This is only a preview of the February 2024 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
Items relevant to "Active Mains Soft Starter":
Items relevant to "ADVANCED SMD TEST TWEEZERS":
Items relevant to "Active Subwoofer For Hi-Fi at Home":
Articles in this series:
|
ADVANCED
SMD
TEST
T EEZERS
Part 1 by Tim Blythman
The SMD Test Tweezers and their successor, the Improved SMD Test
Tweezers, are both simple but useful tools. We have developed an
enhanced version with many more features and other improvements,
such as a larger screen and an easier-to-use interface.
I
f you have not already built an
SMD Tweezers kit, you may be wondering what the fuss is about.
After publishing our simple design
from October 2022 and the subsequent
refresh in May 2023, we were left with
no doubt that both variants were very
popular, with many kits sold.
Both these designs used a tiny 8-pin
8-bit microcontroller run from a single CR2032 coin cell to probe components by applying voltage via a resistor. The original Tweezers measured
resistance, capacitance or diode forward voltage and displayed the readings on a tiny OLED screen.
The Improved Tweezers used the
same hardware but a microcontroller
with more Flash memory, allowing
us to add extra features, such as the
ability to flip the display to suit being
used in either hand and an expanded
capacitance range.
Advancements
Both those variants of the Tweezers
were designed with small size, low
cost and simplicity in mind. They
both used just about the cheapest
microcontroller and smallest display
possible. Given their popularity, we
had to produce a follow-up, and knew
it needed to be good.
To be clear, this is not an incremental change over the first two designs,
but a vast improvement. You can
see from the list of features that the
Advanced Tweezers will do much
more than its predecessors.
One of the things we looked for
in a new microcontroller for the
Advanced Tweezers was a 12-bit
ADC (analogue-to-digital converter)
peripheral. This would provide extra
resolution over the 10-bit ADC that is
standard on most 8-bit PIC microcontrollers, such as those we used for the
previous Tweezers.
We reviewed some of the newer
8-pin PICs in October 2023, and have
since started using the PIC16F18146
in some projects. However, we
chose not to use an 8-bit PIC for our
Advanced Tweezers.
Instead, we have chosen a 28-pin
16-bit micro, the PIC24FJ256GA702.
It also has a 12-bit ADC peripheral,
so we still get the improved resolution. It also has some other interesting peripherals that we’ve put to
good use.
It isn’t much more expensive than
an 8-bit micro, but it is undoubtedly
a lot more capable. Importantly, it has
much more RAM and Flash memory,
so we can include many more modes
and settings. That extra memory also
means that the blocky font used on
the earlier Tweezers versions has
been replaced by one that is larger
and much more readable.
We’ve also used some interesting
techniques for probing and sensing.
So let’s introduce the various test
modes that are available.
Modes
The earlier Tweezers variants only
had a single mode which would
try to identify the device under test
and display its value. For a resistor
The Advanced SMD Test Tweezers are a bit bigger than
the earlier version but only because they incorporate a larger
display and extra pushbuttons. They also have new measuring modes,
including an oscilloscope, voltmeter, I/V curve plotter and a tone/square
wave generator.
26
Practical Electronics | February | 2024
or capacitor, it would show resistance or capacitance. For a diode, it
would work out the forward voltage
and polarity and display both. Dual
anti-parallel diodes, such as bi-colour
LEDs, would not be detected as they
conduct in both directions.
The Advanced Tweezers add many
more modes, which we will briefly
introduce before going into more
detail during the usage section of the
article (in the second part).
Like the older variants, several
modes are for characterising components such as resistors, capacitors
and diodes. Instead of attempting
to identify a device under test, the
Advanced Tweezers reports all of its
assessments together. This is made
possible by using a larger OLED display and removes the possibility of
the Tweezers identifying a component incorrectly.
There are still dedicated modes
for resistors, capacitors and diodes,
which each display only one value
in a large, clear font, but you do
need to select them. These modes are
especially handy when dealing with
surface-mounting capacitors, which
typically don’t have any distinguishing markings.
The diode mode also provides a
low, steady bias current which is
only interrupted by the reading cycle.
This has the advantage that you know
immediately that the LED is working
and what colour it is when it lights.
Checking the value of hard-to-read
surface-mounting parts is one of the
great advantages of the Tweezers format. It is especially handy for capacitors and LEDs, which often have subtle polarity markings.
There is now also a digital voltmeter mode, which shows the voltage across the probe tips, up to ±30V.
In oscilloscope mode, it can sample
at up to 25kSa/s with varying voltage
and time scales. It also offers some
basic trigger modes. It’s not likely to
make your bench ‘scope obsolete, but
it could be handy for probing signals
in the audio range. The ‘scope mode
uses the same ±30V-capable input
stage as the voltmeter mode, so it
offers the same range.
Many current digital oscilloscopes
we offer a serial decoding utility, and
the Advanced Tweezers do too. There
is only one input channel, so we can
only decode a UART data stream. The
Advanced Tweezers can accept and
decode a variety of baud rates and
data formats.
To overcome the limitations of
the diode checker only being able to
handle single diodes, we have implemented an I/V curve plotting mode.
Practical Electronics | February | 2024
Features and Specifications
❎ 10 different modes (see modes and options lists)
❎ Runs from a single CR2032 coin cell
❎ Sleep current <1μA
❎ Resistance accuracy ~1% when calibrated
❎ Voltage accuracy ~2% when calibrated
❎ Capacitance accuracy ~5% when calibrated
❎ Adjustable sleep timeout
❎ Adjustable display brightness
❎ Sleep timer can be paused for continuous operation
❎ Display can be rotated to suit left- and right-handed use
❎ Cell voltage displayed in all modes
❎ Auto calibration of some parameters
❎ Works down to 2.4V cell voltage
❎ Standby cell life: equal to shelf life
❎ Operating cell life: typically several hours of use
Modes
1
2
3
4
5
6
7
8
9
10
Resistance: 1Ω to 40MΩ, ±1%
Capacitance: 10pF to 150μF, ±5%; gives readings up to 2000μF
Diode forward voltage: 0-2.4V, ±2%
Combined resistance/capacitance/diode display
Voltmeter: 0 to ±30V ±2%
Oscilloscope: ranges ±30V at up to 25kSa/s
Serial UART decoder
I/V curve plotter
Logic probe
Audio tone/square wave generator
Oscilloscope options
❎ Voltage ranges: 0-5V, 0-10V, 0-20V, 0-30V, -5 to +5V, -10 to +10V, -20
to +20V, -30 to +30V
❎ Trigger on rising edge, falling edge, both or continuously (auto)
❎ Trigger level settable in 1V intervals
❎ Timebase (per div, 4 divs visible): 1ms, 2ms, 5ms, 10ms, 20ms, 50ms,
100ms, 200ms or 500ms
Serial UART decoder options
❎ Baud rate: 110, 1200, 2400, 4800, 9600, 14.4k, 28.8k, 38.4k, 57.6k or
115.2k
❎ 8N, 8O, 8E and 9N data length/parity
❎ 1 or 2 stop bits
❎ active high or active low
❎ text (terminal) or HEX display
I/V curve plotter options
❎ six-point sampling, live update, centred on 0V/0mA
❎ vertical scale (per div, four on screen): 1mA, 500μA, 200μA, 100μA
or 50μA
❎ horizontal scale (per div, four on screen): 2V, 1V, 500mV, 200mV or
100mV
Tone/square wave generator options
❎ frequency: 50Hz, 60Hz, 100Hz, 440Hz or 1kHz
❎ nominal amplitudes (pk-pk): 300mV, 600mV, 3V or 6V
❎ on/off control (defaults to off)
27
Advanced SMD Test Tweezers
Fig.1: while the 28-pin microcontroller chip is about twice the physical size of the SOIC-8 parts used for the earlier
Tweezers, there are many advantages to having so many available I/O pins. 10 pins are used for probing the tips, giving
much more range. Three more I/O pins handle buttons for control and calibration, while the OLED display can be
powered down completely using another spare pin.
The I/V curve shape will also allow
you to categorise many uknown or
‘mystery’ components.
The logic probe mode can differentiate between a high logic level, a low
logic level and a high impedance. It
also provides a digital trace so that
transient signals and digital waveforms can be seen.
Finally, a Tone Generator allows
square waves to be delivered at several frequencies and amplitudes. It’s
ideal for injecting test signals into
audio equipment or a clock signal
into a digital IC.
If you’re working with audio gear,
you might consider having two sets
of Advanced Tweezers; one to inject
a tone and a second to trace it. The
Tweezers also have the great advantage of being battery-operated, allowing them to be used without needing
to be referenced to ground.
We’ve provided three pushbuttons,
giving more control over what the
Advanced Tweezers are doing and
making them easier to work with. This
also allows us to add more extensive
calibration and configuration options
than the earlier variants.
Circuit details
Fig.1 shows the circuit diagram of
the Advanced Tweezers. It has some
improvements over the earlier versions that give better accuracy over a
wide range of component values and
also provide improved protection to
the microcontroller.
IC1 is a PIC24FJ256GA702 microcontroller, and its numerous I/O pins
allow us to connect to the device
under test (DUT) in various ways.
However, the design heritage shared
with the earlier Tweezers is evident.
Like the earlier Tweezers, a coin cell
The arrangement of the arms and tips is much the same
as that for the Improved Tweezers, using the
same arm PCBs and gold-plated pins
as simple, practical tips.
28
holder (BAT1) provides the nominal
3V supply to the circuit.
The three capacitors, and the single
10kW resistor connected to IC1’s pin
1 are essential for any application of
this microcontroller. The 10kW resistor pulls up the MCLR pin, allowing
normal operation unless a connected
programmer/debugger overrides it.
This pin and the other pins associated with programming IC1 are
connected to CON1 for this purpose.
You’ll note that the PGED and PGEC
programming pins (pins 4 and 5)
are not shared with any other components, making development and
debugging much easier.
The 100nF capacitors bypass the
main chip supply, while the 10µF
capacitor bypasses an internal regulator responsible for powering the
chip’s processor core.
The remaining ten resistors provide
the interface between the DUT (connected to the Tweezers tips at CON+
and CON−) and the microcontroller.
You might note that there is no
direct connection between the tips
and the microcontroller; any path
is always via at least one resistor.
This is another improvement to the
design and affords the microcontroller greater protection from the
outside world. That’s especially
important since we envisage many
Practical Electronics | February | 2024
Fig.2: the Advanced Tweezers uses IC1’s internal ADC to measure voltages,
using the voltage divider equation to calculate resistances and voltages across
diodes. This works much the same as the earlier Tweezers, but with the addition
of extra resistances and a 12-bit (instead of 10-bit) ADC to provide more range
and accuracy.
users probing active circuits with
the Advanced Tweezers.
The 1kW resistors to pins 2 and
26 provide the lowest-resistance
path between the microcontroller
and external circuitry, so we have
protected each of these with a dual
schottky diode clamping each to the
two supply rails. These shunt excess
current away from the I/O pins before
any semiconductor junctions within
IC1 can conduct current.
The three tactile pushbuttons, S1,
S2 and S3 also connect to I/O pins on
IC1. These pins are normally pulled
up weakly to the positive supply
internally to the microcontroller,
but they go low when the button is
pressed so IC1 can sense that.
MOD1 is the 0.96-inch (24mm)
diagonal OLED display. It is nearly
twice as wide and twice as tall as
the 0.49-inch (12.5mm) OLED used
in the earlier Tweezers, making for
a much more legible display packed
with more information.
Two I/O pins are required for its I2C
control interface with the microcontroller. We also use another I/O pin
to power the OLED’s VCC pin. That
means we can completely disconnect
power from the OLED module, guaranteeing it draws no current when
the Advanced Tweezers shut down.
We had problems with some apparently faulty 0.49-inch OLEDs drawing
too much current in standby mode,
so we’re eliminating that possibility
with this new design.
Measuring resistors and diodes
Naturally, much of the Advanced
Tweezers operation depends on the
firmware. Still, before we get to that,
we will explain how the microcontroller uses the sensing resistors in
different ways to measure various
components and voltages.
Practical Electronics | February | 2024
The microcontroller has an internal 1.2V bandgap voltage reference.
We measure this using the ADC
(with the supply as a reference) and
invert the result to calculate the supply voltage. For example, if the 1.2V
reference is measured as 40% of the
reference voltage, the supply must be
1.2V/0.4 or 3V.
Since the internal bandgap reference can vary by up to 5% from nominal, the exact value of the reference
needs to be determined during calibration for improved accuracy.
Fig.2 shows the arrangement that is
used for probing resistors and diodes.
Resistors Ra and Rb could be any two
of the 1kW, 10kW and 100kW resistors
available, while Rc and Rd have the
same options. The micro’s pins can be
driven high, low or left floating (in a
high-impedance mode).
Ra is typically pulled to the supply
voltage by driving it high, while Rb
is left high-impedance. Similarly, Rd
is connected to ground by driving it
low, and Rc is also high impedance.
Current thus flows from the micro via
Ra and into the DUT via CON+, then
back to ground via CON− and Rd.
Tests are then performed with
CON+ pulled low and CON− pulled
high to account for reverse-biased
diodes. For the following explanations, you can assume that any pins
not mentioned are left in a high-impedance state, so they do not affect
the calculations.
The microcontroller’s ADC (analogue-to-digital converter) peripheral is used to read the voltages on
the pins connected via Rb and Rc.
With the ADC scaled to use the supply voltage as its reference, the actual
value of the supply is not important
for resistance calculations.
The calculations are made with
raw ADC values. For the 12-bit ADC
used on the PIC24FJ256GA702, there
are 4096 steps, four times as many as
with a 10-bit ADC.
The calculations make use of the
voltage divider equation. Six tests
are performed using various combinations of the 1kW, 10kW and 100kW
values. These have 2kW, 11kW and
101kW total in series with the device
under test for both polarities.
The best resolution is when the test
and unknown resistors are similar in
magnitude, so our algorithm discards
invalid results and selects which of
the measurements will give the most
accurate final value.
The two tests with 2kW series resistance are also used for diodes. In this
case, the readings are scaled by the previously calculated supply voltage to
determine the diode forward voltage.
If the DUT voltage is close to the
supply voltage, it is assumed that the
DUT is not passing current. This will
be the case for reverse-biased diodes
or when no device is connected. So
a diode is only detected if a voltage
notably less than the supply voltage
is seen in one direction and a voltage close to the supply in the other.
In this case, the polarity and voltage
are reported.
Measuring capacitors
Fig.3 shows the different arrangement
used to measure the value of capacitors. One of the features of the ADC
on this microcontroller is the CTMU
or charge time measurement unit.
This view shows the spacing of the OLED module above the main PCB. Note
the header pin acting as a reinforcing spacer at one corner of the OLED. This
prevents the assembly flexing and causing a short between the two PCBs.
29
Screengrabs from part two, showing the Advanced SMD Test Tweezers in operation
Screen 5: the AUTO SET tunes
three calibration parameters by
performing internal measurements
with the tips open. It depends on the
previous calibration settings being
entered and correct.
While the CTMU has many applications, what matters to us is that
it includes a programmable current
source that can be delivered to an
ADC pin during sampling.
The ‘charge time’ naming comes
from the fact that it can be controlled
by external triggers and used to measure intervals between those triggers
by measuring the amount of charge
delivered to a known capacitor.
Instead, by delivering a known current over a known interval, we can
apply a fixed amount of charge, and
with the equations shown in Fig.3,
we can measure capacitance.
That means we don’t need to resort
to complex calculations involving
logarithms which are often needed
to analyse RC circuits.
The 8-bit PIC devices we used for
the earlier Tweezers avoided logarithms by using an approximation and
limiting the state of charge to regions
where the approximation would be
most accurate.
For this test, Rd is connected to
ground and Ra is connected to the
CTMU current source. An initial ADC
sample is taken, followed by a second
sample after a known interval, with
Screen 14: the initial Meter display
mode, which can read up to 30V with
both negative and positive polarities
(with respect to CON+ and CON-). The
resolution is 10mV to 9.99V and 0.1V
above that.
the current source active between the
two samples.
In both cases, 1kW series resistors
are used. This is because the resistors will drop some voltage due to the
current flowing, and the 1kW resistors will drop the least voltage. Fortunately, it will be the same for the
first and second readings, so it will
cancel out.
Five different currents can be
applied, so we can take multiple
readings. To extend the range further, shorter and longer durations are
used, giving six readings over different orders of magnitude.
Like the resistor measurement, the
readings near the middle of the range
are chosen. High readings are ignored
as the current source tends to saturate as its output nears the supply
voltage. That would result in inaccurate readings.
Since the voltage is the denominator of the equation, lower values are
disregarded because this will diminish
the resolution. Higher values lead to
closer steps between their respective
reciprocals and thus better resolution.
The capacitance calculation
depends on the supply voltage, CTMU
Screen 15: Scope mode is handy, even
though there are only 100 horizontal
and 48 vertical pixels in the trace
area. It samples at up to 25kHz,
is suitable for audio use, and has
adjustable trigger settings.
current and time, so the expected
accuracy is not as good as for resistance or diode voltage. Still, with
calibration, it should be within 5%.
Between measurements in the resistor and capacitor modes, the 1kW
resistors in each group are pulled
low, and the remaining pins are left
floating. Apart from minimising current flowing in or out of floating pins,
this also serves to discharge any connected capacitor, so it is ready for the
next measurement cycle.
One exception is in diode mode. In
this case, the CON+ terminal is pulled
high instead of low to provide a bias
to light an attached LED, allowing it to
be visually checked. A light-emitting
diode connected in the forward direction will illuminate except for the
period when the reading is done, when
it will appear to flicker off briefly.
Scope and meter modes
Another arrangement is used for the
scope and meter modes that allows
them to read voltages outside the
Tweezers’ supply rails. Four more 1kW
resistors are put into play. Of each
pair, one is pulled high at the micro
end and the other low. This situation
Fig.3: the constant-current source of the CTMU peripheral greatly simplifies the measuring of capacitances. It eliminates
the need for the processor-intensive logarithmic calculations needed to derive a capacitor value from the time constant of
an RC circuit.
30
Practical Electronics | February | 2024
Screen 16: we find the UART Serial
Decoder indispensable at times.
Like the Scope mode, it is highly
configurable in terms of baud rates,
bit depth and data polarity. This
shows the TXT view.
is shown on the left of Fig.4, with the
simplified circuit to its right being
functionally equivalent.
Each tip is thus subjected to a 20:1
voltage divider biased to half the supply voltage. Readings are taken by measuring the difference in the voltage
between V1 and V2 and multiplying by
21. With a nominal 3V supply, we can
measure up to around 30V (differential) between CON+ and CON−. Biased
differential inputs allow positive and
negative voltages to be measured.
It’s possible for current to flow
through the unused 1kW and 100kW
resistors if the applied voltage is
greater than the supply voltage. The
current through the 1kW resistors is
shunted to the supply rails by D1 and
D2. The 100kW resistors will conduct
much less current, and this will flow
through the microcontroller’s internal
protection diodes.
These unwanted currents dictate
the useful upper voltage limits of
the scope and meter modes. Voltages
beyond those limits could cause damage to the microcontroller.
Damage could also occur if excess
voltage is applied while the pins are
being driven (as for resistor, capacitor
Screen 17: the Serial Decoder also
offers a hexadecimal mode, useful for
seeing binary data and control codes.
Framing or parity errors are shown,
which can help you to determine the
data format.
and diode modes), since these currents
will now flow through the chip’s internal output transistors instead of the
external and internal protection diodes.
We found that one of our earlier
prototypes was running cells flat
even when not being used; this was
because the damaged microcontroller
was drawing excess current in sleep
mode. If you find your Tweezers are
going through cells excessively, that
could be why.
So care must be taken only to apply
higher voltages in modes when the
Tweezers concern with the older
Tweezers designs, as they did not
have any modes to measure externally applied voltages, and were only
designed for use with passive devices.
Modes that expect digital signals,
such as the logic analyser and serial
decoder, simply pull CON− to ground
via its 1kW resistor. CON+ may be left
floating or weakly pulled up or down
by the 100kW resistor to detect the difference between high, low and high
impedance logic levels.
Firmware
The firmware program on IC1 is
responsible for initialising all the
Fig.4: the Meter and Scope modes use a set of four fixed resistors to provide a
biased divider capable of measuring voltages above and below the Advanced
Tweezers’ supply rails. The circuits on the left and right are equivalent.
Practical Electronics | February | 2024
Screen 18: while Diode mode cannot
report dual diodes such as bicolour
LEDs, the I/V Plotter shows both
polarities. The current and voltage
scales can be zoomed in for more
detail.
peripherals and the OLED display. It
coordinates the measurements, reads
the pushbuttons and controls the display as needed.
Apart from the main program loop,
a timer interrupt is set to fire about
three times per second, triggering display updates at a comfortable rate.
The code is modular, and each of
the individual modes is much like a
self-contained program that is called
upon during the program loop. Each
makes the measurements it needs and
displays the results.
The buttons are checked and flags
are set for each mode to process in
accordance with its operation.
Power consumption
The processor runs at a modest 4MHz
instruction clock (down from the
maximum possible 16MHz) to minimise power consumption and thus,
the load on the coin cell. We could not
maintain the desired screen update
rate at lower speeds than this.
During some of the scope mode’s
sample periods, the clock is sped up
to 16MHz to allow faster ADC sampling rates. There are also periods
where no urgent processing is needed,
With three pushbuttons, calibrating
and changing modes is much easier
than earlier version of the Tweezers.
31
Screengrabs from part two, showing the Advanced SMD Test Tweezers in operation
Screen 19: the Logic Analyser shows
whether it detects a high, low or high
impedance logic level. A scrolling
chart also shows a brief history,
making it easier to see transients and
repeating patterns.
Screen 20: like Scope mode, the
Tone Generator is handy at audio
frequencies or as a simple clock
generator. It can produce square
waves at five different frequencies and
four different amplitudes.
Screen 21: the Auto screen is only
one of ten pages but encompasses
and surpasses the abilities of its
predecessors. It shows resistance,
capacitance, diode polarity and
forward voltage.
The hole at upper left is for a Nylon M2 screw to prevent children from removing the coin cell. While it would be quite
difficult for them to remove it anyway, we want to ensure it is safe.
in which case the DOZE feature is
activated. The processing core runs
at an even lower fraction of its maximum speed, reducing power usage
even further.
There is a timer counting off the
timer interrupt. When this expires,
a routine is called to power off the
OLED and put the peripherals and
I/O pins into a low-power state, after
which the processor goes into the lowest-power SLEEP state.
By completely powering off the
OLED, we avoid any possibility that
it is not in its lowest possible power
state. The OLED modules we used
Parts List – Advanced SMD Test Tweezers
1 double-sided main PCB coded 04106221, blue (28 × 36mm) *
2 double-sided arm PCBs coded 04106212, blue (100 × 8mm) *
3 gold-plated header pins (for tips and OLED support)
1 PIC24FJ256GA702-I/SS microcontroller programmed
with 0410622A.HEX (IC1) *
1 0.96in 128×64 I2C OLED module, blue/cyan or white (MOD1)
2 BAT54S dual series schottky diodes, SOT-23 (D1, D2)
2 100nF 50V X7R ceramic capacitors, SMD M2012/0805 size
1 10μF 6V X7R ceramic capacitors, SMD M2012/0805 size
2 100kW ⅛W 1% SMD resistors, M2012/0805 size
3 10kW ⅛W 1% SMD resistors, M2012/0805 size
6 1kW ⅛W 1% SMD resistors, M2012/0805 size
3 small SMD two-pin tactile switches (S1-S3)
1 surface-mount 32mm coin cell holder (BAT1)
2 100mm lengths of 10mm diameter clear heatshrink tubing
1 5-pin right-angled header, 2.54mm pitch (CON1; optional, for ICSP)
1 label (optional; see Fig.8 next month)
1 M2 × 6mm Nylon screw
2 M2 Nylon nuts
1 CR2032 or CR2025 lithium coin cell
* We will provide details for purchasing these items in Part 2 next month
32
in the earlier Tweezers have a sleep
mode that initially appears quite
effective but sometimes had a current draw that crept up higher than
we expected.
Interrupts triggered by a change
in the switch states are used to wake
up the processor while it is stopped.
It resumes by doing much the same
as when it first initialises, since the
peripherals have all been put into
low-power modes too.
The SLEEP mode keeps the RAM
contents, so resuming from sleep will
thus retain all the same mode settings
and parameters.
Our measurements during SLEEP
recorded a consistent current draw
around 700nA, much lower than the
earlier Tweezers variants. At these levels, the cell’s self-discharge is likely
to be more significant than the actual
circuit current.
We also sought to minimise current
draw during normal operation; this
is typically in the single-digit milliamps, depending on the operating
mode. This is critical, as the amount
of usable capacity for a coin cell (as
measured in mAh) is higher with a
lower current draw.
So higher consumption not only
reduces the time that a given cell
capacity can be used, but also tends
Practical Electronics | February | 2024
Screen 23: the Cap screen works
similarly, displaying just the
measured capacitance in large text.
It’s perfect for working out which part
is which among a pile of unmarked
SMD capacitors.
Screen 22: the Res screen provides
the same resistance information as
the Auto screen but in a larger font,
which is handy for checking and
sorting through different resistor
values.
to reduce that capacity. The internal
resistance of a coin cell is of the order
of 20W, so a current in the milliamps
will also reduce the voltage available
to the circuit by a noticeable amount,
around 0.1V.
Apart from its internal controller,
the OLED only draws current for lit
pixels, so there is the option to adjust
the brightness and thus compromise
between visibility and power consumption. The OLED is typically the
greatest drain on the battery.
The OLED dictates the 2.4V minimum voltage as it tends to fade
and flicker below that level. The
microcontroller will work down to
around 2V, but running this low also
limits the effective sampling range
of the ADC.
We initially used a pretty thick font
for some of the displays. By changing
to a lighter font with thinner strokes,
we reduced the current by over 3mA
in some modes!
We found that the display was perfectly visible indoors at a reduced
brightness, so we have set the default
brightness to be somewhere in the
lower end of its range, prolonging cell
life and reducing the voltage drop.
You can increase the brightness via
GET T
LATES HE
T COP
Y
OF
TEACH OUR
-IN SE
RIES
A
Order direct from
Electron Publishing
VAILAB
L
NOW! E
PRICE £8.99
(includes P&P to UK if ordered direct from us)
Screen 24: the diode screen is similar
to the Diode display on the Auto
screen but a bias is applied from
CON+ to CON− between tests. This
lets you quickly check the polarity
and operation of LEDs.
the settings if necessary, eg, for use
in very brightly lit areas.
Next month
Because this is a reasonably complicated instrument (at least in terms
of its modes and features), we don’t
have space in this issue for the full
construction, calibration and usage
details. That will all be covered in
the final article next month. Some
screengrabs showing the Tweezers in
operation are shown above.
Reproduced by arrangement with
SILICON CHIP magazine 2024.
www.siliconchip.com.au
EE
FR -ROM
CD
ELECTRONICS
TEACH-IN 9
£8.99
FROM THE PUBLISHERS OF
GET TESTING!
Electronic test equipment and measuring
techniques, plus eight projects to build
FREE
CD-ROM
TWO TEACH
-INs
FOR THE PRICE
OF ONE
• Multimeters and a multimeter checker
• Oscilloscopes plus a scope calibrator
• AC Millivoltmeters with a range extender
• Digital measurements plus a logic probe
• Frequency measurements and a signal generator
• Component measurements plus a semiconductor
junction tester
PIC n’ Mix
Including Practical Digital Signal Processing
PLUS...
YOUR GUIDE TO THE BBC MICROBIT
Teach-In 9 – Get Testing!
Teach-In 9
A LOW-COST ARM-BASED SINGLE-BOARD
COMPUTER
Get Testing
Three Microchip
PICkit 4 Debugger
Guides
Files for:
PIC n’ Mix
PLUS
Teach-In 2 -Using
PIC Microcontrollers.
In PDF format
This series of articles provides a broad-based introduction to choosing and using a wide range
of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints
and tips on using, and – just as importantly – interpreting the results that you get. The series
deals with familiar test gear as well as equipment designed for more specialised applications.
The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics.
The series crosses the boundaries of analogue and digital electronics with applications that span the full range of
electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is
something for everyone!
Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either
extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone
instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10
(including components, enclosure and circuit board).
© 2018 Wimborne Publishing Ltd.
www.epemag.com
Teach In 9 Cover.indd 1
01/08/2018 19:56
PLUS! You will receive the software for the PIC n’ Mix series of articles and the full Teach-In 2 book – Using PIC
Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide.
ORDER YOUR COPY TODAY: www.electronpublishing.com
Practical Electronics | February | 2024
33
|