ARDUINO-BASED
DIGITAL AUDIO
MILLIVOLTMETER
by Jim Rowe
Low cost, easy to build, highly accurate: an essential piece of test equipment!
If you’re involved in audio – at any level – you really must have an audio
millivoltmeter in your test gear arsenal. Once you’ve used one, you’ll wonder
how you ever managed without it. It’s useful for setting up and calibrating
audio systems, doing performance measurements and troubleshooting audio
equipment, and much more. This one doesn’t just measure low-level signals.
It provides high-resolution measurements of balanced or unbalanced audio
signals from below –85dBV (56µV RMS) to above +35dBV (60V RMS)! It’s easy
to build and has automatic range switching and can log data to a PC.
W
e decided to design a
new Audio Millivoltmeter because we wanted one which
worked over a very wide range of signal amplitudes with excellent accuracy
and resolution.
We also wanted to have the ability to measure balanced or unbalanced audio signals without the
need for any additional hardware.
But, most of all, we wanted it to be easy
to build and would fit in a compact case.
32
So why build this one instead of our
previous audio millivoltmeter (March
2011 – Low-Cost Digital Audio Millivoltmeter)? Well, for many reasons this
new unit makes that old one obsolete:
• It can measure smaller signals and
much larger signals
• It has much better resolution
• Its frequency response (on both
ranges) is much better (see Fig.1)
• It has a built-in balanced input (no
separate converter required)
• It does not require manual range
selection
• It runs off USB power
• It is quite a bit smaller.
Some of the improvements in this
version are due to our use of an Arduino Nano MCU module for control, while most of the performance
improvements are due to our use of
an LTC2400 24-bit analogue-to-digital
converter (ADC).
Practical Electronics | October | 2020
Features and specifications
• Unbalanced measurement range
• Balanced measurement range
• Frequency range
• Resolution
• Measurement linearity
• Basic accuracy
• Input impedance
• Maximum input level
• Power supply
• Current drain
This gives much higher measurement resolution than the 10-bit ADC
built into most Arduinos.
The result is a unit that’s much
more convenient to use, with higher
performance and it fits into a diecast
box measuring only 119 × 94 × 57mm.
That’s less than half the volume of the
earlier version.
We estimate the total cost for everything you’ll need to build this project
to be under £125. That compares more
than favourably with what you’d pay
for a similar commercial instrument.
To give you an idea of why you
might want to measure down to
–85dBV, if you have a 100W amplifier which can drive 8Ω loads, at full
power then it’s delivering 28.28V RMS
(√(100W × 8Ω) across the speaker. That
equates to +29dBV.
For such an amplifier, a noise level
of −85dBV would therefore mean a
signal-to-noise ratio of 114dB (85dB
+ 29dB). A good amplifier can do that.
So if you had a meter which couldn’t
measure down to −85dB, you couldn’t
come close to getting an accurate measurement of the signal-to-noise ratio of
such an amplifier.
Some potential uses
✔ Audio performance measurements
❏
(signal-to-noise ratio, frequency
response, sensitivity, power output,
channel separation, crosstalk,
amplifier gain)
✔ Crossover adjustment
❏
✔ Equalisation and room response
❏
adjustments (in combination with a
microphone and preamp)
✔ Amplifier calibration
❏
✔ Amplifier and preamplifier
❏
troubleshooting and repair
Practical Electronics | October | 2020
A compact high-resolution digital audio millivolt/voltmeter with
balanced and unbalanced inputs, backlit LCD readout, automatic
range switching and the ability to send its data to a PC.
from <56µV RMS (−85dBV) to 60V RMS (+35dBV)
from <56µV RMS (−85dBV) to 600mV RMS (−4.5dBV)
5Hz-110kHz (+0/−3dB); 20Hz-70kHz (+0/−0.5dB); 50Hz-45kHz (+0/−0.1dB)
24 bits (1 part in 16,777,215)
±0.3dB
approximately ±0.1% after calibration
1MΩ/10kΩ (unbalanced input) or 760kΩ (balanced input)
as per measurement ranges
5V DC via USB mini Type-B socket, either from a USB charger or a
PC USB port
<78mA (390mW at 5V)
The best our 2011 design could
achieve was −76dBV, limiting you to
SNR measurements of no better than
about 105dB for a 100W amp, and considerably worse than that for lower-powered amplifiers, or line-level devices.
How it works
Fig.2 is a simplified block diagram of
the new meter. At its heart is IC3, an
Analog Devices AD8307 logarithmic
amplifier/detector. This is the same
device used in our earlier meter.
The AD8307 has impressive specifications: it can convert AC signals into
a DC voltage equivalent, following a
logarithmic ‘law’ of 25mV per dB (typically linear to within ±0.3dB) and with
a span of just on 100dB. The device
also operates up to around 500MHz,
so it’s just ‘idling’ at audio frequencies.
In the new meter, we are feeding
IC3’s output to IC4, an LTC2400 24bit delta/sigma ADC. This measures
the output of IC3 relative to an accurate 2.500V DC provided by an LT1019
bandgap voltage reference. The resulting 24-bit digital samples are passed
to the Arduino Nano via SPI (serial
peripheral interface).
DIGITAL MILLIVOLT/VOLTMETER FREQUENCY RESPONSE
• Description
The microcontroller then processes the samples to calculate the corresponding measurements, which are
displayed on the LCD module shown
at upper right in Fig.2.
They’re also sent out via the D− and
D+ lines of the USB socket at lower
right, for logging via a PC if required.
The micro indicates when sampling is
taking place by lighting LED1.
The elements on the left-hand side
of Fig.2 have been added to provide
input buffering, low-pass filtering (to
reject RF or other unwanted signals),
range selection and selection between
the unbalanced and balanced inputs.
IC1 is an AD629B high-commonmode-voltage-rejecting difference amplifier, used to convert the balanced
input signals from XLR socket CON1
into an unbalanced signal. Switch S1a
then selects between the unbalanced
signals from either CON2 or the output of IC1, with the other half of the
double-pole switch (S1b) allowing the
micro to detect which input is currently selected.
The signal then goes into the range
switching section, where a reed relay
controlled by the micro via transistor
+1dB
0.0dB
HIGH RANGE
–0.5dB
LOW RANGE
–1.0dB
–1.5dB
–2.0dB
–2.5dB
–3.0dB
1Hz
10Hz
100Hz
1kHz
FREQUENCY
10kHz
100kHz
Fig.1: a frequency response plot for our prototype in the low range (blue)
(measured at 600mV RMS) and high range (red). This demonstrates that
the reading is within 0.5dB of the actual signal amplitude over the entire
audible range and beyond. It’s within 0.1dB from 50Hz to 45kHz.
1MHz
SC
20 1 9
33
CON1
BALANCED
INPUT
SCL
DIFFERENCE
AMP
SDA
IC1
S1a
100:1
DIVIDER
CON2
UNBALANCED
INPUT
LOG AMP/
DETECTOR
(IC3)
SCK
24-BIT
ADC
(IC4)
D9
MISO
SS
+
Q1
REED RELAY
D2
+
SAMPLING
LED
ARDUINO
NANO
MCU
2.500V
REFERENCE
RLY1
INPUT SELECT
SC
20 1 9
BUFFER AMP
& LOW-PASS
FILTER (IC2)
16x2 I 2 C SERIAL
LCD MODULE
LED1
Vbus
D–
D3
D+
USB SKT TO
POWER/PC
1
2
3
X
4
S1b
Fig.2: this block diagram shows the operating principle of the Meter. IC1 converts a balanced signal to unbalanced and
S1 selects between the two inputs. The signal then either passes through RLY1 or a 100:1 divider, depending on whether
Q1 (and therefore RLY1) is energised, giving the unit its two ranges. The signal is then buffered, filtered and fed to the
logarithmic detector before passing to the ADC and onto the Arduino.
Q1 is used to select between either the
input signal divided by 100 (for the
high range, up to 60V), or bypassing
the divider (for the low range).
The signal is then fed to IC2, a dual
op amp with the first stage used as a
unity-gain buffer and the second stage
as a low-pass filter.
This removes, or at least significantly reduces, any noise (including digital switching artefacts from the control
circuitry) which may be induced into
the analogue signal.
The full circuit
You’ll find more details in the main circuit diagram (Fig.3). The signal from the
balanced input at CON1 is filtered using
a common-mode choke (T1) and a 47pF
capacitor to remove RF signals, before
being coupled via two high-voltage capacitors to the inputs of IC1, the balanced-to-unbalanced converter. This
allows for balanced common-mode signals up to 400V peak from earth.
A 2.5V bias signal is applied to the
REF– and REF+ inputs of IC1 (pins 1
and 5), biasing its input signals to half
of the 5V supply, to allow for a symmetrical signal swing before it runs
into clipping.
The signal from the unbalanced input (CON2) is also RF filtered using
inductor L1 and a 22Ω series resistor
and 22pF capacitor to ground.
The output from selector switch S1
is AC-coupled to the precision 100:1
voltage divider, the upper portion of
which is shorted out when the contacts
of RLY1 are closed for measuring lower
Reproduced by arrangement with
SILICON CHIP magazine 2020.
www.siliconchip.com.au
Digital Audio Millivolt/Voltmeter
34
Fig.3: the full circuit follows much the
same pattern as Fig.2, but you can see
that some details were left out of the
earlier diagram, such as the input RF
filtering. VR1 allows the 100:1 divider
to be accurately trimmed, while VR2
Practical Electronics | October | 2020
level signals. Trimpot VR1 is used to
‘fine-tune’ the divider for calibrating
the Meter’s HIGH range.
The way the divider works, and the
reason for selecting these exact component values, is shown in more detail in Fig.4.
The values are selected so that trimpot VR1 can be used to set the divider ratio to precisely 100:1 without restricting its rotation to a narrow portion of its range. VR1 can compensate
for within-tolerance variations in the
four 0.1% tolerance fixed resistors.
Note that as well as forming the
lower leg of the divider for the Meter’s
HIGH range, the 10kΩ 0.1% resistor
also forms the input resistance for
the Meter’s LOW range, for the unbalanced input.
That’s because when RLY1 is
switched on to short out the divider’s
upper arm for the LOW range, the lower part of the divider still provides the
DC bias for input pin 3 of IC2a.
Pin 21 of the Arduino (the D3 digital input) is used to monitor the position of S1, while pin 20 (digital output
D2) controls the range selection relay
(RLY1) via NPN transistor Q1. Diode
D1 protects transistor Q1 from damage due to the back-EMF generated by
the coil of RLY1 when it switches off.
Schottky diodes D2 and D3 protect
IC2a from overload damage, by clamping its pin 3 input voltage within a
few hundred millivolts of the supply
rails, even if the input signal amplitude is too high for the Meter to measure accurately.
The purpose of IC2a is to buffer the
signal from the divider to provide a
low-impedance source for the following low-pass filter, which is built around
the other half of the dual op amp, IC2b.
This is a second-order (−12dB/octave) ‘multiple feedback’ low-pass filter with a −3dB point of around 52kHz.
This was chosen to give a very flat response up to 20kHz, then a steep rolloff above audio frequencies.
This filter is important since, as stated earlier, the log converter (IC3) has
a wide bandwidth of up to 500MHz.
So any digital noise or RF picked up
before this point will add to the signal being detected and give erroneous
readings. Therefore, we want to ensure
that all ultrasonic frequency signals
are severely attenuated.
This filter type and its values were
chosen carefully for this role, as a
multiple-feedback filter has a significant advantage over the more common
Sallen-Key type in that it still provides
excellent attenuation for signals above
the op amp’s bandwidth, and it is far
less reliant on said bandwidth to provide the expected filter attenuation.
A second-order multiple-feedback
resistor needs just one more resistor
than a Sallen-Key type, which is well
worth it for its superior high-frequency attenuation.
The inputs of IC2a and IC2b are biased to the 2.5V rail, both through its
connection to the bottom of the switchable voltage divider ladder, as well as
it being fed directly to pin 5 of IC2b.
Again, this biases the AC signal fed
to these rail-to-rail op amps so that
it swings symmetrically within the
5V supply.
The audio signal is then AC-coupled
to input pin 8 of the AD8307 log detector. A 100Ω series resistor provides
additional RF filtering, in combination
with the 470pF capacitor between its
pins 8 and 1. Pin 1 is grounded via a
220µF capacitor, as we are not feeding
differential signals to this chip. The
INL input sits at the chip’s DC bias level
while the INH input swings above and
below that voltage.
Trimpot VR2 allows us to adjust
IC3’s ‘intercept’ point, calibrating the
Meter’s LOW measurement range. A
1µF capacitor smoothes the logarithmic output voltage from pin 4, and
calibrates the output of the log detector.
The components around IC2b form a
second-order multiple-feedback low-pass
filter, followed by another passive RC
low-pass filter, to reject high-frequency
signals before IC3 detects them.
Practical Electronics | October | 2020
35
Fig.4: the details of the precision
100:1 divider. Starting with the
choice of a 10kΩ 0.1% resistor
in the bottom leg (which can
have a value from 9.99kΩ to
10.01kΩ), that means we need
a total resistance in the upper
leg of 990kΩ±990Ω. Taking into
account the tolerance of the fixed
resistors in that upper leg, a 5kΩ
potentiometer gives sufficient
scope for adjusting for precisely
the right attenuation factor.
this is then fed to the analogue input
of IC4, the 24-bit ADC.
JP1, connected to pin 8 of IC4, changes the ADC’s internal sampling frequency to provide a ‘notch’ for rejecting either 50Hz or 60Hz ‘hum’ in the
signal from IC3.
So for UK use it would be set in the
upper (50Hz) position, while in the US
and other countries with 60Hz mains
power, you’d set it in the lower position.
REF1 provides a very stable 2.5V reference to IC4, necessary for it to operate
with the high precision possible for a
24-bit ADC. This means its resolution is
149nV (2.5V ÷ 224), so the limiting factor
in its performance will be system noise.
The reference has an initial tolerance of ±0.05%, which equates to
±1.25mV. REF1’s output also provides
the 2.5V biasing for IC1 and IC2 mentioned earlier.
The reference output is stabilised by
a Zobel network (5.6Ω and 10µF), as
recommended in its data sheet.
The AD8307 logarithmic
amplifier/detector
The Arduino Nano communicates
with the ADC (IC4) with the standard
SPI pins (ie, pins D10, D12 and D13)
while communication with the LCD is
via an I2C bus at pins A4/SDA and A5/
SCL. Sampling LED1 is driven from the
D9 digital output.
Construction
Most of the circuitry and components
of the new Meter (including the Arduino Nano) are mounted on a PCB
measuring 109 × 84mm, and which
is coded 04106191.
The only components not mounted
on the PCB are the LCD module, the
input connectors and input selector
switch S1. These mount on the box
front panel and connect to the PCB via
short lengths of wire.
Some of the components on the PCB
are of the through-hole variety and
somewhat larger than the SMD components. So it’s best to fit the smaller
SMD parts first.
+INPUT
–INPUT
The location and orientation of all
parts are shown on the PCB overlay
diagram (Fig.5), but you can also refer
to the photos. Note though that there
may be some slight differences between
the prototype and final PCBs.
There are no fine-pitch SMD parts;
all of them are reasonably generous
in terms of size and pin spacings, so
they are not difficult to handle. Start
by fitting all the SMD passives (resistors and capacitors), except for those
which are right next to one of the SMD
ICs, as these would otherwise make fitting the latter more tricky.
The usual technique is to tack one
side of the component onto its pad,
make sure it is sitting flat on the board
and properly aligned, then solder
the opposite pad (after waiting long
enough for the first joint to solidify).
Then wait a little longer and refresh
the first joint with a little extra solder
or flux paste.
With those passives all in place, you
can install the five SMD ICs. In each
case, they must be oriented correctly,
so find the pin 1 dot or divot on the top
face, and make sure it’s facing as shown
in Fig.5. If you can’t find the dot, pin 1
is normally also indicated by a chamfered edge on just that side of the IC.
Again, locate the IC and tack one pin
down before soldering the other seven
pins, then refresh that initial joint. The
pins are spaced far enough apart to be
soldered individually. If you accidentally form a solder bridge between two
pins, add a little flux paste and then
clean it up using solder wick.
SIX 14.3dB GAIN, 900MHz BANDWIDTH AMPLIFIER/LIMITER STAGES
AD8307
INT
SET
INTERCEPT
Logarithmic amplifier/detector ICs are a fairly spe3 x PASSIVE
CURRENT
ATTENUATOR
cialised but quite useful device. You can get an
MIRROR
CELLS
idea of how they work from the diagram at right,
2 A/dB
which gives a simplified view of what’s inside the
NINE FULL-WAVE DETECTOR CELLS WITH
OUT
DIFFERENTIAL
OUTPUT
CURRENTS
–
ALL
SUMMED
AD8307 device.
25mV/dB
The incoming AC signals pass through six casENB
BANDGAP REFERENCE
INPUT – OFFSET
12.5k
caded wideband differential amplifier/limiter stagAND BIASING
COMPENSATION LOOP
es, each of which has a gain of 14.3dB (about 5.2
times) before it enters limiting. This gives a total
OFS
COM
gain of about 86dB, or around 20,000 times.
The outputs of each amplifier/limiter stage are fed to a series of (2.24V). This logarithmic relationship is linear to within ±0.3dB
nine full-wave detector cells, along with similar outputs from three over most of the range.
The output current (IOUT) increases at a slope of very close
cascaded passive 14.3dB attenuator cells connected to the input
to 2µA per dB increase in AC input level, and when this current
of the first amplifier/limiter.
The differential current-mode outputs of all nine detector cells passes through a 12.5kΩ load resistor inside the chip, the result
are added together and fed to a ‘current mirror’ output stage, which is a DC output voltage of 25mV/dB. This slope can be fine-tuned
using an external adjustable resistor in parallel with the 12.5kΩ
effectively converts them into a direct current.
Because of the combination of cascaded gain and limiting in internal resistor.
The ‘set intercept’ (SI) pin allows you to adjust the DC offset in
the amplifiers (plus an internal offset compensation loop), the
amplitude of this output current is proportional to the logarithm the output current mirror, which sets the effective zero-level point
of the AC input voltage. This holds true over an input range of of the chip’s output current and voltage; ie, the origin from which
just on 100dB, from about −93dBV (22.4µV) up to +7.0dBV the output slope rises.
36
Practical Electronics | October | 2020
You can now fit the remaining SMD
passives, plus the two SMD diodes,
ensuring their cathode stripes face as
shown in Fig.5.
Next, fit transistor Q1. It has three
pins, so its orientation should be obvious. Make sure its leads are sitting
flat on the PCB before you solder it
in place.
The last SMD component is L2,
which is quite large. Spread a thin
smear of flux paste on both pads before you start.
You will need a hot iron to form
good solder joints due to the thermal
masses of both the PCB and the part.
Make sure you add enough solder and
heat it long enough to form good fillets.
Through-hole parts
Before proceeding, we need to wind
choke L1 and transformer/commonmode choke T1. These are both
wound on 5mm-long ferrite beads, using 0.25mm-diameter enamel-coated
copper wire.
L1 has three single turns, while T1
has three bifilar turns, wound by first
folding a 200mm length of the wire
in two, and then using the ‘doubled
pair’ to wind their three turns together.
Once both chokes are wound, cut
off the wire ends about 8mm from the
ends of the ferrite beads, scrape off
about 4mm of the enamel and then
lightly tin the wire ends so they will be
easy to solder into the PCB pad holes.
Just before you solder in the four wires
for T1, use your DMM to make sure that
the wire pairs do not ‘cross over’; the leftmost upper and lower wires should be
joined together, as should the right-most
upper and lower wires.
You can now proceed to fit the remaining through-hole parts. Start with
diode D1 (as usual, be careful with its
orientation). It’s then a good idea to install the six PC pins, if you are going to
use them. These make it easier to use
clip leads to connect your DMM to the
board during testing and calibration.
These are for TPGND, TP2.5V, TP5V
and TP1-TP3.
Next, mount the reed relay, again
taking care with its polarity. Follow
with the two multi-turn trimpots,
which are different values (so don’t get
them mixed up), followed by the 4-pin
header for CON3, the 3-pin header for
JP1 and the 2-pin header used to facilitate the connection of LED1.
Now is also a good time to install the
1µF through-hole capacitor, near IC4.
Before you mount the Nano board,
you will need to fit a short length of
wire shorting out its onboard diode
D1, on the underside; see the sidebar
photo and text for an explanation of
why this is necessary and how to do it.
Practical Electronics | October | 2020
Fig.5: this PCB overlay diagram (and photo below) shows where the components
are mounted on the PCB, including the prebuilt Arduino Nano microcontroller
module. Most of the components are larger SMD types which are not difficult to
hand-solder. Some components, such as CON1, CON2 and S1 are mounted on the
lid (front panel) and wired back to the board using short leads.
Now solder the Arduino Nano module to the rows of pads on the board,
with its USB connector over the outside
edge. Make sure it’s pushed all the way
down before soldering; it’s a good idea
to solder two diagonal pins first, check
that it’s flat and then solder the rest.
Finish up by mounting the three
large capacitors.
The final step at this stage is to
solder the leads of LED1 to the pins
of the 2-pin header fitted to the PCB,
taking care to connect them to the
correct pin (the longer anode pin
goes to the inner pin marked ‘A’).
The leads should be soldered to the
pins so that the underside of the
LED’s body is 28mm above the top
of the PCB.
Your Meter’s PCB assembly should
now be complete and ready to be fitted
into the box, once it has been prepared.
Before you do so, though, plug the
4-pin female socket onto CON3 and
place the shorting block in the correct position on JP1, to suit your local
mains frequency.
Preparing the box
Most of the holes you’ll need to drill
or cut in the box are in the lid, which
becomes the Meter’s front panel.
There are only three holes to be cut
in the base of the box: two circular
37
holes in the right-hand end for access
to trimpots VR1 and VR2, and one rectangular hole in the centre of the box
rear to allow access for the power/PC
USB connector.
You’ll find the location and sizes of
all of these holes in the two drilling
diagrams (Figs.6 and 7). Most of the
holes are circular and can be drilled,
although the 23mm-diameter hole for
Parts list – Digital Audio Millivoltmeter
1 119 × 94 × 57mm diecast aluminium box [Jaycar Cat HB-5064 or similar]
1 double-sided PCB, 109 × 84mm, code 04108191 (RevH)
1 Arduino or Duinotech Nano MCU module
1 USB Type-A to mini Type-B cable
1 16x2 backlit alphanumeric LCD module with I2C serial interface
[eg, SILICON CHIP ONLINE SHOP Cat SC4198 or similar]
1 panel-mount miniature DPDT toggle switch (S1) [Jaycar ST035, Altronics S1345]
1 panel-mount 3-pin female XLR connector (CON1) [Jaycar PS1930, Altronics P0804]
1 panel-mount BNC socket (CON2)
1 4-pin header, 2.54mm pitch (CON3)
1 4-pin female header socket, 2.54mm pitch (to connect LCD module)
1 2-pin header, 2.54mm pitch (for LED1)
1 3-pin header with jumper shunt (JP1)
1 SPST DIL reed relay with 5V/10mA coil (RLY1) [Jaycar Cat SY-4030 or similar]
2 5mm-long ferrite beads, 4mm outer diameter (L1,T1)
[Jaycar Cat LF-1250 or similar]
1 300mm length of 0.25mm-diameter enamelled copper wire (for L1 and T1)
1 100µH SMD RF inductor (L2) [Jaycar Cat LF-1402 or similar]
4 25mm-long M3 tapped spacers
4 6mm-long untapped spacers
8 12mm or 15mm-long M3 panhead machine screws
2 9mm-long M3 countersunk head machine screws
2 M3 hex nuts and star lockwashers
4 16mm or 20mm-long M2.5 countersunk head machine screws
4 9mm-long untapped spacers, >2.5mm inner diameter
4 M2.5 hex nuts
6 PCB pins (optional; for TPGND, TP2.5V, TP5V and TP1-TP3)
Semiconductors
1 AD629BRZ high common-mode-voltage difference amplifier, SOIC-8 (IC1)
1 MCP602-I/SN dual rail-to-rail input/output op amp, SOIC-8 (IC2)
1 AD8307ARZ logarithmic amplifier/detector, SOIC-8 (IC3)
1 LTC2400CS8#PBF 24-bit ADC, SOIC-8 (IC4)
1 LT1019ACS8-2.5#PBF precision 2.500V voltage reference, SOIC-8 (REF1)
1 BC817-40 NPN transistor, SOT-23 (Q1)
1 3mm red LED (LED1)
1 1N4148 silicon small-signal diode (D1)
2 1N5711W-7-F schottky diodes, SOD-123 (D2,D3)
Capacitors (all SMD ceramic, 3216/1206 size unless otherwise stated)
2 220µF 6.3V X5R, SMD 3226/1210 size
2 100µF 6.3V X5R
2 22µF 10V X5R
3 10µF 16V X7R
1 10µF 250VDC metallised polypropylene,radial leaded [Panasonic ECQ-E2106KF]
1 1µF 50V through-hole ceramic or MKT
1 1µF 16V X7R
2 220nF 275VAC metallised polypropylene, radial leaded [Panasonic ECQ-U2A224ML]
1 220nF 16V X7R
(Code 220, 0.22 or 220n)
7 100nF 16V X7R
(Code 100, 0.1 or 100n)
1 2.2nF 16V X7R
(Code 2.2, .022 or 2n2)
2 470pF 100V C0G/NP0
(Code 470, .0047 or 470p)
1 47pF 100V C0G/NP0
(Code 47, .00047 or 47p)
1 22pF 250V C0G/NP0
(Code 22, .00022 or 22p)
Resistors (all SMD 1% 0.25W, 3216/1206 size unless otherwise stated)
1 910kΩ 0.1% 1 75kΩ 0.1% 1 51kΩ
1 10kΩ
1 10kΩ 0.1%
1 3.0kΩ 0.1% 1 4.7kΩ
1 2.2kΩ
1 1.5kΩ
2 1.2kΩ
1 1kΩ
1 100Ω
3 470Ω
1 22Ω
1 10Ω
1 5.6Ω
1 5kΩ multi-turn horizontal trimpot (VR1)
1 50kΩ multi-turn horizontal trimpot (VR2)
38
XLR connector CON1 is best made using either a hole saw or by drilling a
circle of small holes and then cutting
between them using either a rat-tailed
file or jeweller’s saw.
The best plan for cutting the 65 ×
15mm rectangular hole for the LCD
screen is to drill a 6mm diameter hole
inside each corner, to allow you to use
a small metal-cutting jigsaw to cut
along each side.
Then you can tidy up the edges using a small file.
For the rectangular hole in the rear
of the box, I first drilled a 9mm diameter hole in the centre, then used jeweller’s files to expand it out into the
final rectangular shape.
Once all of the holes have been
made, remove all burrs from the inside
and outside of each hole using one or
more small files.
As a final step in preparing the box
for assembly, you should fit a professional-looking panel on the lid.
We have produced a front panel
artwork for this project, which can be
downloaded from the October 2020
page of the PE website as a PDF file.
You can then print, laminate and attach it to the lid using thin doublesided adhesive tape or a smear of silicone sealant.
The final step is to cut out the holes
in the dress front panel to match those
in the lid itself, using a sharp knife.
Final assembly
Glue an 80 × 40mm rectangle of
0.5mm-thick clear plastic sheet to the
rear of the lid, just behind the LCD
window. This is to keep dust out and
protect the LCD screen from accidental scratches. It can be cut from a clean
takeaway container lid or similar.
Then mount the LCD screen to the
underside of the lid using four 16mmlong M2.5 countersunk-head screws
with four 9mm-long untapped spacers
and four M2.5 nuts, as shown in Fig.8.
Next, fit XLR connector CON1 to the
lid using two 9mm-long countersunkhead M3 screws with lock washers and
nuts on the rear.
After this, fit BNC connector CON2
using its matching lock washer, solder lug and nut, then input selector
switch S1.
To ensure that the switch is fixed
in place horizontally, you can drill a
small blind hole in the rear of the lid
to accept the spigot on the edge of the
switch’s flat washer.
Now up-end the lid/front panel and
solder stiff wire leads to the rear lugs
of CON1, CON2 and S1. These don’t
have to be very long; just long enough
to pass down through their matching
holes in the PCB when it’s fitted.
Practical Electronics | October | 2020
The only one that needs special
treatment is that for CON2, which
should ideally be made using a 25mm
length of shielded microphone cable.
Take care when separating the
screen wires at each end, to prevent
accidental shorts.
Once these extension leads have
been fitted, you are ready to mount the
PCB to the rear of the lid/front panel.
The PCB is mounted using four
25mm-long M3 tapped spacers, together with four 6mm-long untapped
spacers, as shown in Fig.8. First attach all four pairs of spacers to the
corners of the PCB, using 12mm-long
M3 screws passing up through the PCB
and the untapped spacers, and then
into the 25mm tapped spacers.
The complete PCB-and-spacers assembly is then attached to the rear of
the lid/front panel, using four 12mmlong M3 screws.
While doing this, ensure that the
extension wires from CON1, S1 and
CON2 pass through their matching
holes in the PCB. And before you finally tighten up the screws, make
sure that the body of LED1 is protruding through its matching hole in the
front panel.
Now solder the ends of the extension wires from CON1, S1 and CON2
to their matching pads on the rear of
the PCB.
If all has gone well so far, you should
find that the pin ends of the 4-pin SIL
header fitted to the end of the LCD
module are now very close to those of
the socket plugged into CON3.
You should only need to bend the
module’s header pins down slightly
to meet the pins from CON3’s socket,
and then you can solder them together.
Your Meter is now complete, apart
from the final fitting of the front panel
assembly into the box.
But before you do this, it’s a good
idea to load the Meter’s firmware
sketch (program) into the Arduino
Nano. This is done using the Arduino
IDE, running on a suitable PC, with
the Meter connected to a USB port of
the PC via a standard USB Type-A to
mini Type-B cable.
Programming the Meter
The firmware program to be loaded into the Meter’s Arduino Nano is
called AudiomVmeterMk2_sketch.
ino, which you can download from
the October 2020 page of the PE website. Save it in a folder where you’ll
be able to find it later.
Now is also a good time to make sure
that you have the latest Arduino IDE
(integrated development environment)
installed. If not, you can get it from:
www.arduino.cc/en/main/software
Practical Electronics | October | 2020
showing that the Meter is receiving
5V power.
Assuming that you are running Windows, open the Control Panel and select ‘System and Security’ and then
This software allows you to compile and upload the code to the Arduino board.
Plug the Meter into your PC, and
its LCD backlight should light up,
37.5
B
37.5
B
A
65
HOLES A: 3.
A
HOLES B: 2.5
HOLE C: 6.5
A
HOLE D: 9.0
15
31
39
33
33
32.5
B
32.5
16
B
8
CL
47
47
A
24
29
9.5
29
24
33
33
12
23
D
C
12
A
A
9.5
A
CL
ALL DIMENSIONS IN MILLIMETRES
Fig.6: most of the holes that need to be made in the case go in the lid. Holes A are 3mm
diameter, B are 2.5mm, C 6.5mm and D 9mm. You’ll probably need a hole saw to cut
the 23mm, although you could use a 20mm stepped drill bit and then enlarge to 23mm
with a large tapered reamer. Note that holes ‘B’ need to be countersunk after being
drilled. See the text for suggestions on how to make the large rectangular cut-out.
RIGHT-HAND
END OF CASE
25
3mm DIAMETER
2
17
3mm DIAMETER
ALL DIMENSIONS
IN MILLIMETRES
REAR OF CASE
CL
19.5
9
11
CL of the case to access the calibration
Fig.7: two holes need to be drilled in the side
potentiometer screws, while a small rectangular cut-out on one of the long sides
provides access to the USB socket, both for power and optionally for logging
measurements to a PC.
39
The pre-assembled display PCB mounts
so that the LCD lines up with the cutout
in the lid (which becomes the front
panel). Here we also show the four
mounting pillars and the input select
switch along with the XLR and BNC
sockets, with their connecting wires
already soldered in place and ready to
connect to the main PCB.
‘Device Manager’. This should allow
you to see the Virtual COM Port that
the Meter has been allocated. It should
also allow you to set the baud rate for
communication with the Meter. Set it
to 115,200 bps.
Now start up the Arduino IDE and
load the sketch that you downloaded
earlier. In the IDE’s Tools menu,
set the Board selection to ‘Arduino Nano’ and the Processor to ‘ATMega328P (Old Bootloader)’, then
set the COM Port to whichever one
your Meter is connected to, as determined earlier.
Open the sketch and in the Sketch
menu, click on ‘Verify/Compile’.
When you get the ‘Compiling Done’
message, go to the Sketch menu again
and click on ‘Upload’. The compiled
sketch should then be uploaded into
the Nano MCU’s Flash memory.
After a few seconds, the Meter
should start up, giving you a brief
12mm LONG
M3 SCREWS
CON1
message on the LCD announcing itself. It will then start sampling from
whichever input S1 is set to select.
At this stage, the Meter may not
be giving sensible readings, since it
has yet to be calibrated. But you can
check the various DC voltages on the
PCB test points.
For example, you should find a
voltage very close to 5V between
TP5V and TPGND, while the voltage
at TP2.5V should read 2.500V with
respect to TPGND.
If those check out, you can now
install your Meter in its box, by lowering it in and then screwing the lid
with the four M4 countersunk screws
supplied with it.
Calibration
For accurate results, your Meter must
be calibrated. You’ll need access to
an audio oscillator or a function generator, together with a DMM capable
M2.5 x 16mm LONG
COUNTERSUNK SCREWS
TO ATTACH LCD MODULE
CON2
S1
9mm
LONG
UNTAPPED
SPACERS
S1
ARDUINO
NANO
25mm LONG
M3 TAPPED
SPACERS
2
LCD WITH I C
INTERFACE
(BEHIND)
10 F
250V
6mm LONG
UNTAPPED
SPACERS
MAIN PCB
12mm LONG M3 SCREWS
40
of making accurate and reasonably
high-resolution AC voltage measurements in the range from 500mV to
10V (RMS).
Power up the audio oscillator or
function generator and set it to provide a 1kHz signal with an amplitude
of 600mV RMS (1.697V peak-to-peak).
Check this level using your DMM,
and adjust the generator if necessary.
Then power up the Millivoltmeter
and connect the oscillator’s output
signal to the Meter’s unbalanced input (CON2), with S1 set appropriately.
After a few seconds, the Meter
should show a stable reading in both
millivolts and dBV, with the legend
‘(L)’ at lower right. This indicates the
Meter has switched to its lower range.
At this stage, the reading will probably differ a little from the correct value of 600mV and −4.437dBV. So use a
small screwdriver or alignment tool to
adjust trimpot VR2 (INTERCEPT ADJUST), to bring the reading as close
as possible to that correct value. This
calibrates the Meter’s low range.
The next step is to calibrate the
Meter’s high range. Change the output level of the audio oscillator or
function generator to 10.000V RMS
(28.28V peak-to-peak), checking this
using your DMM again.
If your oscillator or function generator can’t provide an output that high
(which is quite common), you may
have to use a small amplifier to boost
its output.
RLY1
M2.5 NUTS
VR1
Fig.8: this ‘cut-away’ side
profile view of the assembled
unit shows how the various
parts attach to each other
and the back of the lid, and
also gives you an idea of the
connections needed from the
panel-mounted parts to the
PCB below.
Practical Electronics | October | 2020
‘Left and right’ views of the assembled project immediately before it is mounted in the diecast case. The input sockets and
selector switch are all connected to the PCB via short lengths of either tinned copper wire or, in the case of the BNC socket
(CON2), shielded cable. The photo at right compares with the diagram on the opposite page.
An amplifier capable of doing just
that, very accurately, is described starting on page 29 of this issue.
Now connect the oscillator’s output
signal to the Meter’s unbalanced input (CON2) again, and after a couple
of seconds, the Meter should display
a new reading.
This time, the legend at the end of
the lower line should read ‘(H)’, to
show that it has now switched to the
higher range.
The new reading is likely to be fairly
near the correct value of 10.000V and
20.00dBV, but not spot-on. Correct it
by adjusting trimpot VR1 (CALIBRATE
HI RANGE).
Once this has been done, your new
Digital Millivolt/Voltmeter is calibrated and ready for use.
Logging measurements
All you need to do to log measurements to your PC is open up the Arduino Serial monitor, using the same
settings as described above for programming the Nano.
With the unit connected to your
PC, each time it takes a measurement, it will also be written to the serial monitor.
When you have finished, you can
save the log for later analysis (for example, using mathematical functions
in a spreadsheet).
This view of the right end of the PCB shows the two 15-turn trimpots, VR1 (left
– 5kΩ) and VR2 (right – 50kΩ) which are used to set the HIGH range calibration
and intercept adjust, respectively (see text). These pots line up with access holes
drilled in the end of the case.
Ensuring that a low-cost Arduino Nano works reliably
There are Arduino Nanos... and there are
Arduino Nanos!
During the development of this project,
we discovered on two occasions that the
‘El cheapo’ Arduino Nanos had started to
malfunction.
In both cases, diode D1 in the Nano’s
power supply had ‘blown’ and changed
into a high resistance, lowering the supply voltage to less than 2.8V.
This diode (an SS1 or an MBR0520) is
not really required when the Nano is powered from USB. It’s purely to protect the
USB port of the PC when the Nano is powered via a higher voltage supply fed directly
into its Vin pin.
Since the Nano and its associated circuitry (here, the Millivoltmeter) are always
going to be powered from the USB connector, there’s no reason why the diode
can’t be simply shorted out, to ensure reliable operation.
Practical Electronics | October | 2020
The problem is that the diode is fitted to
the underside of the Nano’s tiny PCB. This
makes it quite inaccessible if the Nano has
already been fitted to your Meter’s main PCB.
In fact, I had to virtually destroy the first Nano
to remove it from the main PCB to get at the
blown diode.
So we suggest that if you are going to
be using a low-cost Nano in your Millivoltmeter, you should first short out D1 with a
short length of wire, before mounting it on
the main PCB.
This should ensure reliable operation and
avoid the need for surgery at a later stage.
The photo at right shows where D1 is located, just below the Mini USB connector.
The diode is usually marked ‘B2’, although
on the one in the photo it looks more like ‘D2’
because there’s a tiny crater in the middle of
the B where the smoke came out.
It’s quite easy to short out the diode with a
short length of tinned copper wire, bent into a
tiny inverted ‘U’. If you use the same soldering iron you use to fit SMD components, it
can be done quite quickly if you’re careful.
Just make sure that the wire link doesn’t
protrude upwards very far, or it might touch
the top copper of your main PCB when the
Nano is mounted on it.
41
