DIGITAL LASER DATA TRANSMISSION
There's something rather futuristic about talking 'over' a laser
beam, which is what this inexpensive project allows. It will easily
give a communication distance of several hundred metres, and with a
parabolic light reflector, up to several kilometres. It transmits
high quality audio and the link is virtually impossible for anyone
else to tap into.
So clearly, this project is ideal for setting up a speech channel
between two areas, say adjacent houses, or offices on opposite sides
of the street. Or you could use it as a link between the work shop
and the house. For duplex (two way) communication, you'll obviously
need two laser 'channels'.
An important feature of transmission by laser beam is privacy.
Because a laser beam is intentionally narrow, it's virtually
impossible for someone to tap into the link without you knowing. If
someone intercepts the beam, the link is broken, signalling the
interception. Fibre-optic cables also have high security, as it's
very difficult to splice into the cable without breaking the link.
However it's theoretically possible; so for the highest security,
you probably can't beat a line-of-sight laser beam.
You can also use an infrared laser. While this gives even better
security, as you can't see the laser beam without special IR
sensitive equipment, it also makes alignment more difficult.
Main component of this project is laser diode. A laser diode needs
a certain value of current, called the threshold current, before it
emits laser light. A further increase in this current produces a
greater light output. The relationship between output power and
current in a laser diode is very linear, once the current is above
the threshold, giving a low distortion when the beam is amplitude
modulated. For example, the 65Onm 5mW laser diode used in this
project has a typical threshold current of 3OmA and produces its
full output when the current is raised by approximately 1OmA above
the threshold to 4OmA. Further increasing the current will greatly
reduce the life of the laser diode, and exceeding the absolute
maximum of 8OmA will destroy it instantly. Laser diodes are very
fragile and will not survive electrostatic discharges and momentary
In this project we transmit two type of data from the circuit. One
is analogue data and second is dtmf data . In the analogue data we
transfer the data from the mike and this sound is transmit through
the laser and then receive on the speaker at the other side.
In dtmf mode we use one 12 key as a input keyboard in the circuit
and when we press any number then this data is available on the
other end and display on the seven segment display.
In addition of these two signal we also applied a signal from the
mobile phone and we follow the same process of transfer the data in
the from of sound and numeric data from the keypad of the mobile
CIRCUIT DIAGRAM OF THE TRANSMITTER
IN this circuit we use two circuit one is dtmf generator and second
is operational amplifier. In the dtmf generator circuit we use ic um
91214 as a dtmf generator. Working voltage of this ic is 3.3 volt
dc. So that we use one 3.3 volt zener diode as a regulator and
provide a regulated power supply to this circuit. Output signal is
available on the pin no 7 . this output signal is coupled to the
input of the amplifier through selector switch. One 3.58 mhtz
crystal is connected to the pin no 3 and 4 to give a carrier
frequency to the circuit. IN this mode we use 3.58 mhtz crystal as
a main carrier source of the dtmf generator.
All the switches are connected to the input of the dtmf generator to
provide a multiple signals. All the switches are connected in four
rows and three coloum. When we press any key then one row and one
coloum is activate automatically.
Data from the dtmgf generator is further connected to the pin no 2
of the ic 741 through capacitor .04 micro farad. Here capacitor
block the dc voltage and pass only signal to the amplifier circuit.
Pin no 6 is the output pin no of the ic 741. Pin no 3 is connected
to zero voltage through voltage divider circuit. Here we use two 10
k ohm resistor as a voltage divider components. Two 10 k ohm
resistor provide a zero reference voltage to the pin no 3 of the ic
741. Output of the ic 741 is further amplify by the two transistor
circuit. Here we use one is npn and second is pnp transistor .
Collector of the npn transistor is connected to the positive voltage
and collector of the pnp transistor is connected to the negative
voltage. Output of the transistor is available on the emitter point
and this output voltage is connected to the laser diode through one
current limiting resistor and one zener diode.
Analogue signal from condenser mike is also feeded to the input of
the op-amplifier using selector switch. Condenser mike convert the
sound signal into electrical singal and this signal is coupled to
the pin no 2 of the ic 741 through .04 microfarad capacitor.
Resistor 10 k ohm provide a dc voltage to the condenser mike.
Gain of the amplifier or ap-amp is depend on the feedback connec
mmmmted to the circuit. In this project we use one 10 k ohm resistor
in series with the 100 k ohm variable resistor. With the help of
this variable resistor we control the gain of the op-amplifier.
Now whatever we speak or press the switch this data is
superimposed on the light of the laser. Laser respond this signal
and then this signal is focus on the photodiode in the line of
In the receiver circuit we
use one photodiode as a main sensor. Photodiode receive the data in
the light and then converted into electrical signal. This electrical
signal is coupled to the pin no 2 of the ic 741 amplifier through
.04 microcfarad capacitor. Pin no 3 is connected to the zero
voltage. For zero voltage we connect two resistance one from the
positive line and second from the negative line to the pin no 3 for
a zero reference voltage. Output is available on the pin no 6 .
Output signal which is available on the output pin is further
amplify by the transistor circuit and then this signal is . Now
signal is connected to speaker through capacitor in series
Now when we apply a dtmf
data from the keyboard then this data is again receive on the
photodiode and then again pass through the op-amp circuit and then
decode by the dtmf decoder ic. Here we use 8870 as a dtmf decoder
Output of the ic 8870 is a bcd output. Pin no 10 and 18 is connected
to the positive supply. Pin ni 5,6,9 is connected to the ground pin.
Input data is available on the pin no 2 and 3. One crystal is
connected to the pin no 7 and 8 to demodulate the carrier
frequency. Output signal available is bcd output. This BCD signal is
further decoded by the seven segment decoder and seven segment
decoder display the bcd code into seven segment display. Pin no 15
od the 8870 is ack pin output of bcd signal. When signal is
available on this pin then it means that ic 8870 decode the signal
Working voltage of the circuit is 9 volt dc. Op-amp is working on
the 9 volt dc and 8870 and 7447 is working on the 5 volt Dc. We
use one regulator circuit with 9 volt power supply to reduce the
voltage from 9 volt dc to 5 volt dc.
LASER DATA COMMUNICATION AND CONTROL SYSTEM.
using this portion with laser commnication, we can control different
electrical or electronics equipments with the help of the control
signal. In communication circuit we use one dtmf decoder circuit. In
that dtmf decoder circuit we use ic 8870 as a dtmf decoder circuit
and one ic 7447 as a bcd to seven segment decoder.
use one another dtmf decoder circuit, ic 8870 decoder and sense the
output in the form of the bcd signal from the dtmf decoder circuit.
this block diagram we show that how we control the many electrical
appliances with the help of the control circuit.
Sognal from the op-amp is connected in parallel with the display
circuit to the control circuit. Output from the mike amplifier is
connected to the 8870 decoder circuit.
Input signal from the op-amp is connected pin no 2 and 3 of the of
the 8870 decoder and output is available on the pin no 11,12,13,14
of the ic 8870 . Flash signal is available on the pin no 15 of the
ic 8870 and this signal is further switch on the npn transistor.
Collector of the npn transistor is connected to the pin no 18 and 19
of the ic 74154. One resistance and capacitor is connected to the
pin no 17 and 18 of the ic 8870 is reset the ic 8870. Output of the
8870 is connected to the pin no 22,23,21,20 of the ic 74154. Pin
no 12 of the ic 74154 is connected to the negative of the circuit
and pin no 24 of the ic is connected to the positive supply of the
circuit.. Pin no 18 and 19 of the ic is enable control pin. It
means that when we apply a negative voltage to this pin then ic
provide a output in decimal form. IC 74154 is basically a bcd to
decimal decoder. Output from the decoder is connected is in
negative form and we use one inverter circuit to convert this
negative output to positive output.. Here we use two ic 4049 as a
inverter ic . IC 4049 contains total six inverter inside . Output
available on the output of the ic 74154 is total 10 output, so we
use total two ic to convert 10 outputs from the decoder.
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
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 a 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 availbel on the pin no 1 and
output circuit is connected to the pin no 1 is on and off.
Output available on the output is digital output, so to interface
this output to the electrical circuit we require a triac or relay
For this purpose we use one l.e.d and transistor circuit in series
to the output of the flip flop. When output is on then flip flop is
on and then relay connected to the circuit is automatically and
pass the 220 volt to the load circuit and due to this electrical
circuit is on.
CIRCUIT AND THEORY ARRANGEMENT
CHECKING AVAILABILITY OF COMPONENTS
COMPONENT INSERTION AND SOLDERING
REWORK OR TROOUBLE SHOOTING
MATERIAL REQUREMENT :-
SOLDERING WIRE FLUX-1
DESOLDERING WICK- 1 Meter
BREAD BOARD- 1
CONNECTING WIRES- ACC. TO
SPEAKER( 8 OHM)-2