. Here we use Ga-As laser. In this laser

beam there is one semiconductor laser diode. Working voltage of this diode

is approx. 3.6 volt. This diode is very sensitive and on high voltage it is

immediately burnt out. Output from the amplifier is further again converted

into regulated 3.6 voltage with the help of one 3.6 volt zener diode. Now

when we press any switch from the keyboard then IC generate a frequency

and this frequency is amplified by the operational amplifier and this signal is

further transmit by the laser beam in the form of the light. In receiver circuit

We use one photodiode. Photodiode is very sensitive to light . When light

focus on the photodiode then photodiode sense the light and convert it into a

very small voltage. This small signal is amplified by the operational

amplifier and further connected to the DTMF decoder circuit. DTMF

decoder circuit receive signal from the operational amplifier and convert

this signal into a BCD signal. For the DTMF decoder circuit we use IV

8870/9170. This a 16 pin IC. Pin no 10 and 18 is connected to the positive

supply. One crystal is connected to the pin no 7 and 8 to demodulate the

frequency. Signal from the operational amplifier is applied to the pin no 2

and 3 of this IC. This IC receive the signal and demodulate the frequency

and this frequency is available on the pin no 11,12,13,14 in the form of

BCD signal. If we use this BCD signal in further then we can control many

electrical appliances through this circuit. But here we display the numeric

number only with the help of seven segment decoder circuit. Here we use IC

7447 as a seven segment decoder circuit. IC 7447 receive the BCD signal

from the 8870 and convert this signal in the numeric display. Now when we

press the numeric code from the transmitter end then transmitter transmit the

signal in the form of light. This light is receive at the other end in the form

of the laser light. This laser light is decode into signal and this signal is

amplified and converted in to numeric form with the help of decoder circuit.

CIRCUIT DIAGRAM OF THE OPTICAL FIBRE TRANSMITTER

CIRCUIT DIAGRAM OF THE OPTICAL FIBRE RECEIVER.

COMPONENT LIST

IC UM9121B

IC CM8870

IC 74154

IC 7805 Power Regulator

IC 741 Op. Amplifier

IC 7447 Seven Segment Display

Resistors

Capacitors

Relay

Diode

LED

Keypad

Laser Torch

Transformer

Speaker 0.5W

CIRCUIT DIAGRAM

Fig. Circuit Diagram for the Transmitter

Fig. Circuit Diagram for the Receiver

CIRCUIT DESCRIPTION

TRANSMITTER

P.C.B. LAYOUT

The entire circuit can be fabricated on a veroboard. However, actual-

size PCB layout for the circuits of figs is given in fig. The component layout

for the PCB of fig. is given in fig.

Shielded wire may be used for connection of microphole. Low profile

sockets may be used for ICs for easy replacement and fault finding.

Assembly of the components may be done in the following sequence: IC

sockets, resistors, capacitors, diodes, transistors, LEDS, etc., observing

correct polarity of components. While soldering, clean the flux using

isopropyl alcohol. Heat sink may be used for IC10 (7805).

Before putting the ICs in their respective sockets, it is better to

measure the resistance between their pins and ground point using

multimeter. It should not be very low.

Observe PCB for dry joints, solder splashes and bridges between

tracks. After that connect the power supply from transformer X1. Use good

quality transformer and relay of proper rating as required for the load.

Layout of desired circuit preparation is finest and most important

operation in any printing circuit be and manufacturing process. For of all

layout of component side is to be made in accordance with available

component dimension. The following point are observed while forming the

layout of PCB :

Between two component sufficient space should be maintained.

High voltage/max. dissipated components should be mounted at a

sufficient distance from semi-conductors and electrolytic capacitors.

The most important. Point is that the component layout is making proper

component. With copper side circuit layout. Printed Circuit Board (PCB)

are used to avoid most or all the disadvantages of conventional bread

board. This also avoid the use of thin wired for connecting the

components.

They are small in size sufficient in performance.

The two most popular boards are singles sided board and the double

sided/board. The single sides PCB are widely used.

For general purpose application where the cost it is to be low it the

layout is simple.

After etching the PCB in kept into clear water for about half-an hour.

In order to get PCB away from any acidic properties will be the cause of

poor performance of circuit. After the PCB has been thoroughly caused,

point is removed by soft price of cloth dipped in thinner or turbine. Then

PCB is checked as per the layout now the PCB is ready for uses.

SOLDERING

Soldering is the process of joining two metallic conductors the joint

where two metal conductors are to be joined or fused is heated with a device

called soldering iron and then as allow of tin and lead called solder is

applied which melts and converse the joint. The solder cools and solidifies

quickly to ensure is good and durable connection between the jointed metal

converting the joint solder also present oxidation.

How to Solder

Soldering is very important for assembling any electronic circuit. A

properly soldered joint or connection in electronic circuit is the important

steps to be followed circuit in good and concerned soldering.

1. Use of coned type of soldering iron and solder avoid the use of

excessive fault.

2. Keep the soldering iron hot during the working period and let it rest

on its stand when not in use.

3. All components leads and wires should be thoroughly cleared to

remove dust and rust before soldering.

4. Enough heat is applied to the joint so that the solder metal flows

freely over the joint.

5. Over heating of components in PCB is avoided, over heating may

result in damage to components on PCB.

BIBLIOGRAPHY

1. Basic Electronics : By V.K. Mehta

2. Electronics Projects : By K.A. Sakthidharan

3. Integrated Circuit : By K.R. Botkar

4. Switch And Protection : By B.V.S. Rao

5. Twisted Electronics Project : By M.D. Agrawal

6. Power Electronics : By S.M. Rai & M.S. Qureshi

7. World Wide Web

APPENDIX

1. IC 7805

2. IC MT8870

3. IC 74154

4. IC 7447

5. IC UA741

6. IC UM9121B

IC 7805 :

Three Terminal Positive Fixed Voltage Regulators

These voltage regulators are monolithic integrated circuits designed as

fixed voltage. These regulators employ internal current limiting, thermal

shutdown, and safe-area compensation. With adequate heat sinking they can

deliver output currents in excess of 1.0A. Although designed primarily as a

fixed voltage regulator, these devices can be used with external components

to obtain adjustable voltages and currents.

Output Current in Excess of 1.0A

No external components required

Internal thermal overload protection

Internal short circuit current limiting

Output transistor safe – area compensation

Output voltage offered in 2% and 4% tolerance

Available in surface mount D2PAK and standard 3-lead transistor

packages

Previous commercial temperature range has been extended to a

junction temperature range of –40°C to +125°C

DESCRIPTION

The 7805 series of three terminal positive regulators are available in the

TO-220/D-PAK package and with several fixed output voltages, making them

useful in a wide range of applications. Each type employs internal current limiting,

thermal shut down and safe operating area protection, making it essentially

indestructible. If adequate heat sinking is provided, they can deliver over 1A

output current. Although designed primarily as fixed voltage regulators, these

devices can be used with external components to obtain adjustable voltages and

currents.

Fig. Block Diagram of IC7805

Absolute Maximum Rating :Parameter Symbol Value Unit

Input Voltage (for VO = 5V to 18V) (for VO =24V)

VI

VI

3540

VV

Thermal Resistance, Junction to Cases (TO-220)

RJC 5 °C/W

Thermal Resistance, Junction to Air (TO-220)

RJC 65 °C/W

Operating Temp. Range TOPR 0 - +125 °CStorage Temp. Range TSTG -65 - +150 °C

Electrical Characteristics (TA = 25°C unless otherwise noted)Parameter Symbol Min. Type Max. Unit

Output Voltage TJ =+25°C VO 4.8 5.0 5.2 VLine Regulation (Note 1) VO =7V to 25V

Regline - 4.0 100 MV

Load Regulation (Note 1)IO = 5.0mA to 1.5A

Regload - 9 100 MV

Quiescent Current TJ =+25°C IQ - 5.0 8.0 mAQuiescent Current Change IO = 5.0mA to 1.0A

IQ - 0.03 0.5 mA

Output Voltage DriftIO = 5.0mA

VO/T - -0.8 - MV/°C

Output Noise Voltagef=10Hz to 100MHz, TA=+25°C

VN - 42 - V/VO

Ripple Rejectionf=120Hz, VO=8V to 18V

RR 62 73 - dB

Dropout VoltageIO = 1A, TA=+25°C

VDrop - 2 - V

Output Resistancef=1KHz

r0 - 15 - m

Short Circuit CurrentVI = 35V, TA=+25°C

ISC - 230 - MA

Peak CurrentTA=+25°C

IPK - 2.2 - A

NOTE : Load and line regulation are specified at constant junction temperature. Changes in VO due to heating effects must be taken into account separately. Pulse testing with low duty is used.

IC7447 BCD TO 7-SEGMENT DECODER/DRIVER :

The SN54/74LS47 are Low Power Schottky BCD to 7-Segment

Decoder/Drivers consisting of NAND gates, input buffers and seven AND-

OR-INVERT gates. They offer active LOW, high sink current outputs for

driving indicators directly. Seven NAND gates and one driver are connected

in pairs to make BCD data and its complement available to the seven

decoding AND-OR-INVERT gates. The remaining NAND gate and three

input buffers provide lamp test, blanking input / ripple-blanking output and

ripple-blanking input.

The circuits accept 4-bit binary-coded-decimal (BCD) and, depending

on the state of the auxiliary inputs, decodes this data to drive a 7-segment

display indicator. The relative positive-logic output levels, as well as

conditions required at the auxiliary inputs, are shown in the truth tables.

Output configurations of the SN54/ 74LS47 are designed to withstand the

relatively high voltages required for 7-segment indicators. These outputs

will withstand 15 V with a maximum reverse current of 250 mA. Indicator

segments requiring up to 24 mA of current may be driven directly from the

SN74LS47 high performance output transistors. Display patterns for BCD

input counts above nine are unique symbols to authenticate input conditions.

The SN54/74LS47 incorporates automatic leading and/or trailing-edge

zero-blanking control (RBI and RBO). Lamp test (LT) may be performed at

any time which the BI /RBO node is a HIGH level. This device also contains

an overriding blanking input (BI) which can be used to control the lamp

intensity by varying the frequency and duty cycle of the BI input signal or to

inhibit the outputs.

• Lamp Intensity Modulation Capability (BI/RBO)

• Open Collector Outputs

• Lamp Test Provision

• Leading/Trailing Zero Suppression

• Input Clamp Diodes Limit High-Speed Termination Effects

NOTES:

(A) BI/RBO is wire-AND logic serving as blanking Input (BI) and/or

ripple-blanking output (RBO). The blanking out (BI) must be open or

held at a HIGH level when output functions 0 through 15 are desired,

and ripple-blanking input (RBI) must be open or at a HIGH level if

blanking of a decimal 0 is not desired. X = input may be HIGH or

LOW.

(B) When a LOW level is applied to the blanking input (forced condition)

all segment outputs go to a LOW level regardless of the state of any

other input condition.

(C) When ripple-blanking input (RBI) and inputs A, B, C, and D are at

LOW level, with the lamp test input at HIGH level, all segment

outputs go to a HIGH level and the ripple-blanking output (RBO)

goes to a LOW level (response condition).

(D) When the blanking input/ripple-blanking output (BI/RBO) is open or

held at a HIGH level, and a LOW level is applied to lamp test input,

all segment outputs go to a LOW level.

IC 74154

Pin no. 16 is connected to the positive supply and pin no. 8 is connected to

the negative supply of the circuit. Output from the IC 4049 is next connected

to the D type flip flop circuit to switch on/off any electrical circuit. For this

purpose we use IC 4013. IC 4013 is a flip flop circuit and dual flip flop

circuit.

In dual flip-flop.

Pin no. 14 is the positive supply and pin no. 7 is the negative supply. Pin

no. 3 and pin no 11 is the input clock pulse of the IC 4013. Pin no 1 and pin

no. 13 is the output pin of q output, we never use an output from pin no 2

and pin no 12. When positive data is available on the pin no 1 then pin no 2

is become negative and this time this pin no 2 is connected to the data input

pin 5. Now data is available on the pin no 5 is negative and when again we

provide a clock pulse then negative data is available on the pin no and output

circuit is connected to the pin no 1 is on and off.

An Effective Photodiode ReceiverMost all light wave receiver designs I have seen in electronic hobbyist

books and Amateur Radio publications are truly pathetic. The best range I've

seen claimed is a couple thousand feet, more often just a few hundred or

even tens of feet. In my opinion anything under several miles does not merit

the time of a Radio Amateur worth his or her salt. Bear in mind that for the

ARRL VUCC laser award you have to do five two-way QSOs, one of those

all the way across a grid square - something like 60 miles or more! A well

designed receiver is essential. Photomultiplier receivers are well capable of

this performance, but I believe that a properly designed photodiode receiver

will also do. The photodiode receiver design I am presenting here works as

well or slightly better than the photomultiplier receivers I used to set the

current 57.7 mile He-Ne laser DX record. This highly optimized design

when bench tested side-by-side with one of my PMT receivers worked

better. (See figure 2.)

Pd, the photodiode, is a EG&G Vactec VTP1188S. It is available as

stock number 95F9029 from Newark Electronics for $2.36 (good

photodetectors don't have to be expensive!) Its plastic package also serves as

a lens. The package is about 0.3" in diameter and 0.3" tall. The typical

sensitivity is 0.55 A/W (Amps output for Watts of light input). This does not

mean that the device will put out 0.55 Amps under a 1 Watt light bulb

photodiodes don't generate that much current. 0.55 micro Amps for

1 microwatt is more realistic. The 0.55 A/W figure is typical for any silicon

photodiode. The sensitivity of silicon photodiodes does not vary much from

some standard figures due to the laws of physics. The sensitive area of the

actual VTP1188S photodiode chip is 11mm2. Much smaller area

photodiodes are available and result in higher speeds due to less capacitance.

But the smaller the active area size is, the more critical the optics are. You

have to get as much signal to efficiently illuminate the photodiode as

possible, which would be more difficult if the chip is very tiny. Much larger

area photodiodes are also available. They also offer us no advantage. They

cost a lot more, don't put out any more Amps/Watt, and generate more noise.

The VTP1188S is by no means the only photodiode that would work for our

application, but it is an excellent performer, readily available, and

inexpensive.

Rl, the load resistor, serves mainly to give capacitor Cc a discharge path to

ground. Its value is not critical, but best performance was found at 50 ,

plus or minus 10 M. This is a much higher resistance than most Amateurs,

or even electronic professionals are used to dealing with. You will not find a

resistor value above 10 M from any of the normal parts sources, such as

Newark or Digi-Key. Specialty resistor manufacturers such as Victoreen

make the high resistance, low capacitance resistors commonly used in

photodiode transimpedance amplifiers. Values are available in the G

Unfortunately, these resistors are expensive ($5 to $15) and high minimum

orders ($100) are required. Fortunately, the circuit shown here is not too

critical and a string of 10 M resistors will work just fine. Cheap 10 M

resistors are available from Digi-Key as their part number 10ME-ND, five

for 28 cents, or 100 for $4.60. Strings of these resistors should be used for Rl

and Rf. I have tested this circuit with both Victoreen resistors and strings of

the Digi-Key resistors and saw no difference in the resulting signal-to-noise

ratio.

Cc, the coupling capacitor, serves to let AC signals from the

photodiode through to the amplifier while blocking DC signals. This is a

very important feature of our circuit. Capacitive coupling is very unusual for

transimpedance photodiode amplifiers - they are normally DC coupled.

After working for over 2 1/2 years for one of the top photodiode

manufacturers I had never seen this done! If you don't capacitively couple,

the DC signals generated by sunlight, moonlight, incandescent light, etc.,

will totally overload the amplifier. The value of Cc is not at all critical.

0.1F works fine, but 0.01 F and 1.0 F work fine also. It should be a

reasonably good capacitor, that is, not leaky.

Rf, the feedback resistance, is another high value resistor. 50 M in the form

of a string of 10 M resistors works fine. Again, this value is not critical.

This is a very important part of a transimpedance amplifier. The output

voltage of the amplifier is determined by multiplying the input current times

the value of this feedback resistor. This amplifier will put out 0.5 Volts (a

very large signal) with an input of only 10 nanoamps (10-8 Amp)! The

photodiode will generate 10 nanoamps with a light signal of only 18

nanowatts (0.000000018 Watt)! The "power" of a transimpedance amplifier

is incredible.

Cf is a very small capacitor, in the order of 1 to 10 pF. Without it a

phenomenon known as "gain peaking" will occur. At some relatively high

frequency the gain of the amplifier will peak above what it is supposed to be

as determined by the feedback resistor. Cf also is used to reduce the normal

amplifier gain at high frequencies. Gain outside of the desired bandwidth,

which is 300 to 3000 Hertz, the standard communications audio bandwidth,

just contributes to unwanted noise. This capacitor can be adjusted for the

best sounding signal when receiving a very weak signal. A better way to

adjust it is to connect a sensitive AC voltmeter to the output of the amplifier.

Make a signal source by powering a LED from a audio signal generator set

to 1000 or 2000 Hertz. Shine the signal into the photodiode and adjust the

physical arrangement and the signal generator output voltage to get a very

weak signal (a few mV). Turn the signal generator on and off while

adjusting the capacitor for the largest ratio of signal-plus-noise (signal

generator on) to noise (signal generator off.)

IC 1 is an op amp. The "input noise current" rating of this op amp must be

very low for this kind of circuit. Fortunately, there are a number of good op

amps that aren't too expensive. I began by testing nine op amps ranging in

price from $1 to $20. They included the Burr-Brown OPA627 and OPA111 -

premier op amps for photodiode transimpedance amplifiers, the PMI OP07,

the Analog Devices AD743, Linear Technology's LT1028, LT1037, and

LT1055, the new National LMC6001, and even the lowly 741. The input

noise current for some of the better op amps tested was rated under one

femtoamp, that's 0.000000000000001 Amp! This rating is important as the

current from the photodiode is going to be amplified 50 million times by our

circuit. In some tests op amp ‘A’ did better than op amp ‘B’, in other tests

just the opposite. A transimpedance amplifier circuit is deceptively simple,

there actually are dozens of nuances and a lot of interplay. The testing was

done by measuring the signal- plus-noise to noise ratio in a 300 to 3000

Hertz bandwidth while receiving a weak signal from a 1 KHz square wave

modulated red LED. The results in a nutshell: the OP07, OPA111, OPA627,

AD743, LT1055, and LMC6001 all performed well. The losers were the

LT1028 and LT1037 - presumably their extremely high bandwidths

contributed to the noise. The 741 did surprisingly well, but there are much

better choices for just a few dollars more. I have settled on the AD743JN

because of its excellent performance and its price and availability ($5.53

from Newark). The Burr-Brown parts are top performers but are very hard to

get and are expensive. An excellent alternate to the AD743JN would be the

LT1055CN8 at $3.04 from Digi-Key.

When designing a circuit for absolute lowest noise even the op amp

supply voltage needs to be taken into consideration. The high quality op

amps we are dealing with are normally spaced by the manufacturer for

operation at +/- 15 Volts. Voltage causes current, current causes heating,

heat causes noise. I measured lower noise at +/- 9 Volts, which is also

convenient for battery operation.

The above covers all of the critical components in the laser receiver

with the exception of circuit layout and shielding. Those used to dealing

with RF and microwaves may think they have seen it all, and that there can't

be much to a circuit that just amplifies audio. Nothing could be farther from

the truth when you are dealing with gains in the tens of millions and currents

in the picoamps! It is important to use a printed circuit board with plenty of

ground plane. Keep the input well isolated from the output. Keep leads short.

Keep the board and components clean and de-fluxed else pA will leak across

the insulation. This much gain at audio frequencies will result in 60 Hertz

pickup, so shielding is needed. I built the transimpedance amp alone on a

4" diameter round circuit board with the photodiode in the center. On the

back of the circuit board I soldered a 3.4" diameter Coffee-mate(r) jar steel

lid. I did the same on the front of the circuit board using a lid with a 0.5"

hole in the center for the photodiode to look through. The finished round

board fits into a 4" diameter plastic pipe along with a 4" diameter lens. The

filter and speaker amp are on a separate round circuit board.

The filter is a TOKO THB111A 300-3000 Hertz active bandpass

filter. It is available from Digi-Key for $21.39 (their part number TK5425-

ND). It is 30 dB down at 100 Hertz and 7.9 KHz. Its main purpose is to

attenuate 60 and 120 Hz optical interference street lights, room lights, etc. It

also attenuates non-optical pickup of 60 Hz electric fields. And it also limits

the upper bandwidth to reduce noise, which would be heard as a hiss in the

speaker.

IC2 is the renowned National LM386 audio power amplifier. The

capacitor across pins 1 and 8 sets the chip to its maximum gain of 200. This

chip is available in three versions. The LM386-1 is optimized for 6 Volt

operation at which it will put out 325 mW of audio power into an 8 ohm

speaker. The LM386-3 is optimized for 9 Volt operation at which it will put

out 700 mW. The LM386-4 is for 16 Volt supplies and will drive a 32 ohm

speaker to 1 Watt. The -1 and -3 versions can be run as high as 15 Volts.

Only the -4 requires the 0.05 uF capacitor and 10 ohm resistor on pin 5.

These components can be left off circuits using the -1 and -3 chips. I

strongly recommend not running the LM386 off of the same 9 Volt battery

used to power the transimpedance amp. The LM386 draws significant

current which may disturb the incredibly sensitive photodiode amplifier. It

would be ideal to use a LM386-3 and power it from a separate 12 Volt

NiCad or Gel Cell. Alternatively, a separate 9V alkaline transistor radio

battery would do.

The Edmund Scientific folding stand magnifier lens is ideal for the

receiver antenna (their model G38,599 at $9.50). The 4.3" diameter glass

lens with a 8.5" focal length can be popped out of the plastic stand. Its

diameter is perfect for mounting inside of a length of 4" ABS "DWV" pipe.

This pipe can be purchased from a hardware store. It is black ABS plastic

with a 4" inside diameter and a 4.5" outside diameter. The lens can be

pinned between the end of the pipe and a plastic pipe coupling, also

available from the hardware store. The same mounting method is used for

the detector/transimpedance amp circuit board and the filter/speaker amp

circuit board. The speaker and on-off switch can be mounted on a matching

pipe cap. The entire laser receiver, including batteries, will fit in a neat and

inexpensive 4.5" diameter by 2 foot long housing. A second pipe cap can be

used to cover the lens end of the receiver during storage and transport.

A way to fasten the round pipe to a tripod is needed. The receiver can

be mounted to a metal plate with a 1/4-20 threaded hole in it. 1/4-20 is the

thread on a camera tripod screw. "Minerallac Straps" can be used to mount

the pipe to the metal plate. Minerallac straps are "U" shaped metal clamps

that snap around pipe and allow it to be fastened down by a single bolt.

Electrical supply houses or very well stocked building supply stores will

have these. Two straps can be used to fasten the receiver to the metal plate.

 

ATMOSPHERIC LIGHT PROPAGATION

Obviously, the atmosphere is a good medium for transmitting visible light.

Atmospheric transmittance can be considered excellent from 500

nanometers to about 950 nanometers. Meteorologically speaking, on an

"exceptionally clear" day, visibility is considered to be 50 to 150 kilometers.

It is not quite as simple to predict just how far you can communicate on a

beam of light. There is a formula though:

Range = meters.

Po = The power output of the laser in watts.

Ar = The area of the receiver lens or mirror in square meters.

To = The transmissivity of the receiver optics.

Ta = The transmissivity of the atmosphere.

Pt = The threshold power of the receiver system in watts.

D = The laser beam divergence in radians.

Lets try some practical figures. Po = 0.002 watt (2 mW), typical for a small HeNe or Diode pen pointer laser.

Ar = 0.008 meter (4 inch diameter lens).

To = 0.84 (84%), no filters, two typical 8% reflections off of the lens surfaces.

Ta = 0.9 (considered a clear day).

Pt = 5 x 10-12. Approximate Noise Equivalent Power of the EG&G Vactec

            VTP1188S photodiode in a 300-3000 Hz bandwidth.

D = 0.0015, (1.5 milliradians) for a typical Helium Neon laser or Diode laser pen pointer.

Range = 1,017 kilometers, or 633 miles!

Is this figure realistic? Not taken into account is the noise that the receiver

circuitry adds to the signal. If we make Pt ten times worse we get a range of

328 kilometers, or 204 miles. Probably more realistic, but still fantastic.

Atmospheric distortion and the problem of finding two points this far apart

and still line-of-sight would likely be the real limiting factors.

Note the importance in this range formula of keeping the laser beam

divergence small. Halving the divergence doubles the range. Small

divergence is a quality to look for in a laser. Note too the importance of

antenna area. A 2" diameter lens gives half the range than a 4" diameter lens.

 

OPERATING EXPERIENCES

The "Early Days"

I was a member of the Los Padres Microwave Group in the late 70's and

early 80's. This was a West Coast VHF/UHF/Microwave contest group

founded by W6OAL, WB9KMO, and myself. My job was to provide and

operate the microwave-and-higher contest gear. After two years of one-way

testing we completed a two way HeNe laser QSO with the K6MEP contest

group in the June 1979 ARRL VHF QSO Party. We were located on Frasier

Mountain, Ventura County, Santa Barbara section. The K6MEP group was

on Reyes Peak, 15 miles away in the same county and section. The

equipment at K6MEP was a 3 mW Spectra Physics He-Ne laser owned by

W6OAL. I modified the laser by adding a chopper modulator and a tripod

mount. I built the receiver around a 931A PMT with a 3" lens. The gear the

Los Padres group used was a 4 mW chopper modulated He-Ne laser I built

around a surplus plasma tube. The receiver was a 931A PMT with a 10 inch

Fresnel lens. The choppers were made from small DC motors and tin can

lids. The chop rate provided 1 KHz modulation. The resulting audio

modulated beams were Morse code CW modulated by interrupting by hand.

This proved to be one of the more difficult aspects of the contact as the hand

interruption had to be made in inverse Morse code. It took a little getting

used to! By far the most difficult problem was aiming the lasers. First,

finding the other party is difficult at 15 miles. Second, keeping the laser

aimed is equally hard. With a one milliradian beam divergence, discounting

any path distortion, the beam at 15 miles away would be 79 feet in diameter.

Moving the laser by just one degree would move the 79 foot diameter beam

at the far end by a whopping 1382 feet! Contact was established by slowly

sweeping the lasers back and forth across the suspected target point. Two

meter liaison was essential. The person at the receiving end would signal on

2M when he saw a flash of red light. This was not sufficient to stop the

beam in time but it did give the people at the transmitter end a good idea of

where to sweep at a slower rate. Finally the beam would be somewhat steady

at the receiving end. At night it was very bright red. Tests during the day

were also successful, but the beam did not appear as bright and looked

somewhat pink due to the competing sunlight. All equipment was mounted

on standard SLR camera-type tripods. Heavier tripods, such as used for

video cameras, would be highly desirable. Laser two way contacts were

achieved by the Los Padres Microwave group with the same equipment a

half dozen times since the 1979 contact, although it wasn't until the June

1982 ARRL VHF contest that the ARRL would grant us contest credit.

 

Recent Experiences and Techniques

Since the original late 70's - early 80's shots, I have built several generations

of equipment. Some important lessons learned were incorporated into

improved equipment:

1) The support for the laser must be extremely stable. I've abandoned

camera tripods and made my own heavy tripods.

Here's the problem... If you are using a laser with a typical 1.5

milliradian beam divergence, then the beam will spread out at a rate of

1.5 feet for every thousand feet of travel. Therefore, 50 miles away

the beam is about 400 feet in diameter. If the laser or tripod twists by

only 0.1 degree, the beam at the receiving end will move 460 feet -

totally missing the target! If the laser is mounted on a baseplate a foot

long, moving one end of that plate only 0.0017" will have the same

effect. Camera tripods aren't up to this kind of stability.

I have made tripods with 3/4-inch steel pipe legs. The legs are

spread out at a wider angle than a camera tripod to give more stability.

The legs converge at a heavy aluminum plate, 1-foot square by 5/8-

inch thick. The legs screw into 3/4- inch pipe threaded holes in the

plate. This works.

2) There must be some very fine adjustment screws for precise aiming of

the laser. A tripod pan-and-tilt head is out of the question. (The

effective beamwidth of the laser receiver is much wider than that of

the laser so a regular camera tripod would suffice for it.)

In early Helium Neon laser transmitters I mounted one end of

the laser on what was effectively a swivel. The other end had a 10-32

screw for elevation adjustment and another for azimuth adjustment.

With the arrangement I had, one turn of a 32 thread-per-inch screw

would move the beam 2.6 beam diameters at the receive end. This is

quite acceptable. In later lasers I used micrometers. These usually run

at 40 threads-per-inch, not tremendously finer than a plain screw, but

they are very well made and turn very smoothly. Used micrometers

show up a swap meets and garage sales, but even new economy

models aren't too expensive.

3) Aligning the laser, or, "initial signal acquisition", is the single biggest

problem. An extremely bright light, like a handheld spotlight, is

needed. Better yet - a Xenon strobe light. This is used in conjunction

with radio liaison and rifle scopes mounted on the laser.

You can't understand how hard it is to get the laser lined up initially so the

other end can see it. I have done most of my shots to a mountain, like Mount

Pinos, probably the tallest in Southern California (over 8800'). You would

figure that if you go out a few dozen miles from that mountain and look at it

that you would see this nice prominent peak to point at. No way. All you see

is a hazy outline of ridges, none more prominent than the other.

A compass helps, but it doesn't get you close enough. You can try to go by

landmarks. On one memorable laser shot we were convinced the city lights

we were seeing were from Bakersfield, near where we expected to see the

laser coming from. But the lights were really from Lancaster, 60-degrees off

target! How could one be so far off? When standing on a mountain, in the

dark, directions become confused. This is where you need to use and trust a

compass.

Some suggest using surveying equipment and sighting off of the North Star.

Great if you can afford it. Such gear probably costs more than the lasers and

receivers.

What works is first pointing the lasers as well as you can by compass

bearings. Now you should know where too look within 10-degrees. The

other end shines headlights or a handheld spotlight in your direction. This

will work for a few dozen miles. For longer distances you need a Xenon

strobe light. I mounted a strobe in an 18-inch diameter spun aluminum

parabolic reflector to make a killer light beacon.

Once some light from the other end is seen you can center the cross-hairs of

the rifle scope mounted on the laser on it. You need to have the rifle scope

mounted firmly to the laser and aligned to the beam. Now the laser can be

turned on. It's very unlikely that the laser will be seen yet at the other end.

This is where the real work begins. You move the laser back and forth, up

and down, very slightly with the fine adjustment screws. Eventually the

other end will see a flash of light as the beam sweeps by them. At that point

they yell into the liaison radio to stop. Again, it's unlikely that they will see

the light, but you will be very close. A little more fine adjustment will hit the

target.