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Rotor Pole
Temperature Sensor
Network
Sponsored by Avista
Danu Hankins, Jay Hartshorn, Justin Postma
[Pick the date]
Contents
Executive Summary....................................................................................................................................... 2
Background ................................................................................................................................................... 3
Problem Definition ........................................................................................................................................ 3
State of the System ....................................................................................................................................... 3
Signals ....................................................................................................................................................... 3
Hardware .................................................................................................................................................. 4
Technical Specifications ................................................................................................................................ 4
Transmitter Circuit .................................................................................................................................... 4
Receiver Circuit ..................................................................................................................................... 4
Display Circuit ....................................................................................................................................... 4
Present Design .......................................................................................................................................... 4
Sensor Design ............................................................................................................................................ 5
Receiver Design ......................................................................................................................................... 8
Display Design ......................................................................................................................................... 10
Program Flow .......................................................................................................................................... 12
................................................................................................................................................................ 12
Power .......................................................................................................................................................... 13
Slot Harmonics ........................................................................................................................................ 13
Permanent Magnets ............................................................................................................................... 13
Future Work ................................................................................................................................................ 15
Electrical and Computer Engineering ..................................................................................................... 15
Board Layouts ..................................................................................................................................... 15
SCADA Interface .................................................................................................................................. 15
Wireless Transmission Testing ............................................................................................................ 15
Power Source Implementation ........................................................................................................... 15
Mechanical Engineering .......................................................................................................................... 15
Housing ............................................................................................................................................... 15
Mounting............................................................................................................................................. 15
Appendix A: File Repositories ..................................................................................................................... 16
1
Executive Summary
We have designed a proof of concept for a temperature sensor network to monitor
temperatures on pole faces of the synchronous generators at the Noxon hydroelectric dam. There is no
temperature measurement on these pole faces currently. The lack of damper windings on these poles
causes the poles themselves to act as damper windings during any transients. With the increasing
number of transients that Noxon experiences as the load leveling station for Avista there is concern that
the temperatures are close to exceeding the rating of the insulation. The solution will be scalable to
allow for monitoring of temperature hotspots on poles and will interface with the monitoring PLCs
already at the dam. Implementing this solution will provide important metrics for the safe and efficient
operation of the generators at Noxon.
2
Background
The Noxon hydroelectric dam has no damper windings on its rotor poles. As one of the load
leveling generators for the power grid it goes through power transients more often than other
generating stations. When it goes through these transients the rotor poles themselves act like the
damper windings and eddy currents run through them. This heats up the poles which are not measured
by any sensor currently. The temperatures potentially approaching or exceeding the insulation’s
temperature rating is a cause for concern for safe and efficient operation of the generators. By
providing metrics on the rotor pole temperatures Avista will be able to operate the generators
efficiently while maintaining safe operation.
Problem Definition
Our project is to design a proof-of-concept for a temperature sensing network that will take
temperatures from the surface of the rotor and send that data to the Avista PLC. The temperature
sensor network had to meet a few specifications from the client including:




+/- 5 degree Celsius accuracy.
Being able to provide two temperature readings per minute to the PLC.
Being able to interface with the existing Avista PLC.
The insulation on the rotor is rated to 155C
Without a mechanical engineer on the team we constrained ourselves to assuming that we could mount
the sensor onto the rotor and the circuitry on the spider arms of the generator. This means we could
focus on the functionality of the system rather than trying to find a way to mount it safely within the
generator housing.
In addition to these constraints and specifications we added another specification, scalability. We want
our concept to be able to take in multiple temperature sensor readings so Avista can monitor multiple
potential hotspots on the rotor.
State of the System
The following sections are a summary what our proof-of-concept is capable of and what kind of
hardware is present. A more detailed explanation of how the signals and hardware function can be
found later in this report.
Signals
The system currently has three distinct circuits that comprise it: transmitter, receiver, and
display.
The transmitter circuit can currently interpret the readings from an RTD into a temperature
reading. This reading is combined with other metadata such as: pole number, sensor number and error
flags. This data package can be transmitted via a wireless signal to the receiver circuit.
The receiver circuit can receive transmissions reliably within a 25 foot unobstructed, line-ofsight, distance from the transmitter circuit. The receiver circuit can queue up data packets to be
transmitted via RS232 to the display circuit.
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The display circuit can take in data from the receiver circuit. This data can be processed to
derive the following metrics: maximum temperature, which pole and sensor that temperature comes
from, number of reliable signals, number of expected signals, and the average temperature of the
system. This data can be output to an LCD display and a USB output which can be interfaced with
HyperTerminal. The maximum temperature data can also be sent out via a current Digital to Analog
Converter (DAC).
Hardware
The system is currently all on breadboard. There are finished board layouts, made in Kicad, for
the transmitter and receiver circuit. These circuit boards have not been debugged yet.
The transmitter circuit can be powered by a battery or other voltage source. There is no
implementation of a power source solution that will work on the rotor yet. There are math models of
potential sources are mentioned later in this report.
Technical Specifications
This is a list of the major components and electrical ratings for each circuit.
Transmitter Circuit




120 mW
6-35V
MRF24J40MA Microchip 2.4 GHz Wireless Transceiver
PIC24FJ64GA002 Microchip microcontroller
Receiver Circuit
 Powered by display circuit
 MRF24J40MA Microchip 2.4 GHz Wireless Transceiver
 PIC24FJ64GA002 Microchip microcontroller
Display Circuit
 24V Supply
 LCD Display
 PIC32MX534F064H Microchip microcontroller
Present Design
The present design consists of three separate boards, see figure 1. The purpose of the circuit
located on the rotor, denoted as Temperature Transducer/Transmitter or “sensor” for short, is to
measure the temperature of a single rotor pole and transmit the reading off wirelessly. The figure only
shows a single sensor for clarity. The actual implementation will likely have more sensors to obtain a
temperature profile of the rotor, e.g. one sensor per four rotor poles. The sensor’s power supply is
shown separately for emphasis. The sensors are to be “self-powered”, i.e. no batteries. The
Communication Relay, “receiver” for short, serves two purposes. The primary purpose of the receiver is
to convert the wireless signals into wired signals. The conversion is necessary because the wireless
signals can’t penetrate the rebar within the concrete wall reliably. The second purpose of the receiver is
to fetch the data wirelessly from each sensor in a cyclic fashion and buffer the sensors’ data.
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Figure 1: Present design
The third and final board, Display/Output or “display” for short, fetches the data from the
receiver via wired communication. The purpose of the display is to serve as an interface to the
programmable logic controller (PLC) and notify the operator of the generators condition, i.e. summary
of the sensors’ temperatures.
Sensor Design
One of the unforeseen challenges with the sensor design is the power source. Many different
concepts where investigated. These range from wind energy, vibration energy, solar cells with a light
source, direct power from field windings, slot harmonics and a concept that creates a DC machine on
the generator. The team decided to regard any potential power source as an unregulated AC or DC
voltage source with a wide voltage range. A buck converter power supply was chosen because it can
accept a wide input voltage range and maintains a stable output voltage with minimal components, see
figure 2. The power supply for the sensor is designed to accept 6-35V AC or DC which is then regulated
to power the circuit at 3.3V.
Figure 2: Buck Converter to regulate Rail Voltage
A Resistance temperature detector, RTD, is used to measure the temperature of the rotor pole
which can be located up to one meter from the actual sensor. The RTD acts as a temperature
dependent resistor that has a near linear correlation between resistance and temperature over a wide
temperature range, namely -220C to 700C. The sensor utilizes a 4 channel Sigma-Delta Analog to Digital
Converter, ∑Δ ADC, to measure the RTD(s), see figure 3. Until now, the sensor has been associated with
a single RTD but it is actually possible to measure up to four. The additional ADC channels can be left
unused but the channels provide some flexibility in implementation. For example, there can be up to
four RTDs per rotor pole or the RTDs can be spread to four adjacent rotor poles and use a single sensor
to measure and transmit their temperature data.
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Figure 3: Sensor Design, Located on the Rotor
The ADS1248, a ∑Δ ADC, was selected for several reasons. The resolution of the ∑Δ ADC is 24bits which is very high in comparison to other types of ADCs. The reason the ∑Δ ADC can have such a
high resolution is because the design is based on oversampling. Oversampling in this context means the
channel is sampled many times and then averaged to obtain a higher resolution than other types of
ADCs. The averaging of the converter also acts as a low pass filter to reject unwanted high frequency
noise because the temperature of the rotor poles does not change quickly due to its large thermal mass.
In addition to the intrinsic low pass filtering of the ∑Δ ADC, there is a built in digital low pass filter to
reject frequencies down to tens of Hertz. It should be noted that the ADC is 24-bit but the Effective
Number of Bits, or ENOB, depends on the sample rate and configuration of the converter. Our team
chose to have the ADC sample through the channels @ 40 samples per second which results in an ENOB
near 16-bits.
An ADC works by comparing the input channel to a reference voltage and outputting the input
channel’s voltage as a percentage of the reference. The ∑Δ ADC used for the sensor operates in a similar
fashion but the reference is created using two external high precision resistors and two matched current
sources built into the ADS1248. The principle of operation is shown in figure 4. The schematic gives the
resulting ADCvalue equation following the figure.
Figure 4: Ratiometric Resistance Measurement
𝐴𝐷𝐶𝑣𝑎𝑙𝑢𝑒 =
𝑉𝑐ℎ𝑎𝑛
𝑉𝑟𝑒𝑓
, 𝑉𝑟𝑒𝑓 = (𝐼+ + 𝐼− )𝑅𝑟𝑒𝑓 , 𝑉𝑐ℎ𝑎𝑛 = 𝐼+ (𝑅𝑟𝑡𝑑 + 𝑅𝑤𝑖𝑟𝑒 ) − 𝐼− (𝑅𝑜𝑓𝑓𝑠𝑒𝑡 + 𝑅𝑤𝑖𝑟𝑒 )
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If 𝐼+ = 𝐼− , then 𝐴𝐷𝐶𝑣𝑎𝑙𝑢𝑒 =
𝑅𝑟𝑡𝑑 −𝑅𝑜𝑓𝑓𝑠𝑒𝑡
2∗𝑅𝑟𝑒𝑓
The only precision components necessary are the reference and offset resistors. The
importance of this is that the RTD measurement is independent of the RTD’s wire resistance which is
significant when 16-bit resolution is expected. Also the measurement is independent of power supply
voltage assuming the ADC rail voltage is within its specification. The immunity to power supply voltage
means there is no need for additional filtering of the buck converter output voltage. This simplifies the
layout and reduces the number of components.
The processor for the sensor is necessary for two reasons. The most important function of the
processor is to implement the wireless protocol stack in software and control the RF transmitter, see
figure 5. The transmitter used in the circuit is actually a transceiver, meaning it can receive and transmit
messages. It has been referred solely as a transmitter because the primary purpose is to transmit the
temperature data. The transmitter also sends other metadata with the temperature data such as: rotor
pole number, sensor number, and any error flags. There are many wireless transmitters available on the
market but our team chose one of Microchip’s transmitters because they cost $10, half that of
competing transmitters. The tradeoff is that the wireless protocol must be implemented in a
microcontroller. Both the wireless protocol, Microchip’s MiWi which is similar to Zigbee, and the
protocol stack implemented in software is free so long as Microchip’s processors are used.
Figure 5: Microchip MRF24J40MA 2.4GHz Wireless Transceiver
The secondary function of the processor is to linearize the RTD measurement and convert the
result back to temperature. The RTD is near linear for a large range of temperature but there are
standard tables available, i.e. Din 43760 for our RTD, that show measured values for RTD resistance vs.
temperature, see figure 6. The temperature range of interest shows that the nonlinearity can be as
much as 0.5 ohms which corresponds to roughly 1.3C. The error becomes significant if the range of the
RTD goes beyond 0 to 250C which is possible. The solution to this problem is to linearize the ADC
reading inside the processor using a lookup table that contains all of the values from the Din 43760
standard table. The processor still performs linear interpolation between each of the points in the
lookup table but the error introduced is reduced by an order of magnitude when compared to using a
single linear approximation for the entire temperature range.
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Figure 6: RTD nonlinearity
The remaining parts of the sensor are the LED status indicators. The indicators are used to aid in
troubleshooting. One LED is used to indicate that a wireless connection has been established with the
receiver. A second LED is used to indicate whether the ADC has cycled through all of the channels at
least once so data is valid. The last LED toggles every time the sensor transmits data wirelessly to the
receiver.
Receiver Design
The receiver is powered from one of the rail voltages of the display. The input voltage is 15Vdc
to account for voltage sag across the wires between the receiver and display and to keep the current
draw low. The rail voltages required to power the circuit are then obtained by bucking down the voltage
to 5V and regulating the 5V to 3.3V, see figure 7. Both 5V and 3.3V are required to power the
components on the receiver.
Figure 7: Receiver Power Supply Design
Two of the components on the receiver are exactly the same as on the sensor; the processor
and RF receiver, see figure 8. The processor is necessary to implement the wireless protocol in software
and control the RF receiver. The basic task of the processor is to continually request temperature data
from sensors in a cyclic fashion, i.e. the receiver asks for temperature data from sensors one-by-one.
The result is a wireless channel that is not full of signal collisions because only one device is allowed to
send a wireless message at a time. In addition to requesting temperature data the receiver buffers the
data until the display requests the information for a particular sensor.
8
Figure 8: Receiver Design, Located inside the Generator Housing
The wired communication, with the display, is performed using a protocol similar to RS232. The
difference in protocols between our design and the RS232 standard is the meaning of the RTS and CTS
signal used for handshaking. An example of the communication handshaking is shown in figure 9. If
device “B” asserts RTS then device “A” knows that it can send data to device ”B”. The wired transmitter
block shown in figure 8 serves the purpose of translating the logic level voltages to +/-8V typically. The
increase in voltage swing allows for a more robust communication link between the receiver and
display, i.e. less susceptibility to electrical noise.
Figure 9: Wired Communication between Receiver and Display Circuits
The LEDs are similar to the sensor and indicate the status of the receiver. One LED indicates
whether a RF channel has been created successfully. The receiver is responsible for setting up the
wireless channel that its sensors will operate on. Another LED toggles every time a wireless message is
received from one of the sensors. The last LED toggles for every message sent or received over the
wired communication channel.
9
Display Design
The display is the largest of the three boards. There are three configurations to power the
circuit using two power sources. A standard wall outlet using a switch-mode power supply to step down
to 24Vdc and the PLC 24Vdc supply are the two power sources, see figure 10. Two buck converters
followed by a linear regulator create the 5V and 3.3V rails used by everything except the DAC. The 15V
rail is generated to power the receiver. The reason that three configurations are possible is because the
DAC can be powered independently from the remainder of the circuit. The choice of DAC power supply
depends on the PLC interface. The three configurations are as follows:
~ Display and DAC powered by the switch-mode supply
~ Display and DAC powered by the PLC supply
~ Display powered by the switch-mode supply, DAC powered by the PLC supply
Figure 10: Display Power Supply Design
The components of the display are shown in figure 11. The display fetches temperature data
using wired communication in a similar manner as the receiver does using wireless communication. The
display requests information from the receiver’s buffer for sensors one-by-one and receives the
temperature data shortly thereafter. The primary task of the processor is to summarize the
temperature data from all of the sensors and output the data to the PLC. The processor calculates the
maximum temperature with corresponding rotor pole number, the average temperature and whether
the temperature data is reliable. The reliability is determined from error flags that are sent along with
the temperature data. The error flags indicate the status of the sensor such as ADC timeout, RTD short
or open circuit, Voltage rails out of range and ADC over-temperature. In addition to the error flags
provided by the sensor, the receiver appends error flags such as data not yet requested from sensor,
sequence numbers to indicate whether the sensor data is old and whether wireless communication has
failed with a particular sensor.
The interface to the PLC is through a 4-20mA signal that is an industry standard. A current
source is used because it is less susceptible to noise than a voltage source. The processor takes the
maximum temperature value and converts that value into a current value using a linear equation. The
equation is set to output 4mA @ 20C and 20mA @ 40C for demo purposes. The range can easily be
adjusted to a larger range by changing two constants in a software header file. The DAC is electrically
isolated from the display because the ground potential of the PLC may be different from that of the
display. A second reason is reliability, if the power supply of the display were to fail and burn up the
display then the PLC is still protected.
10
Figure 11: Display Design, Located near the PLC
The purpose of the LCD display is to output a summary of the rotor’s temperature. The
maximum temperature with corresponding pole number and the average temperature are displayed for
an operator walking by the PLC. There are two additional numbers displayed to indicate the status of
the temperature sensor network. The number of sensors that the display expects to receive data from,
i.e. “known” for short, is displayed on the LCD and the number of sensors that are considered “reliable”
based on error flags is displayed. If the number of known sensors does not match the number of
reliable sensors there are two possible causes. If the generator is running then some of the sensors
have failed. If the generator is not running then none of the sensors will be powered and the display
interprets this as no reliable data in which case the current output is set to 0mA.
The USB interface is not used in the present design for any real purpose. It was added to the
design to interface with a program called Supervisory Control and Data Acquisition, SCADA. SCADA
collects data from various types of sensors on each generator, e.g. vibration, magnetic field, etc. The
team chose to use a USB component from Future Technology Devices International, FTDI, because the
design does not require any knowledge of the USB protocol. The Integrated Circuit, IC, from FTDI
communicates with the microcontroller using I2C then translates this to USB which is sent to a
computer. The computer is loaded with a driver that creates a Virtual Comm Port, VCM. The VCM
allows the designer to think of the communication between the microcontroller and computer as serial
port and use programs such a HyperTerminal.
The buttons for the display circuit are not used in the present design. The purpose of the
buttons is to create a way for an individual to pair sensors to the receiver and display. The wireless
communication between the sensors and receivers require unicast messages for reliable data transfer.
Unicast messages send out a wireless packet to a specific device and await an acknowledgement. If no
acknowledgement is received within some period of time the send attempt is repeated for a specified
number of attempts prior to returning control to the main program. For reliability purposes, unicast
messages are superior to broadcast messages which send the data without any acknowledgements. The
cost of using unicast messages is the overhead of sending a devices address. In the MiWi protocol there
are two numbers that specify a specific transceiver; the long address which is set in software and must
be unique to each device on a wireless network and the PANID which specifies an extra number that all
devices on a single network have in common.
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A robust sensor network requires the use of unicast messages and thus each sensor and the
receiver must have a method storing the long address and PANID between software resets. The long
address and PANID can be hard-coded into each sensor and receiver with no issue until one of the
devices fails and needs to be replaced which requires the receiver and the new sensor code to be
recompiled. A better solution is to allow for the devices to be paired after the code is compiled and
released. This requires the microcontrollers to program their own flash memory when pairing is
performed. The question is how does the sensor and receiver know that they need to pair? The answer
is by using buttons that an individual can press on both the sensor and display circuit. The buttons allow
the sensor to pair with the proper receiver because there can be more than one wireless sensor network
per Dam, i.e. most Dams have more than one generator and each will have a separate sensor network.
Program Flow
A simplified program flowchart is shown if figure 12. The actual diagram that matches the C
code is included in a flash drive.
Figure 12: Basic Program Flowchart
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Power
Powering the sensor continues to be one of the great challenges of the project. Currently we
have math models in Mathcad for two proof-of-concepts for how to power the sensor.
Slot Harmonics
The major assumptions that were made in calculating the peak to peak voltage output of the
slot harmonics are the magnetic flux density (B) is assumed to be constant and evenly distributed
throughout the magnetic flux path. The machine is saturated enough that the approximation for the
length of the flux path in the air gap is accurate and that the relative permeability of iron is two
thousand times that of air.
The parameters that are used in the calculation have been highlighted in the Mathcad file. These
are the rpm of the machine, the diameter of the machine and the total number of poles in the machine,
the width of the pitch (a slot and a tooth), the width of a slot, the air gap between the pole and the
tooth and the dimensions of the wire loop and number of turns of wire.
The first calculation is the reluctance of the path through the iron in both the stator and rotor
(this has little effect on the output of the voltage). Next is the calculation of the air gap as a function of
position on the rotor, it assumes that the flux lines will make a quarter circle to the nearest tooth. Then
using the air gap function the net permeance across the area of the wire is calculated as a function of
the position of the center of the loop. These functions are graphed across one pitch to demonstrate the
results.
The magnetomotive force (MMF) is calculated using the closed loop integral of the magnetic
field, H, with the flux path. This is done with the length of the iron path divided by the relative
permeability of iron plus two times the average gap length throughout the wire loop all multiplied by
the magnetic field. Then the net flux through the loop is calculated by dividing the magnetomotive force
by the total reluctance of the flux path, as in the reluctance of the iron plus twice the reluctance of the
air gap.
The voltage is calculated with an approximation of Faraday's Law that states that the time rate
of change of magnetic flux will produce an electromotive force in a loop. So by taking the difference of
the flux at the extremes and dividing it by the time between these extremes we can calculate the
electromotive force induced in a loop. The extremes are taken at the center of the slot and the center of
the tooth. The resulting voltage produced is the peak voltage that will be induced.
Permanent Magnets
The major assumption in this model is that the leakage flux between the magnets is ignored,
although it could be significant within the experiment that we had done. It also assumed a similar
calculation for the length of the flux path through the air gap even though we were using weaker
permanent magnets.
The parameters used in the calculation are highlighted in the Mathcad file. They are the radius
of the machine which is used to calculate the velocity (ω) the magnetic flux density (B) The dimensions
of the wire loop, the width of the magnet and spacing between them, the distance between the magnet
and the loop, and the length of the flux path through the iron.
The calculations are similar to that of the Slot Harmonics calculations, starting off with the
calculation of the reluctance of the iron path. The gap equation is expanded out to cover the distances
13
of one pair of magnets, a north and south pole pair, that will complete a flux loop. The permeance is
calculated in the same method and the corresponding results are shown. The magnetomotive force and
the net flux calculations are also calculated in the same method as before.
The calculation of the voltage output differs from before because our experimental result had
the output rectified and averaged by charging up a capacitor. Therefore the flux difference is between
the average flux as the loop passes the magnets and zero for when the magnets are not near the loop.
The time is the total time it takes for the magnets to pass by the loop. The resulting voltage is then the
flux difference calculated over the time multiplied by the number of loops linked.
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Future Work
Electrical and Computer Engineering
Board Layouts
The complete board layouts need to be designed, fabricated, and debugged. Also, the
transmitter circuit needs to have the power source implemented in the final board.
SCADA Interface
The USB interface needs to be able to interface with Avista’s SCADA system. The present design
utilizes a USB port to interface with a PC. However, Avista’s SCADA system requires a DNP interface. As
a result the display circuit’s USB port needs to be swapped with the DNP interface. Avista’s PLC also
needs to be able to correlate the current output DAC’s signal to a temperature reading.
Wireless Transmission Testing
Wireless transmission was only tested with unobstructed line of sight. Further testing needs to
be done to verify that successful transmission of data from the transmitter to the receiver can occur in
the environment of the generator.
Power Source Implementation
Math models for power sources have been completed. Scale testing for the power source
operating with the transmitter circuit is needed.
A 24V power supply for the display circuit needs to be implemented. A suitable switch-mode
power supply has been found, part # VOF-6-24 made by CUI inc. The implementation involves designing
an adapter board to safely power the display circuit.
Mechanical Engineering
Housing
Primarily, the transmitter circuit needs housing for the environment on the rotor. The housing
needs to be sturdy to handle the centripetal forces acting on it. It also needs to take into consideration
that the system is trying to transmit a wireless signal. The housing cannot interfere with that
transmission.
Mounting
The transmitter circuit and housing will need to be mounted to the rotor. The RTD units will also
have to be mounted to the pole faces and care should be taken to make sure they are not exposed to
passing air as that would alter the temperature readings.
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Appendix A: File Repositories
1. All microcontroller program files and Mathcad documents can be found on the included flash
drive.
a. Layout files – “Senior_Design_Files\New Stuff (use this)\Layout_Schematics”
b. Bill of Materials – “Senior_Design_Files\New Stuff (use
this)\Layout_Schematics\Bill_of_Materials_for_Boards”
c. C code – “Senior_Design_Files\New Stuff (use this)\Code\breadboard_code_4_10_13”
d. Datasheets – “Senior_Design_Files\New Stuff (use this)\IC_datasheets”
e. Microchip software – “Senior_Design_Files\New Stuff (use this)\Microchip_software”
f. Analysis files – “Senior_Design_Files\New Stuff (use this)\Analysis”
g. Other reference files – “Senior_Design_Files\Old Stuff (use for reference only)”
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