Silicon Chip – April 2019

82 Silicon chip Australia’s electronics magazine siliconchip.com.au

then place the stripboard over the top

and solder the pins. This ensures the

two rows remain aligned.

You will also need to add the 360kW

resistor to the trimpot on the original

board. Detach the lead connected to

A2, and fit the resistor between A2

and the trimpot lead.

We did this by cutting the trimpot

pin and then desoldering the stub. You

can now plug the stripboard ‘shield’

into the corresponding Arduino pins,

wire up the geophone sensor, and

you’re ready to install the new soft-

ware.

If you haven’t already built the Ar-

duino Seismograph, refer to the April

2018 article for instructions.

Revised software

The new software is very similar to

that used in the April 2018 project.

Some extra code has been added to set

up the ADC reference voltage and to

sample and record the extra channel.

The WAV header data has changed

because there are now five channels

rather than four. There is an extra line

in setup() to set the 1.1V ADC refer-

ence, and extra code in loop() to sam-

ple, filter and output the new channel

to the SD card.

We’re assuming that you have al-

ready installed the Arduino IDE (in-

tegrated development environment).

You can now download the revised

sketch from our website, use the IDE to

compile it and upload it to the Ardui-

no board. The file is named “Arduino_

Seismograph_with_Geophone.ino”.

It’s used in the same way as the

original version. Insert an SD card

into the slot and restart the Arduino

board. Open the Arduino Serial Moni-

tor at 115,200 baud to follow the pro-

gram’s progress and check for errors;

you should see something similar to

that shown in Screen 1.

If there are no errors, allow the

sketch to run for a minute or so. You

can emulate seismic activity by gen-

tly bumping the spot the seismograph

is sitting on.

Press pushbutton S1 to stop log-

ging and write the data to the SD card;

there will be a message on the serial

monitor when this has finished, and

the indicator LED will light up con-

tinuously.

Remove the SD card and open the

files with Audacity. You should see

something similar to what we did,

with five channels displayed. Any ac-

tivity will show up as undulations in

the traces (see Screen 2).

Here we can see movement on the

two bottom channels, both of which

are reading the Z axis. The bottom-

most channel is the geophone sensor,

while the one above this is the MEMS

accelerometer Z axis.

Based on the sensitivity of the geo-

phone sensor with a 1kW damping re-

sistor at around 20V per m/s, full-scale

readings correspond to ±0.0275m/s.

That’s assuming that the attenuation

trimpot is set to provide the maxi-

mum level. At any other setting, it

will take faster motion to give full-

scale readings.

In the April 2018 article, we men-

tioned that, with the default settings,

the readings consume around 30MB of

Fig.3: this
circuit can
easily be built
on stripboard.
Unusually, we’re
mounting most of
the components
on the copper
side of the board.
Make sure the
component leads
can’t short to
anything. The top
three rows are
optional.

Parts List

1 Arduino Seismograph unit (see

April 2018 issue)

1 geophone sensor (20DX or

similar)

1 piece of stripboard (at least 15

rows with at least six pads each)

1 5-pin male header or

1 8-pin male header (with 18+ row

stripboard)

1 6-pin male header

1 3-pin female header socket

1 short length of Bell wire

2 jumper leads to connect geo-

phone sensor to header socket

Capacitors

1 220μF 6.3V electrolytic

1 100nF MKT polyester

Resistors (all 0.25W 1% metal film)

1 51kW 1 10kW 1 1kW 1 360kW

1 1kW mini trimpot (VR2)

So be careful when mounting them to
ensure their leads can’t short to any
tracks or other component leads and
mount the capacitors high enough
that you can get your iron under them
to solder the leads safely.
One wire link is needed (shown in
red); we suggest that you use insu-
lated Bell wire. Note how one lead of
the 100nF MKT capacitor is soldered
directly down into a hole in the A3
row, while the other lead is bent to go
around the 220μF capacitor and con-
nect to one of the GND rows.
We used a small 3-way female head-
er strip and jumper wire off-cuts to
connect the geophone sensor to the
board. The + lead of the geophone sen-
sor should connect to the end nearest
the bottom edge of the board.
Finally, fit the 6-pin and 8-pin male
headers to the underside, to connect to
the Arduino. The easiest way to do this
is to plug the headers into the sockets
on the Arduino board or shield and

The small change needed to the main
shield. The 360kW resistor is soldered
between the Arduino’s A2 pin and
where the trimpot was attached to A2.
This allows the same trimpot setting
to be used in spite of the change
in voltage reference for the ADC
peripheral.