ECEstream: Communication Circuits

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The transmitter board and the receiver board, both powered by a separate 9V battery or a fixed voltage power supply, depending on your needs. The transmitter board has an electret microphone module at one end, and the laser diode at the other end. The electronics modulates the intensity of the laser beam according to the output of the microphone. The laser diode has an inbuilt collimating lens, and is simply a module that connects to the transmitter board.</div>
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The receiver uses a photodiode as the receiving element, and the onboard amplifier powers a small 4-36 ohm speaker. This board is therefore a high gain amplifier with a basic audio output stage.</div>
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<b><span style="color: #3d85c6;">Transmitter</span></b></div>
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<img alt="LASER Transmitter Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjVfc_T2Z6cbcllP1YFfmrnEinj8D0FObdOuFECN877MWC-2Asmmb35xdVGLMEnD0MdA4OweU0usSy3cQ8nWdW4zP6fh3oiYfPyLxH02fDf2sx0MplOFcw3Ez3s89axL6BaEIFGqIFBQMCE/s1600/Laser+transmitter.gif" title="LASER Transmitter Circuit" /></div>
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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!</div>
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However, if used within specifications, the typical life of one of these lasers is around 20,000 hours. In the transmitter circuit (Fig.1) the laser diode is supplied via an adjustable constant-current source. Since the lasing threshold also varies with temperature, a 68ohm NTC thermistor is included to compensate for changes in ambient temperature. Note that the metal housing for the laser diode and the lens also acts as a heatsink. The laser diode should not be powered without the metal housing in place. The quiescent laser diode current is controlled by Q2, in turn driven by the buffer stage of 1C2b. The DC voltage as set by VR2 appears at the base of Q2, which determines the current through the transistor and therefore the laser diode. Increasing the voltage at VR1 reduces the laser current. The setting of VR1 determines the quiescent brightness of the laser beam, and therefore the overall sensitivity of the system.</div>
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The audio modulation voltage is applied to the cathode of the laser diode, which varies the laser current around its set point by around +/-3mA. The modu- lation voltage is from the emitter of Q 1, which is an emitter follower stage driven by the audio amplifier stage of 1C2a. Diodes D4 to D7 limit the modulating voltage to +/-2V, while C4 and C5 block the DC voltages at the emitter of Q 1 and the cathode of the laser diode. The audio signal is coupled to the laser diode via R10, which limits the maximum possible variation in the laser diode current to a few milliamps.</div>
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The audio modulation voltage is applied to the cathode of the laser diode, which varies the laser current around its set point by around +/-3mA. The modu- lation voltage is from the emitter of Q 1, which is an emitter follower stage driven by the audio amplifier stage of 1C2a. Diodes D4 to D7 limit the modulating voltage to +/-2V, while C4 and C5 block the DC voltages at the emitter of Q 1 and the cathode of the laser diode. The audio signal is coupled to the laser diode via R10, which limits the maximum possible variation in the laser diode current to a few milliamps.</div>
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The inverting amplifier of 1C2a includes a form of compression, in which the output level is relatively constant and independent of how soft or loud the audio level is at the microphone. This is achieved by FET Q3 and its associated circuitry.</div>
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The cascaded voltage doubler of C9, D8, D9 and C8 rectifies the audio signal at the emitter of Ql, and the resulting negative DC voltage is fed to the gate of Q3. An increase in the audio signal will increase the negative bias to Q3, increasing its drain-source resistance. Because the gain of 1C2a is determined by R7 and the series resistance of R5 and Q3, increasing the effective resistance of Q3 will lower the gain.</div>
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Since the compression circuit takes time to respond, the clamping network of D4-D7 is still needed to protect against sudden voltage increases. This system is rather similar to the compression used in portable tape recorders.</div>
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he electret microphone is powered through R1 and is coupled to the non inverting input of 1C2a via C6. This input is held at a fixed DC voltage to give a DC output to bias Ql.</div>
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The supply voltage to the transmitter circuit is regulated by ICI, a 5V three terminal regulator.</div>
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<b><span style="color: #3d85c6;">Receiver</span></b></div>
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<img alt="LASER Receiver Circuit" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjN3mQ0JvN-am3fK_YEzY8KlAcvRzpKeYSjkKXigaa0hmMhOHxBEFZv54fnqFdMrD8ZVTa8G9MgBFmzOvcmXD__tnm6s7Pd-dd0U5gU-bjjb2zKxJnf_J4ndguXDqtvNwSEbKo2YuefkvTX/s1600/Laser+receiver.gif" title="LASER Receiver Circuit" /></div>
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The transmitted signal is picked up by the photo detector diode in the receiver (shown in Fig.2). The output voltage of this diode is amplified by the common emitter amplifier around Ql. This amplifier has a gain of 20 or so, and connects via VRI to ICI, an LM386 basic power amplifier IC with a gain internally set to 20. </div>
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This IC can drive a speaker with a resistance as low as four ohms, and 35OmW when the circuit is powered from a 9V supply. Increasing the sup- ply voltage will increase the output power marginally. </div>
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The voltage to the transistor amplifier stage is regulated by ZD I to 5.6V, and decoupled from the main supply by R2 and C2. Resistor R3 supplies forward current for the photodiode. (Incidentally, the photodiode used for this project has a special clear package, so it responds to visible light, and not just infrared.)</div>
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LASER Link Communicator Circuit

The transmitter board and the receiver board, both powered by a separate 9V battery or a fixed voltage power supply, depending on your nee...

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This is a simple Laser communication system. It can transmit and receive signal from any audio device.Communication distance is few meters. All components are not critical. Transistor 2N2222 may be on the cool rib. Laser diode is from laser pointer.</div>
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<img alt="Laser Diode Transmitter" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh4azVnd2mmaLr57OpRMJCjwyRolDffqMB9fIxZstgex9FPMoJjjUZQ3Y6FOhYjv1O1DCdmX5k3GX7g6OYEW0bObjwf2nYLHgPovyQXbdoGes8x_AmECqD1s5EQFgf0da1q58piavRx7IcG/s1600/Laser+transmitter+System.gif" title="Laser Diode Transmitter" /></div>
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<img alt="Laser Diode Receiver" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjt6Hl_xCDo2ED_Ge41Y5LVGk2A188wePw-2KULCRG_i00YG-Qbo4PC3yjRmVHv3WciR_CcOdv9vzO1aLwLNlkF32IGrSH0c3kH_k0LoJcITOzsTVdeOq-_vBro5iYGiwS_VRXVsCiHQ8w3/s1600/Laser+Communication+System.gif" title="Laser Diode Receiver" /></div>
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Laser Communication System

This is a simple Laser communication system. It can transmit and receive signal from any audio device.Communication distance is few meters...

ECEstream

LASER Link Communicator Circuit

Laser Communication System

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