siliconchip.com.au Australia’s electronics magazine April 2019 81
Fig.1: the additions
to the existing
Seismograph circuit
are quite simple.
The geophone is DC
biased with a 0.55V
rail generated by
two resistors and
one capacitor. It’s
loaded with a 1kW
trimpot which also
allows its sensitivity
to be adjusted. The
resulting signal then
goes through an RC
low-pass filter and
into Arduino analog
input pin A3.Fig.2: this frequency response graph from the 20DX
datasheet shows how its normal response (A) is damped
by resistive loading. The 1kW trimpot in our circuit gives
us the relatively flat response shown by line B in red.to provide a flat frequency response (see Fig.2) and also al-
lows its sensitivity to be adjusted, reducing the voltage fed
to the Arduino’s A3 analog input depending on its rotation.
Generally, we suggest you leave VR2 set fully clockwise,
although you may need to back it off a bit if you’re expect-
ing to measure a large quake accurately.
The signal then goes through a low-pass filter with a
-3dB point of 1.6kHz, made from a 1kW resistor and 100nF
capacitor.
Further filtering is performed in the software. The 1kW
series resistor also protects the Arduino from large (clip-
ping) signals from the geophone, while the 100nF capaci-
tor provides a low impedance for the ADC’s sample-and-
hold circuitry.
The 360kW resistor added in series with VR1 matches
its range to the 1.1V internal reference instead of 5V, as
before. We found that this provides more consistent geo-
phone measurements than getting the ADC to switch be-
tween the two different reference voltages dynamically.
Construction
Since this is a simple circuit, we built it on stripboard.
You will need a board with 15 rows, and at least six con-
nected pads available in each row. If you have 18 rows,
then the add-on board will neatly cover one side of an Ar-
duino Uno Rev3 board. The component layout is shown
in Fig.3. No track cuts are required.
We used a vertical (right-angle mounting) mini trimpot
for VR2 in our prototype, but you can also use a horizon-
tal trimpot, as shown in Fig.3
Five rows of stripboard are connected to the POWER
section of the Arduino headers, and six rows go to the
analog section. The empty row between these sections is
used as our bias reference. The three extra rows below the
analog section hold potentiometer VR2 and connect to the
geophone sensor.
Due to the way the board is soldered to the Arduino
headers, the components are fitted to the copper track side.