This is only a preview of the July 2021 issue of Practical Electronics. You can view 0 of the 72 pages in the full issue. Articles in this series:
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Touchscreen
Wide-range
RCL Box
Part 2 – by Tim Blythman
Last month, we described our new Touchscreen RCL Box, a compact
device that lets you quickly and easily select various resistance,
capacitance and inductance values for prototyping and testing.
Now we’re going to go over the construction, testing and operating
procedures. It uses mostly SMD parts, but they’re all easy to work with.
I
n part one, we described how
the RCL box works and listed its
features and specifications. We
also explained how it’s built using
a Micromite V3 LCD BackPack with
a touchscreen and two new boards.
Now, without further ado, let’s start
putting it together.
The Micromite itself
You will of course have to build a Micromite V3 BackPack with its accompanying 3.5-inch LCD touchscreen
module to control the whole shebang.
If you haven’t already done so, refer to
the article in the August 2020 issue –
the PCB is available from the PE PCB
Service, code 07106191.
There is one variation from the
original design to note: we used female
headers (ie, header sockets) on the back
of the BackPack PCB to connect to the
two other boards used in this project.
So when building the BackPack,
it’s probably a good idea to leave the
external I/O and power/serial headers
off initially, and fit them later, after
you’ve built the other board.
There’s also not much point in
mounting the LCD yet. Fit the headers
and test that the Micromite connects to
the LCD, but don’t install the mounting
hardware at this stage.
Note that any ‘optional’ components
fitted to the BackPack may interfere
with the RCL Box operation if they
share pins; these should be removed
if already fitted.
Construction
We suggest that you carefully follow
these instructions and build the boards
in the order given, or you may find it
a bit tricky.
32
While none of the parts are tiny, you
should avail yourself of the usual set
of SMT tools, including a fine-pointed,
temperature adjustable soldering iron,
tweezers, magnifier, solder flux and
braid (wick).
Some flux removal solution or even
isopropyl alcohol will be handy to
clean up any excess flux. In general,
more flux is better than not enough!
The consequence of this is that the
PCBs are left with a messy residue
unless cleaned.
Since both boards have mostly
components only on one side, they
are well suited to reflow soldering. See
our articles on building a Reflow Oven
from April and May 2021.
The design effectively crams four
PCBs into the UB3 Jiffy box, so once
finished, space will be tight. Therefore,
as you progress through the assembly
steps, be careful of components standing higher than needed.
In particular, the relays should
protrude from the board no more than
7mm; use the parts we have specified
(which are around 5mm tall) or check
the data sheet of alternative parts before ordering.
Naturally, positioning of the parts
is critical for correct operation; if
any of the resistors, capacitors or
inductors are mixed up then the
software won’t be able to produce
the correct values.
Resistor PCB
We’ll start by building the resistor
PCB which is coded 04104201 and
measures 115×58mm. Its PCB overlay
diagram (Fig.3) has been repeated
from last month to help you during
the assembly.
First, check that you have the correct
PCB; the two main boards look very
similar. For all the components, we
suggest the following process.
Apply a small amount of flux to the
pads and hold the component in place
with tweezers. Add a small amount of
solder to the iron and apply the iron
to one lead.
For the larger relays, you may be
able to hold them in place with a wellplaced finger; their larger body will
present less risk of being burnt. Once
the component is flat, square and centred, solder the other pin(s).
Start with the resistors. Apart from
one 10kΩ resistor near the Micromite
header, they are all down the centre of
the board. We suggest you start at one
end and work your way along, ensuring that the value printed on the part
matches the silkscreen.
We have repeated the relevant section from last month’s parts list for the
expected SMD component markings
(Table 1).
You should be able to confirm their
resistances, even after they are soldered,
as they are connected to the (absent)
relays at one end, ensuring that their
measured values are not distorted by
being connected to other components.
There are two 100nF capacitors; they
are interchangeable and non-polarised.
Ensure they are fitted accurately, as
there is not much space around them
once installed.
The two ICs have the finest pitch
footprints on the PCB (although they
aren’t very close by SMD standards).
It is vital to ensure that the pin 1 dot
lines up with the silkscreen. If you
cannot see it, pin 1 is also closest to
the 100nF capacitor.
Practical Electronics | July | 2021
The RCL Box
has three sets of terminals
(right side) so you can use the resistance,
capacitance and inductance functions independently of
each other. It’s all under the control of the Micromite Backpack (V3)
which allows you much more flexibility than traditional R, C or L substition boxes.
Proceed with the ICs as for the other
parts, but do not be concerned if a
solder bridge forms, as long as the part
is aligned correctly. Finish soldering
the remaining pins and once the part
is secure, use solder braid to carefully
remove any excess from one side at
a time.
Before you start to add the higherprofile relays, now is a good time to
clean up any flux residue according
to the instructions on your flux cleaning solution.
There are 14 relays to be fitted, all
with their pin 1 markers facing the outside of the PCB. You can confirm this
from the silkscreen and also the fact
that the pin 1 pad is square instead of
rounded. Check your progress against
our photos.
Our relays also have a stripe printed
on their tops which should match the
stripe printed on the PCB silkscreen.
Leave RLY12 and RLY13 until last;
they are oriented differently and
have more space around them; this
gives you better access to RLY10 and
RLY11’s pins when fitting those parts.
The spacing is quite tight, but the
same techniques apply as for the other
components. Using a fine-pointed
soldering iron, come in almost perpendicular to the PCB so as not to burn
and damage adjacent relays. The pins
on the relays are at a generous 0.1in
(2.54mm) pitch.
Do not add the Micromite headers
yet. If you are keen, you might like
to run some jumper wires from a
Micromite to test the resistor PCB in
isolation, although you will naturally
need the software installed to do this
(as described below).
Practical Electronics | July | 2021
Capacitor/inductor PCB
Well recruits, this is what you have
been training for. Not only are there
16 relays on this side of the PCB, but
many of the components also don’t
have any markings. Take care not to
mix them up. But you should find
that the process is much the same as
for the resistor PCB.
Start with the capacitors, checking
the component value as you go. If you
have a capacitance meter, you can use
it to double-check that the correct
parts have been fitted.
As well as the output capacitors,
there are two 100nF parts for bypassing the ICs and a single 10kΩ resistor
to fit. As for the resistor PCB, the two
ICs have the closest pin spacings.
Note that pin 1 on both is closest to
the Micromite header.
Following on from this, fit all the
inductors except the 3.3mH type. It
is larger and can be fitted last, even
after the relays.
With all the low-profile parts fitted,
clean up excess flux before moving
onto the relays.
The low-profile Panasonic TQ2SA5V relays we used are not commonly
available, but they are in stock at
Digi-Key and Mouser.
If you have any doubts, now is the
time to test the part values, as fitting
the relays will make it more difficult
to do so.
Proceed with the relays as you did
for the resistor board. Patience will
definitely help!
Do take note of the orientation
markings; most of the relays face the
same direction, but the two mounted
at right angles face towards each
other. We suggest fitting RLY24 and
RLY30 before the remainder, as they
have the smallest clearances to adjacent components.
Finally, fit the 3.3mH inductor. It
has the largest pads and so may need
more heat. It’s best to apply a thin
smear of flux paste to its pads before
placing it. When finished, clean up
any remaining flux residue.
Mechanical assembly
While the boards we supply are both
covered with a solder mask layer,
providing a degree of insulation if the
boards are laid flat against each other,
you should not rely on this.
The solder mask appears durable,
but is thin and will not resist much
vibration or chafing, and it can even
come from the factory with a few
holes (especially around vias).
So cover the back of one of the
boards with Kapton (or a similar
polyimide) tape, except for around
the Micromite headers and the four
mounting holes.
While CON1, CON2 and CON3
appear to pass through the board,
the tape can sit against the back of
these; this will help to insulate their
pins from the other board. We’ve used
33
TPIC6C595
5V
TX
RX
GND
RST
3
4
5
9
10
14
16
17
18
21
22
24
25
26
3V3
5V
GND
IC2
IC1
TPIC6C595
100nF
CONNECTIONS TO MICROMITE
COIL
COIL
COIL
COIL
COIL
COIL
RLY12
CON1
RLY10
RLY8
RLY6
RLY4
RLY2
COIL
100nF
10k
10M
2.2k
4.7M
1.5M
1k
330
680k
68
150k
15
RLY14
3.3k
33k
3.3
6.8M
1.5k
3.3M
680
1M
150
330k
COIL
33
RLY13
RLY9
RLY11
RLY7
6.8
68k
1.5
15k
6.8k
RLY1
RLY3
RLY5
COIL
COIL
COIL
COIL
COIL
COIL
Fig.3: the PCB overlay diagram for the resistor board, reproduced from last
month. Be careful to orient the relays correctly, as shown here, and add the parts
in the order stated in the text to make your life easier. If you have a magnifier,
you can read the value codes on the individual resistors.
through-hole pads here to provide
more mechanical strength as surfacemounting pads are more easily torn
off the PCB.
Assuming you have built the Micromite V3 BackPack with LCD as
described above, fit the 18-way and
4-way female headers on its back side.
Remember that the Micromite
BackPack usually has male headers
in these positions.
Rather than using multiple threaded spacers with machine screws
front and back, we used a different
technique for the board stack.
Resistor Codes (all 1 of each, SMD 1% 3216/1206 size; SMD markings shown)
10MΩ 106 or 1005
6.8MΩ
685 or 6804
4.7MΩ
475 or 4704
3.3MΩ 335 or 3304
1.5MΩ
155 or 1504
1MΩ
105 or 1004
680kΩ 684 or 6803
330kΩ
334 or 3303
150kΩ
154 or 1503
68kΩ
683 or 6802
33kΩ
333 or 3302
15kΩ
153 or 1502
10kΩ
103 or 1002
6.8kΩ
682 or 6801
3.3kΩ
332 or 3301
2.2kΩ 222 or 2201
1.5kΩ
152 or 1501
1kΩ
102 or 1001
680Ω
681 or 680R
330Ω
331 or 330R
150Ω
151 or 150R
68Ω
680 or 68R0
33Ω
330 or 33R0
15Ω
150 or 15R0
6.8Ω
6R8 or 6R80
3.3Ω
3R3 or 3R30
1.5Ω
1R5 or 1R50
Table 1: reproduced from the parts list in the June issue, this shows the codes
you can expect to be printed on the SMD resistors.
CL
TOP
Mount the LCD to the front panel/
lid piece using four 32mm-long M3
machine screws, with 1mm nylon
washers to separate the acrylic panel
from the LCD and the 12mm threaded
spacers generally used with the BackPack, to secure the machine screws
to the LCD panel.
Add the Micromite BackPack to
the stack, then place 9mm tapped or
untapped spacers onto the exposed
threads. Add the resistor PCB with
its relays facing the BackPack, then
the capacitor/inductor PCB with its
relays facing away and then secure
the whole lot with four hex nuts.
Although we haven’t made the
electrical connections yet, you should
now have a good idea of the overall
size of the PCB stack.
Before soldering anything, you
might like to test fit the stack into the
Jiffy box. This will let you know how
much room there is left. If you’ve used
the 5mm-tall relays we’ve specified,
you should have around 2mm clearance left.
We now need to use a pin header
to connect the two PCBs to each other
and the BackPack headers. To do this,
we remove the individual pins from
the plastic spacer strip, which you
can do using small pliers.
With the boards held together
in the stack, simply slot the pins
CL
TOP
10
B
ALL
DIMENSIONS
IN MILLIMETRES
15
A
15
A
13
A
10
9
12
HOLES A:
6.0mm IN
DIAMETER
HOLE B:
10 x 12mm
CUTTING DIAGRAM FOR
USB SOCKET END OF BOX
18
A
A
DRILLING DIAGRAM FOR
A
BANANA SOCKETS END OF BOX
Fig.5: this shows the location and size of the cut-out for the USB cable, plus the hole locations and sizes for the banana
sockets on the opposite side of the case. If you have a USB lead with a large housing, you may need to enlarge its hole. A
round (drilled) hole is easier to make, but will not look as neat.
34
Practical Electronics | July | 2021
100nF
Programmable LCR Reference
3
4
RLY19
470nF
RLY21
1 F
220nF
47nF
RST
9
5
10
14
16
18
24
GPIO21
25
GPIO22
26
5V
3.3
GND
TX
RX
17
100nF
10nF
2.2nF
470pF
COIL
COIL
RLY17
91pF
COIL
22nF
COIL
COIL
RLY15
12pF
100nF
2.2 F
4.7 F
RLY20
1nF
COIL
220pF
COIL
RLY18
COIL
COIL
COIL
36pF
10 F
RLY23
4.7nF
10pF
RLY16
COIL
RLY24
5V
GND
CON2
IC3
IC 4
TPIC6C595 TPIC6C595
LC PCB 04104202 C 2020 RevB
10k
RLY22
RLY29
COIL
L9 1mH
RLY27
COIL
RLY26
COIL
RLY25
COIL
COIL
RLY30
L8 330 H
L7 100 H
CON3
L1 100nH
L2 330nH
RLY28
L4 3.3 H
L6 33 H
L5 10 H
L10
3.3mH
L3 1 H
Fig.4: the capacitor/inductor board has more relays and some larger
components, so it’s a bit packed. But if you follow our instructions, you
should not find it too difficult. Again, watch the orientation of the relays. The
inductors should have printed values but the capacitors won’t.
Here’s a trick we even see some
manufacturers perform; stacking
multiple capacitors to achieve a
higher capacitance value. In this
case, we have combined a pair of
4.7µF parts to replace a single 10uF
part. It’s not hard to do as long as
you don’t apply to much heat.
5V and GND connections. See Fig.6
for how to wire such an arrangement.
You will need to solder the wires to
the pins on the capacitor/inductor
board, as this connects to the header
on the BackPack board.
Note that such a DC jack must be
installed near the lid of the Jiffy box as
the PCB extends nearly the full width
of the bottom of it. Altronics (P6701)
and Jaycar (PP1985) both carry USBto-DC plug leads made up. Or you
could use a regulated plugpack with
5V output and the correct tip polarity,
to match the socket wiring.
through the PCB holes into the female
header on the Micromite BackPack,
one at a time.
Once you have confirmed that
everything will fit together, solder
the header pins to the PCBs, ensuring
that enough solder is applied to wick
down the stack into the bottom PCB
of the pair.
This can be assisted by squirting a
little flux paste into each hole before
inserting the pin.
Alternatively, if you have no plans
to remove the PCBs from the BackPack, you could omit the female headers and solder male headers directly
to the BackPack.
Then, after mounting the resistor
and capacitor/inductor PCBs, solder
the headers to these two PCBs as well.
You may need longer pins to do
this, or you may choose to run short
lengths of wire between the two
boards instead.
USB socket
For our prototype, we simply made a
cut-out in the side of the box to allow
power to be supplied to the BackPack
using a standard USB cable with a
mini Type-B connector. Its location is
shown in Fig.5. This hole will allow
most USB-mini plugs to pass through
the side of the box and directly into
the Micromite’s USB socket. It may
need to be enlarged if your USB lead
has an unusually large plug.
An alternative that we have used
on some projects is to fit a DC barrel
socket; its wires are run back to the
Banana sockets
You might have noticed that there is
not much space in the Jiffy box; thus,
we’ve had to use low-profile banana
sockets for the six test connections.
The locations of their mounting
holes, on the opposite side to the USB
power cut-out, are shown in Fig.5.
Once fitted, the sockets are simply
free-wired back to their respective
pads on the PCBs. We suggest mounting the sockets in the enclosure first,
to test that they do not foul the PCBs.
Once this is done, solder short
(5cm) leads to each socket, then solder them to the respective pads on
the PCBs. CON1 is for the resistance
connections, CON2 for capacitance
and CON3 for inductance. The LCD
shows their values in this order from
top to bottom, so the sockets should
be wired accordingly.
5V
4
Tx
3
2
Rx
1
USB CONNECTOR
TYPE A MALE
GND
DC PLUG
Fig.6: if you want to add a DC socket for power, here is how
to do it. But be careful that you mount it in a location where it won’t
foul the board stack. The USB-to-DC plug lead is a commonly available,
pre-assembled part (eg, Altronics P6701; Jaycar PP1985).
Practical Electronics | July | 2021
DC INPUT
SOCKET
(ON END OF BOX)
MICROMITE
CON 1 POWER
AND CONSOLE
CONNECTOR
35
Screen1: the larger 3.5-inch display allows a lot of useful
information to be displayed by the Micromite. At right are
the three output parameters, displayed adjacent to their
respective banana sockets. The values can be changed by a
simple tap up or down, via a slider or automatically ramped
by the software.
You may find it easier to remove
the PCBs from the stack while soldering the leads. None of the parts
are polarised, so it actually doesn’t
matter if you swap the wires to the
pairs of sockets.
Micromite setup
There are two ways to load the software on the Micromite; the easiest
is to simply load the RCLBOX.HEX
file directly onto the chip using the
onboard Microbridge or a PIC programmer such as a PICkit 3 or PICkit 4.
The alternative is first to load
the Micromite with MMbasic, then
configure it and upload the BASIC
source code over the serial terminal.
This is the required approach if
you wish to customise the way the
RCL Box works.
To do this, assuming you have a
new Micromite (we’re using MMBasic
version 5.4.8), first open the library.
bas file (extracted from the download
package for this project, available on
the July 2021 page of the PE website)
and upload it to the Micromite (eg,
using MMedit).
Then type ‘LIBRARY SAVE’ at the
Micromite console and press enter.
Next, type ‘WATCHDOG 1’. After
pressing Enter, the Micromite should
restart and the screen will clear. The
terminal should display:
Watchdog timeout
Processor restarted
ILI9488 driver loaded
You can then run the command ‘GUI
TEST LCDPANEL’; you should see
circles appearing on the LCD. Press
Ctrl-C to end the test.
Next, run ‘OPTION TOUCH 7,15’
to enable the touch driver. Then run
36
Screen2: pressing the SETUP button opens the Limit
Settings page. There, Soft limits can be set to avoid nonuseful or dangerous test values. Further settings can be
found by tapping on the RAMP or DISPLAY buttons,
while STORE saves the current setting to non-volatile
Flash memory.
‘GUI CALIBRATE’ and complete the
calibration sequence.
If you like, you can run ‘GUI TEST
TOUCH’ to confirm that the display and
touch panel are working correctly together. Ctrl-C ends this test program too.
At this stage, the display is configured and the main BASIC program can
be loaded. Open the RCL Reference
Box.bas file, send it to the Micromite
and run it. The AUTORUN flag is automatically set, so the software will
start up when powered in future.
The software as loaded now is the
same as what you would get from
the HEX file; the remaining steps are
settings and configuration within the
Programmable RCL Box.
Finishing touches
If you have not already done so, now
would be a good time to fit the acrylic
lid to the display by removing the
four machine screws. Place the 1mm
spacers over the holes and then thread
the machine screws through the acrylic panel and into the tapped spacers.
Note that the acrylic lid piece is not
symmetrical; if it appears that the PCBs
behind are sticking out the side, you
may have it the wrong way around.
As a hint, the end of the Micromite
BackPack with the USB socket goes to
the end with the wider-spaced holes.
Slot the stack into the case and
secure the lid with the four screws
that came with the Jiffy box.
Configuration and use
When powered up, a splash screen appears, followed by the main operating
screen (Screen1). This is where the
resistance, capacitance and inductance values are controlled.
In a large font along the righthand side are the currently selected
resistance, capacitance and inductance values. There are three ways that
these values can be changed.
First, the slider beneath each value
can be used to make quick, coarse
changes. You should have no trouble
picking the exact value needed, but
the up and down buttons to their left
are better to make fine changes.
To the left of the up and down buttons are the soft limits which can be
set. These allow the output values to
be restricted if this is desired. Note
that the up and down buttons are
greyed out when the values are at their
soft limits, warning you that you are
at the extreme values.
At bottom left are the ramp controls,
which can be used to step the outputs
automatically. They are red when the
ramp is inactive, turning green when
activated. The ramps make use of the
minimum and maximum soft limits
as their range.
Above this is there is a small numerical display, which indicates a
characteristic time or frequency based
on a selected combination of the currently enabled resistance, capacitance
and inductance.
The ‘Setup’ button at top right
changes to the first of three pages
for altering settings (Screen2). This
allows the soft limits to be altered,
with up and down controls for the
minimum and maximum values of
each range.
Any changed settings are made
active immediately, but are not automatically saved to Flash. This is done
by the ‘Store’ button, which ensures
that the current settings are saved for
use at power-on.
This has been done to minimise wear and tear on the internal
Flash memory and also provides an
Practical Electronics | July | 2021
Screen3: the RAMP setting page controls the automatic
ramp modes. These can be set to up, down or sawtooth,
with the option to perform a single or repeated ramp.
There are individual settings for resistance, capacitance
and inductance; thus, you can ramp resistance up and
capacitance down simultaneously if that is what is needed.
opportunity for settings to be tested
before saving. If you change the settings to something you don’t like, then
a simple power cycle will reload the
last saved values.
Pressing the ‘Exit’ button returns
to the main control page; note that
this and some of the other buttons
are present on more than one page
to allow ease of navigation.
Pressing the ‘Ramp’ button opens a
page for the settings that control the
ramp modes (Screen3); a setting for
ramp rate is found on the ‘Display’
page (Screen4).
There are settings to ramp up, down
and in a sawtooth pattern (‘Saw’), which
alternates between up and down.
The ramps can also be set to loop
continuously or not (‘Off’). The current setting is displayed in a friendlier
fashion above the buttons.
If an output is set to ramp up but
not loop, it will ramp up to its maximum and then stop. The next time it
is started, it will reset to the minimum
and ramp up again. This simplifies
repeated tests.
The Display page includes the ramp
step time; this can be set from 0.1s to
10s in 0.1s intervals by dragging the
slider along the bottom of the page.
The final setting at the top of the
Display page is the characteristic
time/frequency, which controls what
is displayed at the top left of the main
page. There is a choice of RC, LR or
LC combinations, and the characteristic time constant or frequency can
be selected.
Of course, these may not match the
operation of your circuit as not all
circuits operate at their characteristic
time constant, but they are a useful
thing that the processing power of the
Micromite can add.
Practical Electronics | July | 2021
Screen4: the DISPLAY page contains the setting for what
characteristic time/frequency should be displayed. A choice
of either LC, RC or LR combinations can be chosen, with
either time constant or frequency being available as further
options. The step time for the ramp modes is also chosen by
the slider along the bottom of the page.
BASIC code
In case you wish to delve deeper into
the operation of the BASIC program,
we’ll explain a little bit about how
it works.
After a handful of OPTIONs are set
near the start, several colour values
are defined. If you wish to change the
feel of the interface, changing these
colours is an easy way to do it.
The output values and relay images
list the available values in pairs of
arrays. One contains a list of the output values as floating-point numbers;
these are the RVALUE, CVALUE and
LVALUE variables.
The RIMAGES, CIMAGES and LIMAGES arrays contain nominal 16-bit
values which describe the bit pattern
which is output to the relays.
In the case of the capacitor and
inductor images, these are combined
with a simple addition to allow the
data to be combined for simultaneous latching.
There would be little point changing the image arrays unless you
reworked the circuit itself, but you
could add extra resistance values by
using combinations of more values
than what we have.
Note that these lines are very close
to BASIC’s 255 character limit, so edit
them with care.
Most of the remaining code is to
create the user interface. While we
often complain about how bloated
software can be at times, it’s nice to
have an easy-to-use set of controls;
it’s just unfortunate that it takes so
much code to do so!
The five subroutines starting
with RELAYINIT perform the
interfacing to the shift registers.
If, for example, you were interested in interfacing these boards to
another microcontroller such as an
Arduino or even a Raspberry Pi, then
we suggest looking at these subroutines to understand how to interface
and check the schematic to know
what pins need to be connected.
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
This photo
shows how
the two PCBs
are piggybacked inside
the case.
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