Our SMD Test Tweezers project from the October 2022 issue has been
extremely popular. This did not come as a surprise given that they
are handy, compact, easy to use, easy to build and the cost
is reasonable. We decided to see what features
we could add simply by upgrading the
microcontroller at its heart.
Improved Test
SMD Tweezers
T
he 2022 SMD Tweezers are
a simple but clever design. A
PIC12F1572 eight-pin microcontroller powered from a button cell
is used to probe resistors, diodes and
capacitors and then display its findings on a tiny OLED screen.
The PIC12F1572 does a respectable
job, but the Test Tweezers software
takes up all but 42 bits of the available Flash memory, leaving no room
for expansion.
We used the PIC12F1572 for the
2022 SMD Test Tweezers as it was the
cheapest available at the time, apart
from its close relative with less memory, the PIC12F1571. Until recently,
the PIC12F675 and later PIC12F617
were our 8-pin micros of choice, but
Microchip keeps bringing out new
parts with better performance and
more features at lower prices, so we
try to keep up.
The PIC12F1572 is more capable
than the older PIC12F675. However,
when we looked into upgrading the
Test Tweezers with some software
improvements, we realised that the
PIC12F1572 did not have enough
free memory to add new features or
improve the existing ones. For that,
we would have to move to the latest
generation of PICs.
So when a new family of PICs
became available, we began to investigate what we could add by using them.
the micro; the analogue-to-digital converter (ADC) and watchdog timer that
the software requires are found in most
PIC microcontrollers. The low-power
sleep mode is quite standard too, and
is essential for standby operation when
powered from a cell. This allows the
Tweezers to be left idle but ready to
work at a second’s notice.
The I2C interface to the OLED display is emulated in software by toggling GPIO (general purpose input/
output) pins, a technique often known
as ‘bitbanging’. This all means that just
about any 8-pin microcontroller with
more program memory could be used
for the SMD Test Tweezers.
After developing the original SMD
Test Tweezers, we became aware of the
PIC16F152xx series of microcontrollers. The range spans parts from eight
to 40 pins. While the range has features
that are modest by current standards,
they are still more capable than older
devices such as the PIC12F675.
The PIC16F15213 and PIC16F15214
are the 8-pin parts in the range,
and they are cheaper than the
PIC12F1572, although the current
A new PIC
The original (2022) SMD Test Tweezers
don’t use any exotic peripherals within
Caution
Like any project that uses coin cells,
the Tweezers should be kept well
away from children who may ingest
them. The Tweezers also have quite
pointy tips, another reason to keep
them out of reach of young fingers.
part shortages mean that availability is patchy. (For example,
Mouser expects to have stock of the
PIC16F15214-I/SN by 1 May 2023,
which is annoying, but not the terrible 12-month lead times we endured
at the height of the pandemic!)
Importantly, the PIC16F15214 is
available in the SOIC package and
has twice the Flash memory of the
PIC12F1572. Microchip does a pretty
good job of maintaining pin compatibility between parts, and the PIC16F152xx
series is no exception.
The upshot is that the PIC16F15214
is both cheaper and fully capable of
replacing the PIC12F1572 as the controller for the SMD Test Tweezers, while
also having the larger program space
needed for us to add new features.
Tweezers 2.0
We have implemented three major
updates to the SMD Test Tweezers.
First, we expanded the capacitance
measurement range in both directions
(it can measure both larger and smaller
capacitances than before).
Second, we added a calibration and
setup procedure.
Finally, we improved usability for
left-handed people (or those who want
to hold something like a soldering iron
in their right hand) by allowing the
screen display to be rotated by 180°.
These improvements have all been
made in software, so apart from changing the PIC12F1572 to the PIC16F15214,
the hardware is identical and the general
operation is much the same.
By Tim Blythman
Practical Electronics | May | 2023
39
Features and specifications
∎ Uses identical hardware to the original Tweezers (PE, October 2022)
apart from the PIC microcontroller
∎ Identifies component type (resistor, capacitor, diode or LED) and
measures critical values
∎ Resistors: value from 10Ω to 1MΩ
∎ Diodes: forward voltage up to about 3V
∎ Capacitors: value from (approximately) 10pF to 150μF
∎ Cell voltage with nothing connected
∎ Low power sleep when idle avoids the need for an on/off switch
∎ Instant wake-up by touching probe tips together
∎ Option to select left-handed or right-handed display
∎ Calibration of internal and contact resistance
Circuit details
Fig.1 shows the circuit, which is the
same as last time (PE, October 2022),
besides IC1. All the readings are displayed on a tiny OLED module connected to CON2.
IC1 drives its RA5 (pin 2, IOTOP)
and RA4 (pin 3, IOBOT) pins high and
low and measures the voltage present
on pin 5 with its ADC peripheral. For
example, it can determine the resistance of a resistor connected between
the CON+ and CON− points using the
voltage divider equation.
Diodes will present their forward
and reverse voltages between CON+
and CON− when the micro applies
a voltage. The micro determines the
diode’s orientation, showing its polarity and forward voltage.
Capacitors are first charged by
bringing IOTOP high and IOBOT low
and then characterised by measuring
the rate of discharge when IOTOP is
brought low. The Tweezers can even
measure their own supply voltage
by reading the voltage of its internal
1.024V reference relative to that supply voltage.
These features are already present
in the original SMD Test Tweezers, so
we suggest you refer to the original
October 2022 article for more detail
on how these features work and how
the values are calculated.
Improvements
The upper limit of the capacitance
range was limited by the use of an 8-bit
counter to time the discharge. With
more Flash memory and RAM available,
we can instead use a 16-bit counter.
In theory, this expands the range by
a factor of 256, but in practice, using
this entire range is not possible. The
upper limit is around 150μF now,
equivalent to about 12 bits or a factor
of 16 higher.
The first reason for this is that
higher values would overflow the
32-bit mathematical calculations that
are required. The second is that the
40
time needed to charge and discharge
a larger capacitor becomes unreasonably long, in the order of several seconds between readings.
The only way to overcome this
would be to change the series test
resistor, which would affect the other
readings too.
The relatively high value of the
series test resistor also means that
capacitance readings can be distorted by leakage current. Since
leakage is typically higher in higher-value parts, especially in electrolytic capacitors, the accuracy and
usefulness of these higher ranges
are less than what seems theoretically possible.
So higher-value capacitors can be
measured and will return a reading, possibly after a brief delay, but the accuracy
will not be as good as for lower values.
Low-capacitance measurements
Values lower than 1nF are measured
in an entirely different fashion. This
method is so sensitive that it can measure the capacitance of the touch of a
hand, in the order of picofarads.
It’s called shared capacitance sensing, and we used it to detect finger
touches in the ATtiny816 Breakout
Board of July 2021.
It works by comparing the relative
magnitude of two capacitors by initially charging one and discharging the
other, as shown in Fig.2. When they
are connected, the charge present is
shared between the two in proportion
to their capacitances.
The ratio of the initial and final
voltages relates directly to the ratio
of the capacitances. The theory and
mathematics are explained further in
the ATtiny816 Breakout Board article.
In the case of our new SMD Test
Tweezers, a capacitor connected to
CON+ and CON− is charged up via the
10kΩ resistor. The second capacitor is
actually the tiny internal capacitor that
is used to sample and hold the voltages
read by the microcontroller’s ADC.
This capacitor is nominally 5pF, and
it is discharged by sampling an ADC
channel connected to ground.
Fortunately, the ADC peripheral has
a selection to make an internal ground
connection, so this doesn’t need an
extra pin. The external capacitor is disconnected from its resistor, and the two
capacitors are connected by taking an
ADC reading from the external capacitor.
Improved SMD
Test Tweezers
Fig.1: the circuit for the updated Tweezers is practically the same as the old
version, except IC1 is now a PIC16F15214. It can perform all its tests by
applying different voltages to the IOTOP and IOBOT pins and testing the voltage
on the IOTEST pin.
Practical Electronics | May | 2023
A few constructors had difficulty
finding the brass strips we
recommended for the original
Tweezers. Standard header pins are
a substitute and are easily aligned
for soldering while in their plastic
shrouds. There are even gold-plated
versions available.
We used a low-profile header socket (Altronics P5398) so that we could remove
the OLED module during prototyping, to allow access to the programming pins.
This also required us to cut down the header pins on the underside of the OLED
and remove the plastic spacer block. The alternative is to simply solder the
OLED directly to the main PCB.
An equation similar to the voltage divider equation is used on the
ADC result to calculate the value of
the external capacitor. The way the
capacitors share the charge is analogous to how resistors share voltage in
a divider chain.
The software also makes minor
adjustments to account for some of the
stray capacitance that is present and significant at these magnitudes. We made
tests on real capacitors in the picofarad
range to fine-tune these readings.
The lower limit is fairly arbitrary
and is chosen to avoid the SMD Test
Tweezers detecting stray capacitance
as a component to be measured, which
could cause them not to power down
correctly. At these scales, even the way
the Tweezers are held can change the
reading substantially.
As the ADC reading nears its upper
limit for larger capacitances, the resolution is poor around 1nF, and steps
grow to be as far as 100pF apart. So the
readings using this method are always
shown as pF, and other methods are
used for measurements in nF or μF.
We will detail the calibration and
setup process after the construction
is complete.
Construction
The assembly procedure is identical
to the October 2022 design, but we’ll
go over it again for those who haven’t
seen that article. The SMD Test Tweezers are built using three PCBs, with
the main one coded 04106211, measuring 28 x 26mm and available from
the PE PCB Service. Refer to the PCB
overlay diagrams, Fig.3 and Fig.4,
during construction.
The main PCB is not hard to build,
even if the parts are all surface-mounting types. Gather your SMD tools and
supplies. We recommend a fine-tipped
soldering iron, a magnifier, some flux
paste, solder wicking braid and tweezers, at a minimum.
The small PCB needs something to
hold it in place. If you don’t have an
appropriate vice tool, you can use an
adhesive putty like Blu-Tack instead.
If possible, set up some fume
extraction to deal with the extra smoke
that comes from working with flux, or
work near an open window or outside.
A tip-cleaning sponge is handy too.
Apply flux to the top PCB pads for
IC1 and the three passive components,
then rest IC1 in place using tweezers,
ensuring the pin 1 dot or bevel is
Fig.2: Cx is the device under test (DUT) connected to the Tweezer probes, while
C1 is the ADC sample-and-hold capacitor inside the microcontroller. The
capacitors are connected by sampling Cx with the ADC. If the value of Cx equals
C1, the resulting voltage is half the initial voltage. It’s analogous to a resistive
voltage divider, and the formulas are much the same, with the capacitor charge
replacing the voltage across the resistors.
Practical Electronics | May | 2023
towards the curved end of the board.
Align the part within the pads, clean
the iron’s tip, apply fresh solder and
tack one lead in place.
Adjust the IC if necessary to ensure
it is flat against the PCB and aligned
to the pads. Then solder the remaining pins, cleaning your iron’s tip and
adding solder as necessary.
Use the braid to remove any solder bridges by adding more flux, then
press the braid against the excess solder with the iron. Carefully drag both
iron and braid away when the solder
has been absorbed.
The remaining three components
are not polarised, so their orientations are unimportant. The capacitor
sits near CON−, while the two identical resistors flank IC1 at its other end
and side.
Use a similar technique to IC1. Tack
one lead, adjust the part, then tack the
other lead. You can also go back and
refresh any leads if the joint doesn’t
look right. It should be smooth and
glossy; you can add more flux at any
stage to help improve solderability.
Then solder the single component
to the back of the PCB. Centre the cell
holder to align the two external pins
to their pads. If your iron is adjustable, turn it up while soldering this
larger part. You should also ensure that
the wider opening on the cell holder
faces the rounded edge of the PCB to
allow access for the cell to be fitted
and removed.
As before, apply flux, tack one lead
in place and adjust the position. Then
solder the other lead. For these much
larger pads, it can help to apply extra
solder directly to the pad to create
a robust fillet, which you can see in
our photos.
With the surface mounted parts fitted, you can clean up the PCB using
the flux cleaner designated by the
flux’s data sheet. Methylated spirits or
41
Fig.3: construction of the Tweezers is the same as last time (October 2022) except that IC1 is a different, pin-compatible
microcontroller with more memory. There aren’t many components to fit but make sure that IC1 is oriented correctly.
isopropyl alcohol are good all-round
alternatives for cleaning many fluxes
too, while general-purpose flux cleaners are also available (and generally
work better than plain alcohol).
Just ensure that any flammable solvent has fully evaporated before moving on to the next steps.
Programming IC1
You have two options for loading IC1’s
firmware. First, you can buy a preprogrammed PIC from the PE PCB Service;
or, you can buy and program a PIC yourself. (Skip the following if your microcontroller has been preprogrammed.)
As we noted in the panel, the
PIC16F15214 is a much newer part
than the PIC12F1572, so you will
need a fairly new programmer and a
new version of Microchip’s MPLAB
X IPE (integrated programming environment). It can be downloaded
as part of the MPLAB X IDE from:
https://bit.ly/pe-may23-ide
We’ve tested with versions v5.40
and later. You may also need to download a DFP (device family pack); this
can be downloaded from within the
IDE, and the IPE then detects that the
DFP is installed. You should look for
the PIC16F152xx family.
You will also need a recent programmer such as a Snap or PICkit 4
as the older PICkit 3 is not supported
for these parts.
Connect the programmer to the PCB
at CON1, aligning the arrows that mark
pin 1. You could solder on a header,
but we find that holding a short header
strip in place and pressing it firmly
against the pads to make contact is
usually sufficient.
Select the PIC16F15214 part and
open the 0410621B.HEX file, which
is available for download from the
May 2023 page of the PE website:
https://bit.ly/pe-downloads
You may need to change the settings to allow the programmer to
42
apply power. Then click ‘Program’
and check that the part programs and
verifies correctly.
Tweezer arms
The two arm PCBs should be attached
next, as the OLED module covers much
of the main PCB, limiting access.
Our first version of the SMD Test
Tweezers used small pieces of brass
strip to give the arms finer tips than
just the bare PCBs would provide.
If you can’t find a brass strip, then
we suggest an alternative that will
provide your Tweezers with goldplated tips!
Many header pins are gold-plated
and are a good size for working with
small components. These can be used
instead of the brass strip, but unlike
the brass strip, we found it easier to
solder these to the arms after attaching
the arms to the main PCB.
The other advantage of using the
header pins is that they are a good
fit for breadboards and jumper wires,
making it very easy to connect the SMD
Test Tweezers to other components for
hands-free readings.
We recommend fitting the arms
roughly in line with the edges of the
PCB but slightly tilted inwards with
around 15mm separation at the tip
ends. Like the SMD parts, roughly tack
the arms in place and adjust them to
your liking.
We prefer fitting the arms with the
writing and main contact trace running
down the inside. This helps shield and
isolate the trace from outside contact
or stray capacitance.
Test the action/pressure of the Tweezers when the arms are positioned.
When you are happy, apply a generous
amount of solder on both sides of the
main PCB to secure them firmly in place.
Parts List – Improved SMD Test Tweezers
1 double-sided PCB coded 04106211, 28 x 26mm (main PCB)*
2 double-sided PCBs coded 04106212, 100 x 8mm (Tweezer arms)*
1 PIC16F15214-I/SN or PIC16F15214-E/SN 8-bit microcontroller
programmed with 0410621B.HEX, SOIC-8 (IC1) ●*
1 0.49-inch 64x32 I2C OLED module [eBay, AliExpress – for example, at the
time of writing eBay item 154706482381]
1 surface-mount coin cell holder (BAT1) [Digi-key BAT-HLD-001-ND,
Mouser 712-BAT-HLD-001 or similar]
1 CR2032 or CR2025 lithium coin cell
1 5-pin right-angle male pin header (CON1; optional, for programming IC1
in-circuit)
1 100nF SMD 50V X7R ceramic capacitor, 3216/M1206 size [Altronics
R9935]
2 10kΩ 1% SMD resistors, 3216/M1206 size [Altronics R8188]
2 15 x 2mm short pieces of thin (eg, 1mm) brass sheet for tips (optional)
OR
2 gold-plated header pins for tips (see text) ●
1 40mm length of 30mm diameter clear heatshrink tubing (optional)
2 100mm lengths of 10mm diameter heatshrink tubing (optional)
1 4-way low-profile female header strip (optional, for CON2; can be cut from
Altronics P5398) ●
● These parts have been changed compared to the original Tweezers
* Available from the PE PCB Service
Practical Electronics | May | 2023
Fig.4: the PCB for the
Tweezer Arm section.
To fit the tips, find a strip of about
six pin headers (to maintain the 15mm
separation) and while the pins are still
in the plastic holder, solder the tips of
the arms to the short ends of the headers. Using the holder will keep the pins
parallel and even.
Again, apply a generous amount
of solder when you are happy with
the tips, then carefully and evenly
pull the arms and their tips out of
the plastic holder. We find that some
pointy-nosed pliers are handy to help
in this situation.
OLED screen
The final step is to fit the OLED display module. You can solder the module directly to the main PCB. But since
we had to do a lot more testing for this
new version, we used a low profile
header socket to allow the OLED to
be removed. This is necessary because
the programming pins are also used
to interface with the OLED screen.
We used the PIC Programming
Helper from June 2022 to help with
our testing. But we also needed to do
some testing and tweaking on the final
design, so having a removable display
was handy for these later stages. We
used a low-profile (5mm high) header
socket to keep the unit compact, and
it’s what you can see in our photos.
But we recommend using the direct
mounting method unless you are considering designing your own firmware.
So we’ll describe that.
If the OLED module’s header is not
attached, solder it now, at right angles
to the module’s PCB. Then mount
the module onto the PCB. You might
find that the back of the OLED module touches IC1. In this case, use BluTack or a cardboard shim to keep the
two apart until the module is securely
soldered.
That leaves the long pins protruding
at the back of the PCB. You can trim them
carefully with a sharp pair of sidecutters.
It helps to apply extra solder directly
to the pad of the Tweezer arms to
make a robust fillet.
Practical Electronics | May | 2023
Testing
Fit a CR2025 or CR2032 3V coin cell
into the cell holder, noting the polarity
on the cell holder. After about a second, the OLED should show R HAND
as per Screen 1. If not, check your soldering and that 3V is present on either
the OLED module’s header pins or pins
2 and 3 of CON1.
If 3V is not present, the cell may be
flat or there is a short circuit. Remember to check the reverse of the PCB, as
the cell holder, arms and OLED header
are all very close together.
Before you proceed to use the SMD
Test Tweezers, you might like to go
through the calibration procedure, as
detailed in the panel overleaf.
Operation
With the calibration and setup complete, normal operation will start. You
should see a display indicating the battery voltage preceded by the letter B.
After five seconds, the Tweezers will
enter sleep mode and can be woken
by touching the tips together.
At this point, the new SMD Test
Tweezers work much the same as the
older version, apart from the expanded
capacitance range.
If you close the tips to measure the
Tweezers’ short-circuit resistance, you
should see a value jumping around
between 0Ω and 1Ω if everything is
working correctly.
We measured the current consumption on our prototype as much the
same as the original SMD Test Tweezers. The new Tweezers use around
4mA when working and 5μA when
sleeping. So the cell life will depend
mainly on how much they are used,
tending towards the cell’s shelf life.
Finishing touches
Like we did with the original SMD Test
Tweezers, you might also consider adding some heatshrink to the device to
add some protection and to keep the
battery from being removed.
The 10mm heatshrink can be put
over the arms, leaving just the tips
exposed. It should be pushed up
firmly against the main PCB before
being shrunk with a heat gun.
The wider heatshrink fits over the
main PCB and should overhang the
end enough to prevent the cell from
being removed. Of course, you will
have to remove and replace the heatshrink to replace the cell.
Also, be careful to not shrink the
large heatshrink too tightly around the
OLED, as its glass screen can be fragile. Aim heat along the edges to avoid
heating the OLED and battery, and
only shrink enough to secure everything in place.
Parts availability and upgrading
Apart from the PIC, the hardware is
the same as before. The PCBs can be
bought from the October 2022 section
of the PE PCB Service, and the new
version PIC from the May 2023 page.
While we think the parts cost is
low enough that it’s worthwhile simply building new Tweezers, if you
really want to upgrade a pair you’ve
already built, you can just order the
programmed micro from us and swap
it over.
We suggest you only do this if you
are confident in removing SMDs and
Screen 1: the first display when the
Tweezers are powered on is the HAND
setting, oriented in correspondence
to the setting. Leaving the tips open
selects right-handed operation.
The tips might look a bit wonky, but when the arms are squeezed to bring them
together they become parallel at about the distance you would typically use
them (wide enough to hold a typical SMD component).
43
cleaning up the board to accept a new
chip. This is most easily done with
a hot air station, although it can be
done with a regular soldering iron if
you know how.
Future improvements
The SMD Test Tweezers are somewhat
limited by only having one resistor to
apply voltages to components, which
is in turn limited by the 8-pin PIC. The
Setup and calibration
10kΩ resistor limits the applied current to about 300μA, meaning that the
diode forward voltages reported are
much lower than expected, and LEDs
do not light very brightly.
The accuracy of modern surface-
mounted resistors is excellent and,
as built, the SMD Test Tweezers will
distinguish resistors well enough for
most constructors. Still, the extra
program space available on the
PIC16F15214 gives us room to add
some routines to add some settings
and calibration constants.
I’m right-handed, and have realised I
overlooked an aspect of the design that
probably makes the original Tweezers
very difficult to use for the left-handed.
So the first new setting is the option
to flip the display so that it is legible
when the SMD Test Tweezers are held
in a left hand.
There is also the option to set the
value of the nominally 10kΩ series
resistor between pins 2 and 5 of IC1.
Rather than trying to measure its value,
we recommend testing an external part
of a known value and adjusting the calibration until the SMD Test Tweezers
measure it correctly.
The series value is simply adjusted
in proportion to the desired change
in calculated resistance. For example, if your displayed test resistor is
1% low, increase the series resistor
value by 1%.
This won’t adjust for things like
trace and contact resistance, so there
is a separate calibration step for those.
Still, the preset value we have loaded
into the SMD Test Tweezers firmware
will be quite accurate, as long as your
build is similar to ours.
You may have noticed that the SMD
Test Tweezers do not have any buttons.
So the various settings are configured
using the only input device available:
the probe tips!
We can step through the setup and
calibration by opening and closing the
tips of the Tweezers at various points.
It’s a slow but effective process, made
easier by having a screen to show what
is happening.
Look at the flowchart in Fig.5 as we
explain the process. The setup procedure only runs when the microcontroller is powered up, so it can be triggered
by removing and reinserting the cell.
The right-hand or left-hand operation setting is selected at the instant
power is applied. If the tips are open,
right-handed operation is selected;
otherwise, left-handed operation
is set. A message is also shown to
RELEASE (Screen 2) the tips, and the
microcontroller waits for this to happen so that later calibration steps are
not triggered inadvertently.
If you find it fiddly to insert the cell
while holding the tips closed, join the
tips with a female-female jumper wire
while inserting the cell. The handedness setting is kept in RAM, saving on
wear to Flash memory. Since it is set
every time power is applied, there is
no need for non-volatile storage.
As the remaining calibration steps
can be a bit fiddly, there is the option
to skip them. You enter calibration
by holding the tips together when
prompted (Screen 3) or leaving them
open to skip. If the tips are left open for
about the first 10 seconds after powerup, the settings are the same as the
original SMD Test Tweezers.
The next step is to adjust the value
of the nominally 10kΩ series test resistor. The OLED displays CAL R+ and a
countdown timer (Screen 4). Any time
the tips are closed during this phase,
the displayed value will increase, and
the timer will reset.
This is followed by the CAL R- phase
(Screen 5), which works much the
same but allows the value to decrease.
If any changes are made, the cycle
repeats the CAL R+ and CAL R- steps
until no more changes are made.
The OLED then prompts to save the
value; again, touching the tips together
before the displayed timeout is confirmation that the value is to be saved
and, if this is done, a brief message is
Screen 2: at various times during
setup, you may be prompted to
RELEASE the Tweezers by opening the
tips to ensure that multiple settings
are not inadvertently made.
Screen 3: the first prompt is to
complete the calibration process and is
accompanied by a nominal five-second
timer. If the tips are left open during
this time, calibration is skipped.
Screen 4: when the tips are closed
on this screen, it will increase the
saved value of the series test resistor
in 1Ω steps. See Fig.5 for a flowchart
explaining the process.
Screen 7: this screen shows while the
value is saved to confirm that your
selection has been acknowledged.
Screen 8: you are prompted to close
the tips to calibrate their contact
resistance. If you don’t, the saved
value is not changed.
Screen 9: the contact resistance is
measured around 20 times to get an
average. The value shown here is
higher than the default value of 16Ω.
44
Practical Electronics | May | 2023
shown indicating this. These are seen
in Screen 6 and Screen 7.
Finally, whether or not any changes
are made or saved, the value of the
series test resistor is freshly loaded
from Flash memory and displayed
for user confirmation. The next step
– to set the contact resistance – is simpler, as this is measured rather than
being entered.
Note that the timers shown on
these screens are not high-precision.
The internal timings vary depending on what is displayed (especially
changing numbers, which take time
to render). We’ve tried to make the
countdown timers appear reasonably
consistent as seconds, but they are
not highly accurate.
The prompt seen in Screen 8 is the
start of the contact resistance calibration. When the tips are held together,
Screen 9 is seen. This shows the measured contact resistance, averaged over
several readings. The default value is
16Ω, as measured on our prototype.
If the tips are accidentally opened,
the process aborts, and you will need
to restart the calibration process to
repeat it.
Otherwise, the averaged value is
shown along with a prompt to save it,
as seen in Screen 10. Close the tips to
Having these extra pins means
that we could have multiple current-
limiting resistors and thus a better
choice of test current. As well as
expanding the range, additional test
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
Fig.5: a flowchart representing the
setup and calibration process that
occurs when power is first applied to
the Tweezers. It looks complicated,
but it is simple to go through once
you understand the concepts, and the
Tweezers prompt you with what to do
at each step.
confirm or leave them open to allow
the counter to time out. Screen 11
shows the actual value loaded from
Flash memory.
Screen 5: similarly, this screen allows
the series test resistor value to be
decreased. If any change occurs, these
two steps are repeated until no change
is detected.
Screen 6: this prompts you to confirm
that you wish to save the entered
value to non-volatile flash memory.
Close the tips to do so.
Screen 10: if you don’t get this
message, the Tweezers have detected
that the tips may have been opened,
so the measured value is inaccurate.
Screen 11: finally, the actual value
saved in flash memory is reloaded so
that you can confirm that the saved
value is correct.
Practical Electronics | May | 2023
resistors will also improve the overall accuracy.
►
We are considering a more complicated SMD Test Tweezers design using
a chip with, say, 14 pins. That might
let us add new test modes and improve
the existing ones.
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