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group3:group3final 2008/12/08 21:04 group3:group3final 2008/12/09 01:15 current
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=== Sending Structure === === Sending Structure ===
-The laser's wavelength was decided early on. First, red lasers are very common, so, for the sake of the budget, the cheaper solution would be favored. At the same time, any infrared sources would be very difficult to align over long distances. Finally, most photodiodes and phototransistors are designed to mimic the human eye, which has a spectrum across all visible light, but focused around red.+The laser's wavelength was decided early on. First, red lasers are very common, so, for the sake of the budget, the cheaper solution would be favored. At the same time, any infrared sources would be very difficult to align over long distances. Finally, most low-cost photodiodes and phototransistors, while designed to operate in the near-infrared around 800-900 nm, also have a fairly strong sensitivity to the 650 nm red light produced by low-cost diode lasers.
-The method of mounting a laser in an adjustable manner was often overlooked in the project. From the development of the project in the lab, this was generally accomplished with a clamp, which provided some rotational movement for the laser, and books for the clamp to sit on if height were needed. The first actual solution for the project was the miniature tripod that was used in the final design.+The method of mounting a laser in an adjustable manner was often overlooked in the project. During the development of the project in the lab, this was generally accomplished with a clamp, which provided some rotational movement for the laser, and books for the clamp to sit on if height were needed. The first actual solution for the project was the miniature tripod that was used in the final design.
=== PC Communication === === PC Communication ===
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The send and receive circuits were initially going to communicate with each computer by use of a programmable microcontroller. This would allow for faster transfers, since the microcontroller could partially interpret the incoming data before passing it to the computer. However, the microcontroller did not end up being the speed bottleneck of the design, which was slowed mostly by the phototransistor's rise time. Also, since the microcontroller would theoretically speed up the process by interpreting the laser modulated signal as characters before handing it to the receiving computer, it would not be useful for pictures, where speed was important. In the end, the simplest idea was a direct link to the RS-232 port from the send and receive circuits. The send and receive circuits were initially going to communicate with each computer by use of a programmable microcontroller. This would allow for faster transfers, since the microcontroller could partially interpret the incoming data before passing it to the computer. However, the microcontroller did not end up being the speed bottleneck of the design, which was slowed mostly by the phototransistor's rise time. Also, since the microcontroller would theoretically speed up the process by interpreting the laser modulated signal as characters before handing it to the receiving computer, it would not be useful for pictures, where speed was important. In the end, the simplest idea was a direct link to the RS-232 port from the send and receive circuits.
-There was also the question of how to send and receive pictures. Originally, Hyperterminal would be used, which would be ideal since it came preloaded on all versions of Windows since 1995. Unfortunately, when sending a picture, Hyperterminal relies on being able to receive data back from the receiving computer, handshaking to make sure no bits were lost (known as full duplex). This would be impossible with our system, where one side could only send, and one side could only receive. The options were to either create a double of the send and receive circuit, so the handshaking could occur, or find/write other software that would send a picture without requiring handshaking (known as half duplex). The free program Realterm was discovered that allowed half duplex file transfer.+There was also the question of how to send and receive pictures. Originally, Hyperterminal would be used, which would be ideal since it came preloaded on all versions of Windows since 1995. Unfortunately, when sending a file such as a picture, Hyperterminal relies on being able to receive data back from the receiving computer, handshaking to make sure no bits were lost (known as full duplex). This would be impossible with our system, where one side could only send, and one side could only receive. The options were to either create a double of the send and receive circuit, so the handshaking could occur, or find/write other software that would send a picture without requiring handshaking (known as half duplex). The free program Realterm was discovered that allowed half duplex file transfer.
===== 3.Final Design ===== ===== 3.Final Design =====
-This description refers to the design as it was actually completed and not as it was originally conceived; hence, final design.  The free-space optical system designed by Group 3 is a computer-to-computer system.  This allows for the user interface to be handled by the computers, simplifying design and allowing for better integration with existing hardware.  After all, most people own a computer.  The final design has three basic components: an optical transmission channel to send data over, transceiver electronics to convert between electrical data and optical signals, terminal software used by the computer to control the communication system, and the mechanical assembly on which the components are mounted.  Each section will be discussed in detail.+This description refers to the design as it was actually completed and not as it was originally conceived; hence, final design.  The free-space optical system designed by Group 3 is a computer-to-computer system.  This allows for the user interface to be handled by the computers, simplifying design and allowing for better integration with existing hardware.  After all, most people own a computer.  The final design has four basic components: an optical transmission channel to send data over, transceiver electronics to convert between electrical data and optical signals, terminal software used by the computer to control the communication system, and the mechanical assembly on which the components are mounted.
==== Optical Design ==== ==== Optical Design ====
The optical communication channel is very simple, since this is intended to be a short-range free-space optics system, and is almost identical in its final form to the original design.  As this is only a demonstration system, it operates in a half-duplex mode.  This means that the communication channel is one-way: one end can only send data, and the other can only receive.  As such, only one receiver and one transmitter were needed. The optical communication channel is very simple, since this is intended to be a short-range free-space optics system, and is almost identical in its final form to the original design.  As this is only a demonstration system, it operates in a half-duplex mode.  This means that the communication channel is one-way: one end can only send data, and the other can only receive.  As such, only one receiver and one transmitter were needed.
-An ordinary "dollar variety" 650 nm red diode laser pointer is used as a transistor.  This laser was chosen for its low cost, easy availability, and design simplicity.  The red wavelength is also less strongly absorbed by the atmosphere than shorter wavelengths.  The use of a laser also simplifies the optics of the transmission channel.  If the design had used an LED source instead, collimating optics would have been required to shape the output into a focused beam capable of traversing a long distance.+An ordinary "dollar variety" 650 nm red diode laser pointer is used as a transmitter.  This laser was chosen for its low cost, easy availability, and design simplicity.  The red wavelength is also less strongly absorbed by the atmosphere than shorter wavelengths.  The use of a laser also simplifies the optics of the transmission channel.  If the design had used an LED source instead, collimating optics would have been required to shape the output into a focused beam capable of traversing a long distance.
Unfortunately, this type of laser pointer suffers from reliability issues as well as highly variable beam quality.  A large number of backup and replacement lasers were purchased against the possibility of laser diode failure.  In order to simplify alignment, and account for the poor quality (high divergence) of some of the laser beams, a basic positive lens was used to focus the light onto the photosensor.  The lens selected was a "dollar variety" magnifying glass.  This lens was selected due to its low cost, fairly large area, and easy availability.  The photosensor is located at the focal point of this lens, which was experimentally determined to be approximately 34 cm from its center.  The lens then directs a laser beam entering at a normal incidence to any portion of its surface onto the photosensor. Unfortunately, this type of laser pointer suffers from reliability issues as well as highly variable beam quality.  A large number of backup and replacement lasers were purchased against the possibility of laser diode failure.  In order to simplify alignment, and account for the poor quality (high divergence) of some of the laser beams, a basic positive lens was used to focus the light onto the photosensor.  The lens selected was a "dollar variety" magnifying glass.  This lens was selected due to its low cost, fairly large area, and easy availability.  The photosensor is located at the focal point of this lens, which was experimentally determined to be approximately 34 cm from its center.  The lens then directs a laser beam entering at a normal incidence to any portion of its surface onto the photosensor.
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===== 5.Completed Product ===== ===== 5.Completed Product =====
-The completed product can be seen below, or, in more detail, in the photos above.  The parts list for components used is contained in the budget list. The important and critical devices involved include the laser, the p channel MOSFET, the computers with serial cables, the Mounting and aligning equiptment, the Tube and filter for noise reduction, the lens for focusing the light, the AC filter for signal isolation, the Comparator for Analog to Digital conversion, and the Inverter for inverting the signal. The total cost for construction is a theoretical $63.90. +The completed product can be seen below, or, in more detail, in the photos above.  The parts list for components used is contained in the budget list. The important and critical devices involved include the laser, batteries to supply power, the p channel MOSFET, the computers with serial cables, the Mounting and aligning equipment, the Tube and filter for noise reduction, the lens for focusing the light, the AC filter for signal isolation, the Comparator for Analog to Digital conversion, and the Inverter for inverting the signal. The total cost for construction is a theoretical $63.90. 
{{:group3:dsc00042.jpg?800x600|}} {{:group3:dsc00042.jpg?800x600|}}
 +
 +Close-ups of some of the components are shown below.
==== Transmit Circuit ==== ==== Transmit Circuit ====
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===== 6.Demonstration and Evaluation ===== ===== 6.Demonstration and Evaluation =====
The Demonstration and Evaluation of the project was done over two testing days. The first was an indoors test, and the second was an outdoors test. On both days, the test was a complete success. For the indoor test, the alignment was fast and the project worked right away and only malfunctioned when people walked across the beam during transmission of a file. When this occurred, the photo sent would contain Grey rows where information was lost. For the outdoors test, the alignment was more difficult due to low temperature conditions. The beam spot was also difficult to see due to the failure of 2 lasers the day before the test and the use of a high divergence laser on the day of testing. Nevertheless, the alignment was completed and the project worked again despite the higher amount of background light. This proved that the tube and the red filter were sufficient for blocking light. The Demonstration and Evaluation of the project was done over two testing days. The first was an indoors test, and the second was an outdoors test. On both days, the test was a complete success. For the indoor test, the alignment was fast and the project worked right away and only malfunctioned when people walked across the beam during transmission of a file. When this occurred, the photo sent would contain Grey rows where information was lost. For the outdoors test, the alignment was more difficult due to low temperature conditions. The beam spot was also difficult to see due to the failure of 2 lasers the day before the test and the use of a high divergence laser on the day of testing. Nevertheless, the alignment was completed and the project worked again despite the higher amount of background light. This proved that the tube and the red filter were sufficient for blocking light.
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===== 7.Budget ===== ===== 7.Budget =====
Parts purchased for the project: Parts purchased for the project:
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{{:group3:group_3_budget_2.jpg|}} {{:group3:group_3_budget_2.jpg|}}
-Total Cost: $134.64+Additionally, there was a shipping cost of $2.67 for the Digikey order, and a shipping cost of $4.00 for the Newark order. Therefore, the total cost of the project would then be: 
 + 
 +**Total Cost: $141.31** 
 + 
 +The cost to construct the system is a different matter, where only the parts used are included. Essentially the figure is an estimation of the cost to reconstruct the system on a mass scale. Also, shipping costs from Digikey and Newark would not be included, since many sets of parts could be purchased at the same shipping cost.
-Cost to Construct: $63.90+**Cost to Construct: $63.90**

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