Multimeter
-CheckeR
-Calibrator
It’s amazing how handy even the cheapest
multimeters can be – but, are they accurate?
This Multimeter Checker can verify their accuracy,
and for meters that aren’t so cheap, it will also
allow you to calibrate them and adjust for drift.
Project by Tim Blythman
M
ultimeters are indispensable
tools; perhaps so necessary that
we tend to take them, and their
accuracy, for granted. Sometimes accuracy is not that important, but there are
times when it is.
Back in August 2016, we presented
the Low-cost Accurate Voltage Current
Resistance Reference for checking and
calibrating multimeters.
It provides a DC reference voltage of
2.5V ±1mV (±0.04%), a resistance of 1kΩ
±1Ω (±0.1%) and a current of 2.5mA
±3.5µA (±0.14%). The DC voltage reference comes from a precision voltage
reference IC, and that plus a precision
resistor provides the current reference.
That precision resistor can also be used
on its own as the resistance reference.
The whole thing is compact and ran
from a coin cell, perfect for keeping in
the toolbox to be used whenever needed.
It covers the most common measurements done with a multimeter.
While that was great, it didn’t provide an AC voltage source, so not all of
the typical multimeter ranges could be
checked/calibrated. So, we decided to
develop a new design to add that feature.
For the new Multimeter Checker, we
have a dedicated voltage reference IC
providing 3.3V for DC calibration. This
is also used with a precision resistor
to provide an accurate 100mA current
source. It has another precision resistor
to act as a resistance reference.
Importantly, for calibrating AC voltage ranges, it provides a precise 1V RMS
AC sinewave at one of three frequencies:
50Hz or 60Hz (to match typical mains
frequencies) or 100Hz.
Practical Electronics | July | 2023
Different multimeters use different
methods to measure AC voltage (and
alternating current). That is why some
multimeters are labelled as ‘True RMS’
while others are not.
True RMS multimeters give accurate
AC voltage measurements, whatever the
shape of the waveform. In contrast, some
cheaper multimeters measure the peak
voltage and multiply the reading by a
factor of 0.71, on the assumption that the
waveform is sinusoidal. Of course, this
will not be accurate unless the waveform
is close to being a sinewave.
A square wave, for example, will give
an artificially low reading as its peak is
the same as its RMS value. Similarly,
triangle and sawtooth waves will tend
to give readings that are too high.
Some other meters measure the
average of the rectified AC voltage
and assume a sinewave, which will
have different error magnitudes for
other waveforms.
In our circuit, the AC voltage is generated by an analogue circuit, so it does
not have the digital artefacts that would
be produced by a digital synthesis
method. Its amplitude and frequency
are checked and adjusted by a microcontroller, which compares these to the
DC voltage reference and the frequency
of a crystal oscillator.
As well as enjoying the benefits of
both analogue and digital circuitry, this
allows the AC voltage reference to be set
to 50Hz, 60Hz or 100Hz.
Circuit details
The entire Multimeter Checker/Calibrator circuit is shown in Fig.1. The DC references (voltage, current and resistance)
on the Checker work much the same as
in the earlier Low-cost Accurate Voltage Current Resistance Reference. Still,
we’ll explain how they work together,
because they are also an intrinsic part
of the AC voltage reference.
5V USB power is applied to socket
CON1 and powers, among other things,
3.3V precision voltage reference VREF1.
This MCP1501 low-cost 3.3V precision
reference is critical to the correct operation of all the other parts.
It’s capable of supplying up to 20mA,
which is vital to ensure that the accuracy
Features and specifications
∎ DC voltage reference........................ 3.3V ±0.1%
∎ AC voltage reference........................ 1V ±0.5% RMS
∎ Direct current reference.................. 100mA ±0.2%
∎ Resistance reference....................... 33Ω ±0.1%
∎ AC reference voltage frequency.... 50Hz, 60Hz or 100Hz (±0.3%)
∎ AC reference frequency source..... crystal oscillator
∎ AC reference harmonics................. ≲40dBV
∎ Control via pushbuttons with LEDs, and over USB virtual serial port
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of the reference is not affected by the
connected loads, especially as the analogue generation circuitry is powered
from this 3.3V reference.
VREF1 has a 100nF bypass capacitor at its positive supply, pin 1. The
3.3V output from pin 7 is connected
to TP5 and can be compared with circuit ground at TP6; these two points
are marked DCV on the PCB. The PCB
has separate circuit traces from TP5 to
REF1’s feedback (FB) pin 8. This ensures
that the 3.3V is accurate at the test point,
in spite of any loads.
Precision reference current
Dual low-voltage rail-to-rail op amp IC3
(MCP6272) is powered from the 3.3V
output of VREF1 and has a 100nF supply
bypass capacitor. One half of IC3 (IC3b)
is used to drive the current reference.
The 3.3V from VREF1 feeds into the
non-inverting input of IC3b (pin 5) via
a 1kΩ resistor. Its corresponding inverting input (pin 6) is fed (via another 1kΩ
resistor) from the high side of a 33Ω
precision resistor used to measure the
reference current. Any current through
this resistor causes a voltage to develop
between TP4 and ground.
The output of this op amp (pin 7)
drives the base of NPN transistor Q1,
acting as an emitter-follower, via a 100Ω
resistor. Q1’s collector is connected to
the 5V rail, and its emitter goes to TP3.
TP3 and TP4 are thus the current reference terminals. When TP4 is below
3.3V, Q1 is fed current by the op amp. If
TP4 starts to rise above 3.3V, the current
drive to Q1 starts to get cut off. When
TP4 is at 3.3V, 100mA must be flowing
through the 33Ω resistor to ground.
There will be a minuscule current
flowing from TP4 into the op amp’s pin
6, but it is of the order nanoamps, so it
is much less significant than the 0.1%
precision component tolerances.
Thus, the op amp’s feedback loop
maintains 100mA between TP3 and
TP4 when the two are connected by a
This compact Multimeter Checker
provides outputs to check the most
commonly used features
on most multimeters.
It delivers 3.3V DC,
100mA DC and a 1V AC
RMS pure sinewave that
can be set to 50Hz, 60Hz
or 100Hz and is checked
for both voltage and
frequency by the onboard
microcontroller. The USB
interface can also be used
to manually control the
AC oscillator and set
custom frequencies.
30
multimeter measuring current. TP3 and
TP4 are labelled on the PCB as the DCA
reference points.
The 1nF capacitor between pins 6 and
7 helps suppress any high-frequency
oscillation that might occur due to the
high gain of the op amp.
With 3.3V across the 33Ω resistor plus
the base-emitter drop of Q1 and perhaps
0.1V across the 100Ω base resistor, the
op amp output is typically at 4V, giving about 1V of headroom below the 5V
supply. So anything connected to the
current reference must drop less than
1V or have less than 10Ω resistance for
the current reference to work correctly.
A second, identical 33Ω precision
resistor is provided as the resistance reference, allowing the circuit to provide
an independent set of test pads, TP7 and
TP8, for the resistance feature.
AC voltage reference
Practically all of the remaining circuitry
is used to provide the AC reference.
Since this circuit operates from a single-ended 5V DC supply, we first need
a nominal level around which the AC
signal can swing. For this, we have chosen half of the 3.3V supply, which is
derived by using a pair of 10kΩ resistors to divide the output from VREF1
to produce 1.65V.
The resulting voltage is low-pass filtered by a 1μF capacitor and buffered by
IC3a, with another 10kΩ resistor providing the unity-gain feedback. The output
of this op amp (pin 1) sits at 1.65V, and
this is our AREF rail.
The AC signal is generated by a
phase-shift oscillator based around
another op amp, IC1 (another
MCP6272), and IC2, an AD8403ARZ10
quad 10kΩ digital potentiometer.
IC2 is powered by the 5V rail with a
100nF bypass capacitor. The analogue
ground pins 1, 5, 17 and 21 connect
to circuit ground, along with its digital ground at pin 9, while the SHDN
(shutdown) and RS (reset) pins
are pulled up to 5V by 10kΩ resistors
to allow normal operation of the digital
potentiometer at all times.
Op amp IC1 is powered from the 3.3V
rail, with a 100nF bypass capacitor, to
provide signal symmetry around the
1.65V AREF reference. This is one reason why we have chosen the MCP1501
reference, as it has a sufficient output
current and suitable voltage to power
these components.
This is critical because one of IC1’s
outputs saturates briefly on every cycle,
so if it were powered from 5V, the saturation would occur differently on positive and negative swings, leading to
harmonics (ie, frequencies above the
selected 50/60/100Hz option) creeping
into the output.
A phase-shift oscillator works by reinforcing a signal that is delayed by 360°.
The delay is formed by several RC filter
networks, which add up to 180° of phase
shift, followed by inversion, equivalent
to a further 180° phase shift.
As the RC filter phase shift depends
on frequency, it will only have a delay of
precisely 360° at one specific frequency.
Signal components at other frequencies
are attenuated as they are delayed by a
different amount and interfere destructively as they make their way around
the circuit.
The circuit elements also attenuate all
frequencies to some extent, so one half
of op amp IC1 provides the gain needed
to overcome this, while the other half
provides the phase inversion.
Phase shift oscillator
There are three phase-shift elements
composed of three 1μF capacitors connected to IC2 and three of the digital potentiometer elements inside IC2
(numbered 1-3). These are all wired as
variable resistors (rheostats) and can
vary independently from near to 0Ω up
to around 10kΩ.
Imagine a fairly pure 50Hz 1V AC
RMS signal at pin 1 of IC1; this is what is
expected when the oscillator is working
as designed and set to the 50Hz output.
1V RMS is around 2.8V peak-to-peak.
Op amp IC1b acts as an inverting
amplifier with a gain of 1.5 times. So
the output at pin 7 is expected to be an
inverted version of IC1a’s pin 1 signal,
but with a 4.2V peak-to-peak value.
Since IC1 is fed from a 3.3V supply, the
output saturates at 3.3V peak-to-peak.
The resulting waveform is between a
sinewave and a square wave, so it will
also have some odd harmonics of 50Hz
present, the first of which is at 150Hz.
Fig.2 shows the spectrum of the oscillator’s output at 50Hz. Note that the
only significant harmonic is the third
at 150Hz, although its level is down by
over 40dB compared to the fundamental.
Practical Electronics | July | 2023
Multimeter Calibrator/Tester
Fig.1: most of the components in the circuit are to generate and monitor the AC waveform,
including IC1, IC2 and IC4. IC1 and its connected components form the phase-shift oscillator, with
IC2’s potentiometer elements controlling its frequency and amplitude under the supervision of
IC4. It measures the oscillator voltage using its ADC with reference to the 3.3V precision reference
and adjusts the digital potentiometers to achieve very close to 1V RMS. Similarly, the AC signal
frequency is adjusted using 16MHz crystal X1 as a reference.
Note also that we will still obtain a
3.3V peak-to-peak output from IC1b
even if the signal that comes from IC1a’s
pin 1 output drops as low as around
0.8V AC RMS, or if it was higher than
1V AC RMS due to the previously
described saturation effect.
Practical Electronics | July | 2023
This amplified signal from IC1b (pin
7) passes through the three RC low-pass
filters. If the digital pots are set to around
5.5kΩ, each stage will cause a 60° delay
to the 50Hz component and approximately halve its AC amplitude (as measured at each successive capacitor).
Other, higher-frequency components
will be delayed more and attenuated
even more. For example, the third harmonic of 50Hz at 150Hz will be phaseshifted by around 80° and be reduced
to about a fifth of its original amplitude
by each stage.
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The three stages interact to a degree,
so a simple mathematical analysis of
each stage separately does not quite
match what happens when they are
combined. Before building the prototype, we had to simulate the entire circuit to determine the required component values.
The result is a relatively pure 50Hz
signal, but with quite a low amplitude
coming into pin 3 of IC1a. But as long
as the pin 7 output of IC1b is saturated
on each cycle, the level is steady.
IC1a acts as a non-inverting amplifier
with a gain set by the ratio of the 330Ω
fixed resistor and the fourth variable
resistor in IC4. This gain is selected to
bring the attenuated signal from the RC
filter stages back up to 1V RMS and is
fed to TP1 via a 100Ω resistor to protect
IC1 from external short circuits.
TP2 is connected to the 1.65V reference so that the sinewave between
TP1 and TP2 can be measured without
a DC offset.
So, the AC signal frequency can be
changed by adjusting the three variable
resistor elements in the three RC networks. Similarly, the amplitude can be
varied by adjusting the fourth variable
resistor value.
The resulting waveforms are shown
in Scope 1. The primary output signal
is the blue trace, while the red trace is
the saturated output at IC1b’s pin 7.
Note that it is inverted compared to the
blue trace.
You can see that the orange, yellow
and green traces are phase-shifted and
attenuated by each successive RC stage.
The green trace is amplified to become
the blue trace, thus completing the feedback loop.
Control circuitry
IC4 is a PIC16F1459 microcontroller that adjusts and monitors the AC
reference for accuracy, among other
tasks. It is powered from the 5V USB
supply with a 100nF bypass capacitor between pin 1 (5V) and pin 20
(ground). A 10kΩ resistor between
pins 1 and 4 pulls up the MCLR pin
to allow normal operation when the
circuit is powered.
IC4 needs both an accurate voltage
and frequency reference to do its job.
The 3.3V output of VREF1 goes to JP1,
and with the appropriate jumper fitted
(in the ‘Run’ position), it feeds through
to pin 16 (AREF+) of IC4. Since pin 16
also provides the PGD programming
function, JP1’s other jumper position
(marked ‘Prog.’) connects to programming header CON2.
The other programming signals from
IC4 are also connected to CON2. This
includes MCLR, 5V, ground and PGC
at IC4’s pin 15.
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Fig.2: this spectral analysis of the Multimeter Checker’s AC
output shows that the strongest harmonic is the third, over
40dB below the frequency of interest. The peak at 0Hz is due
to the DC offset and using a grounded oscilloscope, instead of
referring the signal to the 1.65V test point, TP2.
Pins 13 and 14 connect to the AC
reference output at TP1 and the 1.65V
AREF signal, respectively. These are
monitored by the ADC (analogue-to-digital converter) peripheral in IC4 to check
the frequency and amplitude of the output signal.
The frequency reference comes from
16MHz crystal X1, connected to IC4’s
pins 2 and 3 (CLKIN and CLKOUT). A
15pF load capacitor connects from each
side of the crystal to circuit ground so it
will oscillate correctly.
Three LEDs (LED1-LED3) connect to
IC4 via 10kΩ series resistors. The LED
cathodes are grounded, so the LEDs illuminate when pins 8-10 are pulled high.
Two tactile pushbuttons, S1 and S2,
connect to pins 11 and 12. The other
side of each switch is grounded while
the pins are internally pulled up, allowing the micro to detect when the button
is pressed. These LEDs and buttons provide a basic control interface for operating the Multimeter Checker.
Control of digital potentiometer IC2 is
over an SPI serial interface, with pins 5,
6 and 7 of IC4 being connected to pins
14, 12 and 11 of IC2. These lines have
the roles of SCK (clock), SDI (data) and
CS (chip select), respectively.
Since IC2 uses an unusual 10-bit
interface and a high data rate is not
needed, the SPI commands are sent via
bit-banged GPI/O operations. This also
allowed us to simplify the PCB layout
as we did not need to use the dedicated
SPI (MSSP peripheral) pins, but could
use any digital I/O pins.
Pins 17, 18 and 19 are associated with
IC4’s USB peripheral, so pins 18 and
19 are taken to the CON1 USB socket,
and pin 17 is fed 3.3V from REF1. This
means that the Multimeter Checker can
be controlled and monitored by being
connected to a computer’s USB port too.
The PIC16F1459 was chosen as a
suitable part because we could not
quite fit the necessary features onto a
14-pin microcontroller. But the presence of the USB interface means that
we can add some other interesting and
valuable features too.
Finally, we get to the power supply.
We’ve chosen a USB supply for its ubiquity. The 5V supply also gives more
headroom than the 3V coin cell from
the earlier design. After all, the 3.3V
voltage reference would not function
from a 3V cell. It also allows us to produce a higher test current than a coin
cell could supply.
LED4 and a 10kΩ series resistor are
connected across the incoming 5V supply to show that power is present. There
is no onboard 5V regulator; we rely on
the USB source to be within the normal 4.5-5.5V range. All of the onboard
components running from the 5V rail
can handle that.
Firmware
The firmware program that runs on IC4
has three main aspects. The first is the
fairly straightforward task of monitoring
the buttons S1 and S2 and controlling
LEDs LED1-LED3, providing a basic
user interface.
The second is the USB interface. This
appears as a virtual serial port when connected to a computer. Keystrokes from
the computer are stored in a buffer and
handled much like button presses, but
with extra functions.
There is also the option of ‘printing’
status updates to the serial port, so that
the Multimeter Checker can provide
more detailed information via the virtual serial port than can be displayed
with the LEDs.
Finally, IC4 is responsible for setting
and monitoring the AC reference voltage output. It has no control over the DC
voltage or current references, although it
uses the DC voltage reference to check
the AC voltage. The crystal oscillator
used for IC4’s timebase ensures that all
timing is accurate, particularly in measuring the frequency.
The microcontroller samples the
AC voltage waveform and checks
its period (and thus its frequency),
Practical Electronics | July | 2023
Scope 1: the blue trace is the AC output signal at TP1, while the red trace is measured at output pin 7 of IC1b. The orange,
yellow and green traces are measured at the top of each 1μF capacitor to the left of IC1a in Fig.1, from left to right.
peak-to-peak amplitude and average
absolute amplitude (with reference to
the 1.65V midpoint).
Since the 3.3V reference is used as
the scale for the ADC peripheral, the
absolute digital value of the peak-topeak and average amplitude values are
known and fixed in the program.
The sampling works as follows. A
timer interrupt fires 6000 times every
second and takes a sample of the AC
waveform. We chose this rate to allow
integer divisions of 50Hz, 60Hz and
100Hz into that timer. Although that
is not critical, it makes the calculations simpler.
Just over 240 samples are taken, corresponding to two complete cycles at
50Hz. This is so that we can ensure that
at least two positive-going zero crossings
occur within each sample set; these are
the points between which the period is
measured.
While 120 samples for a cycle at 50Hz
does not seem like much precision, the
firmware interpolates where the zero
crossings occur to within 1/16th of a
sample. It does this by calculating how
much the samples before and after the
zero-crossing are above or below the
zero point. This way, the period can be
measured with a resolution of around
one part in 960 for a 100Hz signal, or
better for lower frequencies.
Sampling must occur without interruption, so a set of samples is taken and
then processed. Adjustments are made
if necessary; then it goes back to sampling. By taking both the peak-to-peak
and average amplitude, the Checker can
also confirm that the waveform is sinusoidal, as a waveform with a different
shape will not be able to match both.
Oscillator adjustments
The four digital potentiometers each
have 256 steps. This is what limits the
amplitude accuracy to 0.5% (about 1
part in 200), as the steps are about that
far apart.
In practice, a small amount of dithering occurs, so the average over several
Practical Electronics | July | 2023
cycles will be closer to the target, close
to the accuracy of the 3.3V reference.
The frequency can be controlled more
closely than the amplitude, as three
potentiometers are involved. Rather
than stepping all three together, each is
incremented in turn, giving almost three
times as many steps.
This resolution results in steps of
around 0.1Hz at 50Hz up to 0.3Hz at
100Hz, around 0.3% in the operating
range. Like the amplitude, dither over
several cycles improves the longer-term
average accuracy of the frequency.
We’ll mention the full details of the
USB interface a bit later. It provides a
manual mode that allows direct control
of the digital potentiometers.
Construction
The Multimeter Checker is built on
a small PCB, 65 × 58.5mm, coded
04107221 and available from the PE
PCB Service – see Fig.3. It is mainly
populated with surface mounting
parts, although they are all pretty
large and easy to work with. The
only part with a smaller pin pitch
than 1.27mm is the USB socket, and
all passives are M3216/1206 parts at
around 3.2 × 1.6mm.
We’ll assume you have flux, solder
wick, tweezers and all the other gear for
working with these sorts of parts. Fume
extraction is a good idea when working
with flux.
Start by fitting the USB socket, CON1.
Apply flux to the pads on the PCB and
insert the socket’s locating posts into
their holes on the PCB. Clean the iron’s
tip and add fresh solder. Carefully apply
the tip to each lead in turn without
touching the metal shell.
After soldering each pin, use a magnifier to check that there are no solder
bridges, and if there are, use the wick
to remove them. If you can’t see, clean
off the flux residue with alcohol or a
flux cleaner.
If you find a solder bridge, apply
fresh flux to the leads and press the
wick against the bridge using the iron,
then carefully pull both away. When
the smaller leads look tidy, solder the
larger pads for the shell, turning up the
heat if necessary.
Fit the four ICs and REF1 next. These
are all SOIC (small outline IC) parts of
various sizes, but don’t mix up REF1,
IC1 and IC3 as they all have eight pins.
Note too that IC3 and REF1 face in opposite directions. Check the part markings
against the parts list and PCB silkscreen
as you go, making double sure that pin 1
is correctly oriented in each case before
soldering any pins.
For each part, apply flux, then tack
one lead in place, ensuring the correct
orientation by checking the silkscreen
dot and IC markings. If the pads are all
well aligned, solder the remaining pins;
otherwise, adjust as needed by reapplying heat from the iron.
Like with CON1, check for solder
bridges and remove them as needed.
It’s usually easier to solder all the pins
before removing any bridges.
Q1 is the only transistor on the board,
and it should be fitted as shown in the
photos and overlay. It’s the smallest part
overall, so be careful not to lose it. But
as the leads are widely spaced, it should
not be difficult to solder.
Install the capacitors next. The values
will not be marked on the parts themselves, so work with one value at a time.
The values required for each location are
shown in Fig.3.
Solder one lead, check that the part is
square, flat and even within its pads and
then solder the remaining lead. Refresh
the first lead if necessary. Remember
to add flux to the PCB pads as you go,
regularly cleaning the iron tip and then
adding fresh solder.
The resistors should be marked with
codes representing their values. They
are all the same size; check Fig.3 or
the PCB silkscreen to see which values go where.
We used larger pads for the 33Ω precision resistor in case part shortages
meant that we couldn’t get the high-
accuracy parts in an M3216/1206 size,
33
so don’t be concerned that the part is
much smaller than the pads.
Now fit the four LEDs. They are
all in one corner of the PCB and
have their cathodes to the right, as
indicated by the cathode symbol on
the silkscreen. You can use either
M3216/1206 surface-mounting types
or 3mm through-hole LEDs.
For through-hole LEDs, the anode
lead is usually longer. If using SMD
LEDs, they should have green cathode markings, but it’s pretty easy to
check them with a DMM set on diode
test mode.
Hold the probes on either side of the
LED (making sure it doesn’t fly away!).
If the LED lights up, the red probe is
on the anode and the black probe on
the cathode.
The two tactile switches mount near
the LEDs. Install these in the same
fashion as the other two-lead parts.
That completes the surface-mounted
parts, so you can now clean off the
flux residue. The remaining components are all through-hole types, and
some are optional.
Fit crystal X1 next. You should
not need an insulating pad under the
metal case as the two mounting pads
are covered with solder mask on the
top of the PCB. However, if the solder
mask in that area is damaged, add an
insulator or mount it off the PCB surface. Regardless, verify after soldering
it that its case is not shorted to either
pad underneath.
If you have already programmed the
microcontroller (IC4), you don’t need
to fit CON2, the in-circuit programming header. In this case, you could
also replace JP1 with a short wire link
across the pair of pads on the ‘R’ side
of the jumper.
Otherwise, fit both headers and install
the jumper shunt initially in the ‘P’ position for programming.
Although we have not used them on
our prototype, we’ve scattered a few
3mm holes around the PCB to fit standoffs if you want to mount the Multimeter
Checker to something.
Programming
If you haven’t pre-programmed the
microcontroller, you will have to program it now.
Using a PICkit 3, PICkit 4 or
Snap connected to CON2, load the
0410722A.HEX file onto IC4 using
the Microchip IPE (integrated programming environment). If you are
using a Snap, you likely will need to
supply power to the board; this can
be done using a USB lead connected
to CON1.
When power is applied, LED4 will
light up. So if you don’t see LED4
illuminated, check for power and
that the circuit has been built correctly before proceeding.
After programming, disconnect
the programmer and move JP1 from
the ‘P’ (program) position to the ‘R’
(run) position.
Testing
When the unit is powered up, it will
start in 50Hz mode, and LED1 should
be solidly lit to indicate this. Pressing
S1 will cycle through the 50Hz, 60Hz
and 100Hz modes. LED1-LED3 light
up in turn to show the current mode.
Pressing S2 switches between the
default pure sinewave to a more saturated waveform. You can use this
facility to check how the multimeter
responds to AC waveforms that are not
pure sinewaves.
Parts List – Multimeter Checker and Calibrator
1 double-sided PCB coded 04107221, 65 × 58.5mm available from the PE
PCB Service
1 mini USB Type B socket (CON1)
1 5-pin right-angle header (CON2; optional; only used for in-circuit programming)
1 3-pin header and jumper shunt (JP1)
2 small SMD two-pin tactile switches (S1, S2)
1 16MHz low-profile HC-49 crystal (X1)
Semiconductors
2 MCP6272 or MCP6L2 dual low-power rail-to-rail op amps, SOIC-8 (IC1, IC3)
1 AD8403ARZ10 4-channel 10kΩ digital potentiometer, wide SOIC-24 (IC2)
1 PIC16F1459-I/SO microcontroller programmed with 0410722A.HEX, wide
SOIC-20 (IC4)
1 MCP1501T-33E/SN 3.3V voltage reference, SOIC-8 (REF1)
4 green LEDs, 3mm through-hole or M3216/1206 SMD (LED1-LED4)
1 BC817 50V 800mA NPN transistor, SOT-23 (Q1)
Capacitors (all 10V+, X7R or C0G ceramic, SMD M3216/1206 or M2012/0805)
4 1μF
5 100nF
1 1nF
2 15pF
Resistors (all M3216/1206 1% 1/8W except as noted)
1 15kΩ
12 10kΩ
2 1kΩ
1 330Ω
2 100Ω
2 33Ω 0.1%
34
In this mode, the amplitude is set
to a high level (causing saturation of
the op amp output and clipping). The
LEDs indicate this mode by flickering
rapidly. This waveform may be easier
to verify during initial testing, as it
does not depend on the microcontroller correctly detecting the amplitude.
If the LEDs are flashing slowly
(around 1Hz), the Multimeter Checker
has not been able to verify that the
output frequency and amplitude are
correct. They might flash briefly on a
mode change, but there is a problem
if they continue flashing for more than
a few seconds.
In this case, first double-check that
JP1 is in the run position. This connects
the 3.3V reference to the microcontroller, so if it is still in the programming
position or not fitted, the micro cannot
confirm the AC output level.
One bad solder joint, especially
around IC1 and IC2, will be enough to
corrupt the waveform, so check those
areas too.
If you have an oscilloscope, you can
verify that the waveform at TP1 is a 1V
RMS sinewave offset by 1.65V DC. The
DC level can be eliminated by using
AC coupling on the ‘scope. Be careful
not to ground TP2 unless the supply
to the Multimeter Checker is floating
(for example, it is powered by a USB
battery pack).
USB control
Connecting the USB interface to a computer will provide a lot more information, so do this if possible, especially
if you are troubleshooting. The Multimeter Checker should not need USB
drivers on recent operating systems,
and you can simply use a serial terminal program to communicate.
We usually use TeraTerm on Windows, but programs like Putty, the
Arduino Serial Monitor or MMEdit
can also be used. On Linux, minicom
is one option.
Find out what serial port has been
allocated and open this with your terminal program. You will not need to set
a baud rate as it is a virtual serial port.
Typing ‘1’, ‘2’ or ‘3’ will change
the mode to 50Hz, 60Hz or 100Hz.
You will see the LEDs change as the
mode changes. Pressing ‘S’ selects
the sinewave mode, while the ‘R’ key
sets the saturated output (think ‘rectangular wave’).
Pressing the space bar will produce
a status report over two lines; this can
be seen at the top of Screen 1. The first
line shows the current control variables; ‘A’ controls the amplitude and
‘F’ controls the frequency. The second
line shows the reported amplitude (V)
and frequency (F).
Practical Electronics | July | 2023
Fig.3: most components are
relatively easy to solder;
the USB socket is a bit
tricky because its pins are
pretty close together. During
assembly, the most critical
thing to check is that all ICs
are oriented correctly, with
their pin 1s in the positions
shown. Also ensure that the
solder makes contact with
the pad and pin of each
device and check carefully
for solder bridges between
pins when you’ve finished.
Pressing ‘M’ sets manual control mode. All three LEDs will light
together in this case, and you can set
the A and F parameters manually. The
A parameter is changed with the full
stop and comma keys (think of the <>
above them on the keyboard). Increasing the A parameter will decrease the
output amplitude.
Once the output voltage drops
below 0.8V AC RMS, it may drop off
altogether as there is insufficient gain
around the feedback loop to maintain
oscillation. Still, it will recover once
a valid setting is selected.
You can change the frequency with
the ‘−’ and ‘+’ (or ‘=’) keys. The F
parameter can span between 1 and 750,
corresponding to approximately 45Hz
to over 1kHz. The Multimeter Checker
cannot accurately display frequencies
over about 600Hz, so the use of this
end of the range is not recommended.
Manual mode is terminated by
pressing S1 on the board, or selecting
the 50Hz, 60Hz or 100Hz modes from
the USB interface using the 1-3 keys.
Using it
Before you start using our Multimeter
Checker, you should refer to the calibration section in its manual (if present).
When using our Multimeter Checker,
you can check or calibrate a multimeter in the following modes:
Here we are probing TP1
and TP2 (ACV) with an
Agilent (now Keysight)
U1252A DMM. This result
is within 0.03% of the
expected value, which
shows that the meter’s
calibration is still good, and
demonstrates the accuracy
of the Multimeter Checker
and Calibrator.
Practical Electronics | July | 2023
• DC voltage – connect the probes
between TP5 and TP6 on a range like
20V DC and check/adjust for 3.300V.
• AC voltage – connect the probes
between TP1 and TP2 on a range like
2V AC and check/adjust for a reading
of 1.00V. This should be correct regardless of whether the meter is a True RMS
type or not, as it is a pure sinewave.
• Direct current – connect the probes
between TP3 and TP4 on a range like
200mA and check/adjust for 100mA
output. TP3 is the current source and
TP4 is the sink, so you might get a
negative reading unless the red probe
goes to TP3.
• Alternating current – connect the
probes between TP1 or TP2 with a
100Ω 1% or 0.1% resistor in series.
Set it for a low range and check for a
reading of 10mA.
• Resistance – connect the probes
between TP7 and TP8 on a range like
200Ω and check/adjust for a reading
of 33.00Ω.
• Frequency – connect the probes
between TP1 and TP2 on a range like
200Hz and check for a reading of 50Hz,
60Hz or 100Hz (set using pushbutton
S1 and LEDs1-3). For best results, press
S1 until LED3 lights and check/adjust
for 100.0Hz.
• Duty cycle – connect the probes
between TP1 and TP2 and check for a
reading of 50%. For best results, press
S1 until LED3 lights.
• True RMS readings –
press S2 to activate the
modified wave mode and
check the AC voltage reading
between TP1 and TP2. The
displayed voltage should be
above 1V RMS; our prototype produces 1.27V RMS in
this mode. A higher reading
suggests your meter uses the
average method. In comparison, a lower reading suggests
the peak method (as the peakto-peak voltage in this mode
is 3.3V, a peak-reading multimeter will generally show
around 1.17V).
Summary
While we set out to add an AC voltage
and frequency reference to an otherwise
straightforward DC reference design, we
think that being able to control the operation of the AC source manually will be a
handy feature that many people will use.
The USB interface also gives this handy
little device a range of possible uses.
One thing to watch out for is noisy
USB charger power supplies; they can
cause frequency measurements of the
ACV output to be unstable. In that case,
the best solution is to power it from a
USB power bank. A laptop USB port
usually provides enough clean power
to get stable readings from the Multimeter Checker.
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
Screen 1: a typical output from the
USB serial port. You can trigger
the two-line reports shown here
by pressing the space bar, while
the single-line entries are due to
manual changes in the amplitude and
frequency settings. Mode changes do
not produce any output but will be
seen in changes to the illuminated
LEDs on the Multimeter Checker.
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