Sound-Activated
Security System
Aaron Lebahn and Chris Jenkins
Features
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Controlled by sound
Musical note based "PIN"
Humans remember melodies easier
Can be used in parallel with traditional key
based lock
Hardware Block Diagram
Operation
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User presses button to initiate entry
LED turns on signifying collection
Microphone accepts user whistle
HCS12 processes input
Correct entry "opens" servo motor
Incorrect entry flashes LED
Wiring Diagram
Software Design
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Interrupts triggered by RTI to implement
regular sampling rate
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Static counter keeps track when appropriate
number of samples taken
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Low memory requires that number of
samples are low (128 or 256 samples)
Program Flowchart
Overview of FFT
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Uses implementation of Cooley-Tukey algorithm
modified to work on microprocessor
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Twiddle factors hardcoded using a fixed-point
format
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Fixed point format consists of number shifted by
2^15
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Multiplication temporarily done in 64-bit
FFT pseudocode
(from Wikipedia)
X0,...,N−1 ← ditfft2(x, N, s):
DFT of (x0, xs, x2s, ..., x(N-1)s):
if N = 1 then
X0 ← x0
trivial size-1 DFT base case
else
X0,...,N/2−1 ← ditfft2(x, N/2, 2s)
DFT of (x0, x2s, x4s, ...)
XN/2,...,N−1 ← ditfft2(x+s, N/2, 2s)
DFT of (xs, xs+2s, xs+4s,
...)
for k = 0 to N/2−1
combine DFTs of two halves into full
DFT:
t ← Xk
Xk ← t + exp(−2πi k/N) Xk+N/2
Xk+N/2 ← t − exp(−2πi k/N) Xk+N/2
endfor
endif
Memory Constraints
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MC9S12C128 only has 2kB of RAM
Limits number of samples we can take
Limits amount of memory FFT can use while
running.
Future Improvements
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Allow programmable pass-tune option
Alarm / Lockout system
LCD user interface
Consider using Goertzel's Algorithm
Questions?
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Queries?
Complements?
Inquiries?
Stumpers?