about 40 days. For a lighter weight
version, a lithium coin cell can
be used, which should last about
10 days. After recording, the SD
card is removed and downloaded to
a regular PC.
Ever wonder how hard the post
office or UPS throws your package
marked fragile? By changing the
accelerometer chip to the ST
100-400 g, this device will show
you. Its tripping point can be set
from 1 g to 400 g on all three axes.
It normally records for five seconds
and then goes to sleep. The
recording time can be changed in
the Excel spreadsheet. The
advantage this unit has over static g
monitors is that this one dates and
times the events.
For example, a three foot drop on a hard surface can
produce up to 40 gs. It will date and time the event
happened when its tripping point is exceeded, but will not
show the amount of gs since it does not have FIFO
capabilities. If you need this feature, you can run the unit
in continuous mode.
Electronics
The microprocessor used is a Microchip 16-bit
PIC24J64GA200. Programming pins are made available
on the board for use with PICkit 2 or 3 programmers.
The software is free from Microchip. To change the
programming, you need to know how to program using C.
There is plenty of documentation on how to interface this
chip with SD cards. (Refer to the October 2010
“Implementing a File I/O for the 16-bit Micro
Experimenter” by Thomas Kibalo as one example.) If you
don’t have a programmer, a pre-programmed chip is
available at the NV webstore.
The power for this chip is three volts; the
accelerometer chip has a range of 2. 4-3. 6 volts.
If you want to make your own boards, the Express
PCB files are at the article link. Express PCB provides free
software at www.expresspcb.com.
The SD card sits above the microprocessor and is
easy to remove. The board is round to fit in a 2. 5” PVC
EMT tube.
The microprocessor talks to the accelerometer using
I2C. The accelerometer has a built-in A/D converter and
outputs the results in digital form. The micro is put to
sleep in the seismic and shipping modes, thus it draws
very little power.
The SD card programs the micro with the date, time,
mode, and tripping point. There is an Excel program at the
article link to program the SD card.
Each time an event happens, the
date, time, and results, are recorded
on the SD card. The files are named
Accel1.txt, Accel2.txt, etc.
Each event advances the name
of the file up to 1,000 names. The
LED will flash every four seconds
indicating that an event has taken
place.
The card can be removed any
time by turning off the power and
placing it into a computer for
downloading and viewing.
Constructing
the Board
There are three boards used for this project: a square
board, a round board, and the accelerometer board.
(The accelerometer board comes assembled in a kit, also
available through the NV store.)
Turn the square board over and solder the five 10K
805 surface-mount pull-up resistors. The best way of doing
this is to melt a small amount of solder to the top six pads.
Using tweezers to hold the 805s, touch the tip of the
soldering iron to the pad and place the 805 on the pad
allowing the solder to flow to the resistor. Do this for the
rest of the resistors.
Now, solder the other side of the resistor to the pads
(Figure 5). Make sure you use rosin core solder; I normally
use a . 6 mm diameter. Turn the board over and solder the
SD holder to the top side. Solder each of the pins and
ears in their respective holes. Push the 10 female headers
from the bottom side of the board and solder.
The round board will need to be trimmed to the
outside circle. The easiest way is to use a sander. All the
components are placed on the top of the board.
Solder IC2 to the board. Note the square hole is pin 1
of the chip. Place the chip in front of you with its notch
pointing left. Pin 1 is the lower left pin. Usually, there is a
small dimple above it. There will be a number of areas
vacant, so don’t panic. These were for the previous
seismograph article, and are not included in the Parts List.
Solder resistors R3-R6, R8, R14, and R15; capacitors
C2, C3, C4, and C5; and the crystal. Note the polarity of
the 10 µF cap. Solder the two switches, then the LED with
its long lead going to the square pad.
Push the 10 male headers into the top of the board
where the in-line 10 pads are and solder. There will be
extra pads next to the microprocessor that can be used
for other projects
Pass the two wires of the battery holder through the
strain relief hole and solder them to their pads. If you are
November 2014 35
■ FIGURE 5.
