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group1:group1start 2008/12/12 09:07 group1:group1start 2008/12/12 09:53 current
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-==== 2.3 Comparator Circuit ====+==== 2.3 Comparator Circuit as Receiver/Demodulator ==== 
 + 
 +This sub circuit contains the IR receivers which receive the modulated light from the MFGC. The output of the IR receivers can be observed in the second half of theoretical output diagram above defined by the IR receiver, where it is observed that for the time that the envelope frequency is high, the output of the receiver is low, and for the time that it is low, the output of the receiver is high. The output of the IR receivers is essentially a square wave with a high pulse equal in magnitude to the low pulse of the envelope frequency of the MFGC. The RDC can be observed in the following figure. 
 + 
 +{{:group1:receiver_circuit.jpg|}} 
 +Receiver Circuit 
 + 
 +The square wave output from the receivers were too difficult to analyze within the microcontroller. Therefore the addition of the demodulator circuit was necessary in the later creation time of the project. The demodulator circuit, observed above as a PNP BJT, a 1μF capacitor, two 1kΩ resistors,  and a comparator, acts to change the square wave output into a constant high voltage. Initially, if the beams were to be blocked, the square wave output would merely change frequency, however, with the addition of the demodulator circuit, the final output into the microcontroller would be a constant high voltage which only dropped low if the beam were to be blocked. This allowed for much simpler analysis. 
 + 
 +To convert the square wave into a constant high voltage, the demodulator circuit takes the square wave and biases the BJT. Upon doing this, the capacitor is able to charge and discharge periodically within a given voltage window. This capacitor wave form is fed into the positive input of the comparator. In the negative input, a threshold, constant DC voltage is applied. When the circuit is not blocked, the wave form of the capacitor does not drop below the threshold voltage and therefore the output of the comparator remains high. Once the beam is crossed, the capacitor wave form drops below the threshold and causes the output of the comparator to switch to low. This is illustrated in the following figure. 
 + 
 +{{:group1:comparator_inputs.jpg|}} 
 +Inputs of the Comparator 
 + 
 +The jagged wave form in the above figure is the output of the capacitor which is the first input of the comparator. The second input of the comparator is the observed DC voltage at roughly +1.14 volts. Upon crossing the IR beam, the square wave output of the receiver changes frequency, which in turn causes the capacitor to discharge below the previously mentioned threshold voltage. At this point the output on from the comparator switches from high to low. This corresponding output can be viewed as follows. 
 + 
 +{{:group1:receiver_circuit_theoretical_output.jpg|}} 
 +Receiver Output to Microcontroller 
 + 
 +The output seen above is placed into the input of the microcontroller. It is a constant high output until the beam is blocked, where it switches to low. This event is much more easily analyzed by the microcontroller than the square wave. The demodulator circuit is replicated for the second IR receiver output. The comparators used were actually part of a quad comparator integrated circuit provided by Glen Leinweber.  
 + 
 +Again, the DC sources have filter capacitors on them to remove any transient AC ripple voltages. Four AA batteries are used to power this circuit as well. 
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==== 3.2 Electronic Proof-of-Principle ==== ==== 3.2 Electronic Proof-of-Principle ====
   
 +The initial electronic PoP consisted of a multisim simulation of the MFGC and receiver circuit, and an explanation of the microcontroller. As the parts for the circuit had not arrived yet, the analog portion had not been built and the programming of the microcontroller was still in development. An explanation of the interrupts was given for the microcontroller and how they would be used to observe when the beam was blocked. However, this was under the assumption that a square wave input would be used for the microcontroller, and not the simpler DC signal. Thus, it was too difficult to implement all the ideas in the electronic PoP as later modifications to the circuit proved that an entirely different microcontroller with new code would be used.
 +
 +Given the chance to present later on, the completed MFGC circuit was demonstrated and powered with batteries to transmit an IR signal across the lab to be received and observed on an oscilloscope. This pseudo-PoP allowed the group to demonstrate that the analog components were able to be constructed in accordance with the Multisim schematic, and that it could be powered using batteries. Also, the microcontroller was programmed to output “Leigh”, indicating a beginning understanding on how to place code on the microcontroller and use it to perform operations. Overall, the electronic PoP mainly proved the validity of the then-existing circuit and microcontroller combination.
===== 4. Final Completed Product Description ===== ===== 4. Final Completed Product Description =====
 +
 +The MFGC, the RDC and the DPC as described in the final design can be observed in the following figures.
 +
 +{{:group1:p4.jpg|}}
 +Modulated Frequency Generator Circuit
 +
 +{{:group1:p5.jpg|}}
 +Comparator Circuit as Receiver/Demodulator
 +
 +{{:group1:p6.jpg|}}
 +Microcontroller Circuit
 +
 +As observed in the above figures, the entire circuit fit onto five bread boards. In the figure depicting the RDC, it is important to note that the quad comparator is set to receive three inputs. This is due to the initial design of three beams. However since one of the receivers malfunctioned, only two of the inputs were used, however the third input with the corresponding third demodulator circuit, was left on the bread board. The output from the second stage of the larger circuit in the MFGC screenshot, is also used to bias the base of the BJT in the smaller circuit of the same diagram. Two separate IR receivers were used. One was the ordered receiver from Digikey. Originally, it did not function properly, as it lost its signal from time to time. However during the testing of it implemented with the demodulator circuit, it always kept a constant output when receiving a signal. The second receiver, observed in the DPC figure, comes from an old ENG PHYS 4A06 project, and also receives light modulated at 38kHz. A visible LED is present in combination with a parallel resistor in the above figure to allow for a voltage step down from the +6.5V supply, so that the microcontroller can be powered with about +5V without being damaged. Also note the numerous electrolytic capactiors present in all three circuits, used to filter out transient AC noise from the power supply.
===== 5.Final Demonstration of Completion ===== ===== 5.Final Demonstration of Completion =====

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