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adxl345_fft.py

# Simple demo of the ADXL345 accelerometer library.  Will output the X, Y, Z
# axis acceleration values and its FFT values as csv files.
# Author: Tatsuya Sasaki
# License: Public Domain

# Import the ADXL345 module.
import Adafruit_ADXL345
import csv
import pandas as pd

import time
import sys
import math
import scipy
from datetime import datetime
from scipy.fftpack import fft

SAMPLES = 1024

# Max resolution value ( 3.9mg / LSB )
RESOLUTION = 3.9

accel = Adafruit_ADXL345.ADXL345(address=0x53, busnum=2)
accel.set_range(Adafruit_ADXL345.ADXL345_RANGE_16_G)
accel.set_data_rate(Adafruit_ADXL345.ADXL345_DATARATE_3200_HZ)

l = [None] * SAMPLES

# start measurement
start_time = datetime.now()
for i in xrange(SAMPLES):
    l[i] = accel.read()
end_time = datetime.now()
# end of measurement

time_diff = end_time - start_time
duration_sec = time_diff.seconds + (time_diff.microseconds / float(10**6))

sys.stdout.write('Number of samples: ')
sys.stdout.write(str(SAMPLES))
print

sys.stdout.write('measurement takes ')
sys.stdout.write(str(duration_sec))
sys.stdout.write(' sec')
print

sps = SAMPLES / duration_sec

# raw data output
df = pd.DataFrame(l)
df.columns = ['x', 'y', 'z']
df *= RESOLUTION
df['time'] = pd.Series(scipy.array(range(SAMPLES))*duration_sec/SAMPLES)
df = df[['time', 'x', 'y', 'z']]
df.to_csv("rawoutput.csv")

# FFT output
start_time = datetime.now()
d = {'x': fft(df['x']), 'y': fft(df['y']), 'z':fft(df['z'])}
df_fft = pd.DataFrame(data=d)
end_time = datetime.now()
df_fft['frequency'] = pd.Series(scipy.array(range(SAMPLES))*sps/SAMPLES)
df_fft = df_fft[['frequency', 'x', 'y', 'z']]
df_fft.to_csv("fftoutput.csv")

time_diff = end_time - start_time
duration_sec = time_diff.seconds + (time_diff.microseconds / float(10**6))

sys.stdout.write("FFT takes: ")
sys.stdout.write(str(duration_sec))
sys.stdout.write(" sec")
print
sys.stdout.write("Sampling frequency: ")
sys.stdout.write(str(sps))
sys.stdout.write(" Hz")
print

Hi! Thank you very much for this code.
I am doing a project based on this, but I am new in Python.
I ran the code as it is (I already downloaded the adafruit.adxl345 library), but I get the following error:

Traceback (most recent call last):
File "/home/pi/accelerometer2.py", line 24, in
accel = Adafruit_ADXL345.ADXL345(address=0x53, busnum=2)
File "/usr/local/lib/python3.5/dist-packages/Adafruit_ADXL345/ADXL345.py", line 63, in init
self._device = i2c.get_i2c_device(address, **kwargs)
File "/usr/local/lib/python3.5/dist-packages/Adafruit_GPIO/I2C.py", line 64, in get_i2c_device
return Device(address, busnum, i2c_interface, **kwargs)
File "/usr/local/lib/python3.5/dist-packages/Adafruit_GPIO/I2C.py", line 97, in init
self._bus = Adafruit_PureIO.smbus.SMBus(busnum)
File "/usr/local/lib/python3.5/dist-packages/Adafruit_PureIO/smbus.py", line 105, in init
self.open(bus)
File "/usr/local/lib/python3.5/dist-packages/Adafruit_PureIO/smbus.py", line 130, in open
self._device = open('/dev/i2c-{0}'.format(bus), 'r+b', buffering=0)
FileNotFoundError: [Errno 2] No such file or directory: '/dev/i2c-2'

Would you know how to solve the problem?
Thanks in advance!

Your bus number might not be 2 but 1 (/dev/i2c-1), try to change the line 23 like the following:
accel = Adafruit_ADXL345.ADXL345(address=0x53, busnum=1)

That was so simple! Thank you very much man!!