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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 surges!



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 phone.







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 sight.





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 circuit.

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 circuit.






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 and


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.




By 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.


We 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.



IN 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 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 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 driver circuit.


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.





































3.      DESOLDERING WICK- 1 Meter

4.      BREAD BOARD- 1


6.      IC 741(2)

7.      L.E.DíS-1



10.  SPEAKER( 8 OHM)-2

11.  RESISTANCES(10 K,4.7K,220K,110K,1K)

12.  P.C.B


14.  LASER

15.  SUPPLY- 9V


17.  IC 91214

18.  IC 8870

19.  IC 74154

20.  ZENER 3.3V

21.  IC 7805

22.  DIODES(4007)