This is only a preview of the June 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 1 – by Tim Blythman
Resistance wheels and resistance/capacitance decade boxes are
invaluable tools for prototyping and testing. They allow you to easily
try different resistance and capacitance values in a circuit. Our new
Touchscreen Wide-range RCL Box not only gives you a range of
resistances and capacitances, but also inductances – all at your fingertip!
It can even scan through the range of values automatically.
T
he inspiration for this project
was Jaycar’s RR0700 Resistance
Wheel. It is a compact and handy
tool; we have one in our drawer and
use it often. It has a good range of resistance values consistent with commonly
available parts, and you can easily
step through them by rotating its dial.
Unfortunately, though, it appears to
have been discontinued.
Back in August 2015 we published
a Resistor-Capacitor Decade Substitution Box. This was designed by
Altronics, who have it available as a
kit (Cat K7520).
Further back in April and July 2013,
we also published designs for separate
resistor and capacitor boxes respectively. These each used six knobs to
select the desired value.
Those designs provided an extensive
range of possible values; however, values which are not part of the standard
resistor/capacitor series (E12/E24 for
16
Providing various resistances
The programmable Touchscreen
Wide-range RCL Box is a very different
design to any of the previous devices.
The addition of a Micromite BackPack
with LCD does a lot more than just
allow the device to be controlled via
its touchscreen interface.
It has separate pairs of banana
sockets for resistance, capacitance
and inductance. There are 43 resistance values which can be chosen,
corresponding to the E6 (six values per
decade) values across seven decades,
from 1Ω to 10MΩ (see Table 1).
We have chosen the E6 range as
it incorporates the most commonly
used resistance values. The resistors
are switched by small relays, so the
resistance terminals are fully isolated
from the control circuitry.
Interestingly, we were able to provide these 43 values using only 26
resistors. A set of 14 relays switch these
26 resistors; the relays take up the most
space on the PCB.
While we have not done so, it is
possible to modify the software to
provide even more than the 43 resistance values.
In other words, the 43 E6 values
the software currently provides are
a subset of those which are possible.
This resistance generation technique
gives an accuracy of around ±2% for
the final values using 1% resistors.
But most values are much better than
this; generally, they are close to ±1%,
especially those which correspond to
one of the fixed resistor values used.
Any resistance box introduces some
parasitic resistance, capacitance and
inductance (real resistors have this to
some extent too). The PCB layout is
designed to minimise these unwanted
characteristics where possible.
The LCR box
can individually select
inductance, capacitance and resistance.
Capacitances and inductances
Similarly, 19 capacitor values (from
the E3 series) from 10pF to 10µF are
resistors and E6/E12 for capacitors)
are of limited use.
Also, they are all fairly large units,
fitting into boxes measuring 195 × 145
× 65mm (2015 design) and two 157 ×
95 × 53mm boxes (2013 designs).
By contrast, this do-it-all RCL box
measures just 130 × 67 × 44mm; considerable smaller than either of the
earlier designs, while offering more
capabilities and really easy to drive.
Its only real disadvantage is the need
for a power supply, but these days, we
all tend to have plenty of USB power
sources. You can even use a USB battery bank for portable operation.
Practical Electronics | June | 2021
Features
The inspiration behind
this project: a resistance
substitution wheel. We still use one!
available, controlled by 10 relays. The
inductor range is the smallest, with
11 values, two per decade (from the
‘E2’ series).
These start at 100nH and go up to
3.3mH, covering the most useful range
for most people.
Unlike the resistors, the capacitance
and inductance values correspond to
individual components on the PCB;
thus, the tolerance can be expected to
be close to that of the parts used.
Again, while we have not done so,
extra capacitance and inductance values could be provided if the software
were modified.
The complete circuit
The front end display and interface is
simply the Micromite LCD BackPack
V3 described in our August 2020 issue.
We have mounted two PCBs behind
it to provide the RCL box functions.
The circuit implemented by these
boards is shown in Fig.1 and Fig.2.
Fig.1 shows the resistor switching
functions, while Fig.2 shows the capacitor and inductor switching.
There are effectively two banks
of resistors, one switched by the ‘a’
contacts of RLY1-13 and one switched
by the ‘b’ contacts of RLY1-13. Which
bank connects to the external terminals at CON1 is controlled by RLY14.
With RLY14 off, the resistors switched
by RLY1B-RLY13b are in-circuit, and
when RLY14 is on, those connected to
RLY1A-RLY13a are in-circuit.
Once one ‘bank’ is selected, any of
the resistors in that bank can be paralleled by energising some combination
of RLY1-RLY13.
For example, if RLY1 and RLY14
are energised, only the 1.5Ω resistor
is connected across CON1, giving a
1.5Ω resistance value. But if RLY2
and RLY4 are also energised, the 1.5Ω,
3.3Ω and 33Ω resistors are paralleled,
giving 1Ω across CON1.
Connecting just one resistor at a
time (ie, energising one of RLY1-13,
and possibly also RLY14) gives 26 different values corresponding to each of
Practical Electronics | June | 2021
• 43 E6 resistance values (1 to 10M , ±2%, 1/4 )
• 19 E3 capacitance values (10pF to 10µF, ±10%, 50V)
• 10 E2 inductor values (100nH to 3.3mH, ±20%)
• Independent control of R, C and L values via a touchscreen interface
• Compact design (fits into UB3 Jiffy Box)
• Powered from USB 5V
• Automatically sweep through value ranges
• Frequency display based on RC, LC and RL combinations
• Based on Micromite V3 BackPack with a 3.5-inch LCD touchscreen
• Programmed in BASIC
the physical resistors. For the remaining values, we energise multiple relays
from RLY1-RLY13, as shown in Table
1 (overleaf).
This paralleling of values also
means that the parasitic and contact
resistances are minimised as much as
possible. Also, for some values, the
available power rating is increased.
To drive the relays, we are using two
TPIC6C595 high-current shift registers
(IC1 and IC2).
The Micromite’s output pins could
probably drive the relays directly if
we used 3.3V relays, but the driver
circuits make this less stressful for the
Micromite. IC1 and IC2 each have a
100nF supply bypass capacitor.
Their serial pins are chained, with
SDOUT (pin 9) of IC1 going to SDIN
(pin 2) of IC2.
Serial data is fed into IC1 from
Micromite outputs GPIO5 (pin 4 of
the I/O header) and GPIO9 (pin 5).
These are not the hardware SPI bus
pins; the data rate is low enough, and
updates are infrequent enough, that
this data can simply be ‘bit banged’.
using general-purpose digital I/O pins.
The latch (RCK) lines of both ICs
are driven by Micromite GPIO10 (pin
6), which causes the new serial data
to be used to update the DR0-DR7
outputs of both ICs simultaneously,
switching the relays (assuming the
state has changed).
Similarly, the G/EN pins (pin 8) of
IC1 and IC2 are driven from Micromite
GPIO21 (pin 11). This has a 10kΩ
pull-up resistor to 5V, so when the
Micromite is not driving this pin, all
those outputs are off and hence none
of the relays are energised.
For example, that might be when
the Micromite is being reprogrammed.
This pin must be brought low by the
software to activate the outputs of IC1
and IC2.
Desired Paralleled resistor(s)
value
Desired Paralleled resistor(s)
value
1Ω
1.5Ω
2.2Ω
3.3Ω
4.7Ω
6.8Ω
10Ω
15Ω
22Ω
33Ω
47Ω
68Ω
100Ω
150Ω
220Ω
330Ω
470Ω
680Ω
1kΩ
1.5kΩ
2.2kΩ
3.3kΩ
4.7kΩ
6.8kΩ
10kΩ
15kΩ
22kΩ
33kΩ
47kΩ
68kΩ
100kΩ
150kΩ
220kΩ
330kΩ
470kΩ
680kΩ
1MΩ
1.5MΩ
2.2MΩ
3.3MΩ
4.7MΩ
6.8MΩ
10MΩ
1.5Ω, 3.3Ω, 33Ω
1.5Ω
3.3Ω, 6.8Ω, 330Ω, 680Ω
3.3Ω
6.8Ω, 15Ω
6.8Ω
15Ω, 33Ω, 330Ω
15Ω
33Ω, 68Ω, 3.3kΩ, 6.8kΩ
33Ω
150Ω, 68Ω
68Ω
150Ω, 330Ω, 3.3kΩ
150Ω
330Ω, 680Ω
330Ω
1.5kΩ, 680Ω
680Ω
1kΩ
1.5kΩ
2.2kΩ
3.3kΩ
15kΩ, 6.8kΩ
6.8kΩ
15kΩ, 33kΩ, 330kΩ
15kΩ
33kΩ, 68kΩ, 3.3MΩ, 6.8MΩ
33kΩ
150kΩ, 68kΩ
68kΩ
150kΩ, 330kΩ, 3.3MΩ
150kΩ
330kΩ, 680kΩ
330kΩ
1.5MΩ, 680kΩ
680kΩ
1MΩ
1.5MΩ
3.3MΩ, 6.8MΩ
3.3MΩ
4.7MΩ
6.8MΩ
10MΩ
Table 1 – Available resistance values
17
Capacitor and inductor board
The circuit diagram of the second
board which switches the capacitors
and inductors is shown in Fig.2.
The relay driving arrangement using
IC3/IC4 is essentially the same as for
IC1/IC2 in Fig.1, except this time, the
latch (RCK) pins are brought back to the
Micromite GPIO21 output (pin 11). So
with both boards attached, the Micromite can control them independently.
MICROMITE
V3 BACKPACK
RESET
GPIO3
GPIO4
GPIO5
GPIO9
GPIO10
GPIO14
GPIO16
GPIO17
GPIO18
GPIO21
GPIO22
GPIO24
GPIO25
GPIO26
+3.3V
+5V
GND
1
2
There are 16 relays involved, compared to 14 for the resistors, so all the
outputs of both IC3 and IC4 are occupied – by comparison, there are two free
driver output pins in the circuit of Fig.1.
10 relays are used for switching the
capacitors, with RLY15-RLY23 and
RLY24 doing the same job as RLY1RLY13 and RLY14 in Fig.1.
That is, RLY15-RLY23 connect some
number of capacitors in parallel to the
+5V
+5V
10k
1
7
3
2
10
5
15
CLR
DR7
SDIN
DR6
RCK
DR5
SCK
6
7
8
9
9
TX
RX
GND
DR2
DR1
G/EN
DR0
SDOUT
10
GND
11
16
12
13 RLY6
12
5
RLY1
RLY2
3
RLY3
7
16
2
17
10
18
15
DR7
SDIN
DR6
RCK
DR5
SCK
RLY3
a
RLY4
8
21
9
DR2
G/EN
DR1
SDOUT
DR0
b
RLY6
6.8
33k
RLY3
a
b
15
68k
RLY4
RLY10
13 RLY11
RLY7
12 RLY13
a
11 RLY12
IC2
DR4
TPIC6 C595
TPIC6C595
6 RLY4
19
20
14
3.3
15k
RLY2
100nF
VCC
CLR
b
RLY5
1
14
6.8k
a
RLY2
11
4
1.5
RLY1
+5V
13
22
14 RLY8
IC1
DR4
TPIC6 C595
6 RLY14
DR3
8
b
RLY1
VCC
DR3
5V
a
100nF
4
15
NO or NC contacts of RLY24, and RLY24
connects one or the other set to CON2,
the ‘capacitance’ banana terminals.
So, just as the circuit of Fig.1 can
select or combine resistors to vary the
resistance across CON1, the circuit of
Fig.2 can select or combine capacitors
to control the capacitance across CON2.
Remember, though, that when resistors are paralleled, you get a lower
resistance value; by contrast, when
b
150k
RLY5
RLY8
33
5 RLY5
4 RLY7
RLY9
a
3 RLY9
GND
RLY11
68
330k
RLY6
RLY10
16
b
a
b
150
680k
RLY7
RLY12
RLY13
a
b
330
1M
RLY8
RLY14
+5V
a
CON1
1
b
1.5M
RLY9
RLY14
a
RESISTANCE
2
BLACK BAR
MARKS
RELAY COIL END
680
a
b
b
3.3M
RLY10
a
b
a
b
RLY13
2.2k
6.8M
RLY12
a
Micromite-controlled R-C-L Box
SC MICROMITE
Resistance
Board
CONTROLLED R-C-L BOX RESISTANCE BOARD
1.5k
4.7M
RLY11
Fig.1: the circuit of the resistor-switching section of the RCL box. The
Micromite controls the relays via the high-current shift registers IC1
and IC2. By energising various combinations of the relays, multiple
resistors can be switched in parallel across CON1, giving 43 possible
resistor values from 26 discrete resistors.
1.0k
b
3.3k
10M
2020
18
Practical Electronics | June | 2021
paralleling capacitors, you get the sum
of their capacitances.
To allow the choice of 19 capacitance
values by this arrangement, one capacitor (5.6pF) is permanently connected
to one leg.
While this appears to remove the
option of having no capacitance across
CON2, in practice there is about 4.4pF
of parasitic capacitance already present, so this rounds it up to a neat 10pF.
+5V
MICROMITE
V3 BACKPACK
GPIO3
GPIO4
GPIO5
GPIO9
1
2
7
3
2
4
10
5
15
VCC
CLR
DR7
SDIN
DR6
RCK
DR5
SCK
6
GPIO10
GPIO16
DR3
GPIO17
GPIO18
GPIO21
8
8
9
9
14 RLY24
13 RLY15
DR2
DR1
G/EN
DR0
SDOUT
10
GND
11
16
5
RLY18
4
RLY19
3
RLY17
14
GPIO25
+3.3V
+5V
GND
16
7
17
2
18
10
15
5V
TX
RX
GND
100nF
1
15
GPIO26
RLY15
12pF
a
RLY16
b
22nF
RLY15
RLY17
a
36pF
b
47nF
RLY16
RLY18
RLY19
a
91pF
b
100nF
RLY17
+5V
13
GPIO24
5.6pF
RLY20
12
GPIO22
Inductors
The inductors are switched by RLY25RLY30, with RLY30 switching between
two banks of five inductors. The pairs
12 RLY16
11 RLY30
IC3
DR4
TPIC6 C595
TPIC6C595
6 RLY25
7
GPIO14
If we could have combined capacitors to provide the E6 range, we would
have, but you get oddball values instead. So in fact, only one capacitor is
selected in time, except for the 5.6pF
capacitor of course.
+5V
100nF
10k
1
RESET
In fact, if you can measure the
parasitic capacitance, you can tweak
the values of the 10-100pF capacitors,
increasing the accuracy of the ‘C’ part
of the RCL box.
We’ll discuss that possibility in detail
in the component selection section.
As with the resistors, the software
doesn’t enable all possible capacitance
options. Instead, we limit the choice to
the E3 range to keep things simple.
VCC
DR7
CLR
SDIN
DR6
RCK
DR5
14 RLY26
13 RLY21
19
20
5 RLY22
DR3
21
8
22
9
DR2
G/EN
DR1
SDOUT
DR0
GND
a
220pF
b
220nF
RLY18
RLY22
12 RLY29
11 RLY23
IC4
DR4
TPIC6 C595
TPIC6C595
6 RLY28
SCK
RLY21
4 RLY27
3 RLY20
a
RLY23
470pF
b
470nF
RLY19
RLY24
a
RLY25
1nF
b
1 F
RLY20
16
RLY26
L1 100nH
a
b
L6 33 H
RLY25
a
RLY27
2.2nF
b
2.2 F
RLY21
RLY28
L2 330nH
a
b
L7 100 H
RLY26
L3 1 H
a
b
a
RLY29
L4 3.3 H
a
b
RLY30
a
+5V
10nF
b
10 F
RLY23
CON2
1
L9 1mH
RLY28
4.7 F
RLY22
L8 330 H
RLY27
4.7nF
b
RLY24
a
CAPACITANCE
2
b
L5 10 H
a
b
L10 3.3mH
RLY29
CON3
1
RLY30
Fig.2: the capacitor/inductor portion of the circuit works
almost identically to the resistor circuit shown in Fig.1,
except that only one component of either type is connected
across CON2 or CON3 at any given time.
a
INDUCTANCE
2
SC
2020
b
Micromite-controlled R-C-L Box
Inductance Board
MICROMITE CONTROLLED R-c-lCapacitance
BOX CAPACITANCE & and
INDUCTANCE BOARD
Practical Electronics | June | 2021
19
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.
of inductors are toggled in or out of
circuit by RLY25-RLY29.
As with the capacitors, each inductor corresponds to one output
value, with a range of intervening
values being theoretically possible if
more than one inductor is switched in.
They would be switched in parallel
too. The selected inductance is then
made available at CON3.
Note that with this design, the resistance, capacitance and inductance
are all independent, short of parasitic
coupling between the components.
This small amount of coupling is an
inevitable result of combining these
functions in the same device.
PCB design
Initially, we tried to design a single PCB
to provide all of these functions, but we
found it to be quite difficult to cram it
all into a reasonably sized board.
We considered using a four-layer
PCB but ultimately decided not to do
so, as this would rule out home etching entirely. That might also have led
to a relatively expensive commercially
manufactured board.
But the design lends itself very well
to being split into two double-sided
PCBs, so that is what we did. One PCB
houses the components that provide
the resistor functions, while a second
one has the capacitors and inductors.
In other words, these PCBs correspond precisely to the circuits of Fig.1
and Fig.2. These boards are depicted
in the PCB overlay diagrams, Fig.3
and Fig.4. In essence, the two PCBs
are mounted back to back, forming a
sort-of-four-layer PCB.
It is possible to build just a resistor
box, or just a capacitor/inductor box,
by building one PCB or the other. But
we will describe the construction as
we expect most readers will want,
incorporating all of the features.
20
Pressing the SETUP button opens the Limit Settings page.
Soft limits can be set to avoid non-useful 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.
We have used mostly surface-mount
components as they save some board
space, since they only occupy space on
one side of the board. All the resistors,
capacitors and inductors are 1206-size
(3216 metric or 3.2 × 1.6mm) or larger,
so they are not difficult to work with.
Unsurprisingly, the remaining space
on both PCB is mostly taken up by the
30 relays.
Software features
The software required to provide
equivalent features to a passive resistor
or capacitor box is fairly simple.
The Micromite just needs to be
programmed to produce serial data
for the shift registers corresponding
to the combination of relays for the
desired value(s).
What is more interesting are the
extra features that we have added
now that we have some processing
power available.
The first feature we added to the software is the ability to limit the outputs
to specific values.
This is handy since you can ‘lock
out’ certain component values if they
would either be too low/too high for
the circuit you are testing, and would
either cause damage or prevent it
from functioning.
Even more useful (we think!) is that
we have set it up so that the value the
programmable RCL box is producing
can change automatically.
Troubleshooting and prototyping is
typically a time when both your hands
are busy holding multimeter leads or
wires in place; you won’t have a free
hand to adjust the output on the RCL
box at the same time (unless you have
three or more hands!).
So our design has a mode where it
can automatically sweep each value up
and down, allowing a range of values
to be quickly and easily tested.
Also handy, if you are dealing with
AC or oscillator circuits, is a feature
which calculates and displays the
resonant frequency of the currently selected RC, LC or LR combination. This
may not always align with the circuit
frequency, but can be a handy guide.
Component selection
While we had no trouble sourcing
the necessary parts, it’s worth noting
that the build requires a large number
of parts with different values, one of
This photo shows
how the two PCBs
are piggy-backed
inside the case.
We’ll look at
construction
details next
month.
Practical Electronics | June | 2021
The Display Settings 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.
The Ramp Settings 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.
CON1
CONNECTIONS TO MICROMITE
5V
TX
RX
GND
RST
3
4
5
9
10
14
16
17
18
21
22
24
25
26
3V3
5V
GND
100nF
COIL
COIL
COIL
IC2
IC1
TPIC6C595
TPIC6C595
10k
COIL
COIL
COIL
RLY12
RLY14
10M
2.2k
RLY10
RLY8
4.7M
1k
RLY4
RLY6
330
1.5M
68
680k
150k
3.3
33k
6.8k
6.8M
1.5k
680
3.3M
1M
150
330k
33
RLY13
RLY9
RLY11
RLY7
68k
1.5
15k
RLY2
15
3.3k
6.8
RLY1
RLY3
RLY5
COIL
COIL
COIL
COIL
COIL
COIL
Fig.3: all the components shown in Fig.1 are located on this PCB, which plugs
directly into the Micromite LCD BackPack board via a pin header soldered
along the top. The resistor banana terminals connect to pin header CON1 (or
directly to its PCB pads) via flying leads. On each of the relays, a bar at one end
indicates their orientation on the PCB
100nF
Programmable LCR Reference
3
4
RLY19
470nF
RLY21
1 F
220nF
47nF
RST
9
5
10
14
16
17
24
GPIO21
GPIO22
25
26
3.3
5V
GND
TX
18
100nF
10nF
2.2nF
470pF
COIL
RLY17
91pF
COIL
COIL
22nF
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
RX
GND
CON2
IC3
IC 4
TPIC6C595 TPIC6C595
LC PCB 04104202 C 2020 RevB
10k
COIL
RLY22
RLY29
COIL
L9 1mH
RLY27
COIL
RLY26
COIL
RLY25
COIL
RLY30
L8 330 H
L7 100 H
CON3
L1 100nH
L2 330nH
RLY28
L4 3.3 H
L6 33 H
COIL
Practical Electronics | June | 2021
100nF
COIL
Capacitor selection
The parasitic capacitance across open
relay contacts is around 4pF across
all the capacitor relays (since most
relays will have open contacts at any
one time).
Our measurements indicate that
this is the biggest contributor to stray
capacitance, although it will be subject
to lead and contact variations too; even
moving the leads can change the measured capacitance noticeably!
As mentioned earlier, the baseline
capacitance is set to 10pF by the 5.6pF
capacitor near RLY24, in parallel with
the stray capacitance. This is always in
circuit, and is the reason why the next
values are 12pF, 36pF and 91pF; they
add to the 10pF to produce the (nominal) 22pF, 47pF and 100pF values.
If you have an accurate picofarad
meter, leave the 5.6pF part off and
getting these, and are not concerned
about operation at higher voltages, then
a slightly lower voltage rating (say, 50V)
could be used instead.
The PCB footprints we have used are
slightly oversized (to allow more room
measure the output capacitance once
the build is complete. You can then
subtract this from 10pF and choose
the closest capacitor value you can get.
We’ve specified 100V X7R MLCC capacitors throughout. If you have trouble
COIL
each, and some of these parts cost
practically as much for one or ten as
they are so small.
The exact components you purchase is more critical for the capacitors and inductors.
The actual resistance, capacitance
and inductance values you will get
at the RCL box’s terminals depends
not just on the components fitted,
but also the resistance, capacitance
and inductance of the PCB traces and
relay contacts.
The relays we have chosen add about
75mΩ of resistance, so even with two
in the circuit, that isn’t a big deal. The
PCB tracks add up to at least 68mΩ or
more, as some PCB tracks are longer.
While you could compensate for this,
it is still negligible for most values.
Indeed, the contact and lead resistance
of your connections between the RCL
box and your test circuit could easily
be more than this.
L5 10 H
L10
3.3mH
L3 1 H
Fig.4: this capacitor/inductor PCB is arranged similarly to the resistor PCB, and
they can be soldered back-to-back, sharing the one set of pins along the top.
This allows them both to be plugged into a header socket on the back of the
Micromite BackPack, making a neat module that fits into a small UB3 jiffy box.
21
Parts list –
Touchscreen Wide-range RCL Box
1 Micromite BackPack V3 module with 3.5in LCD touchscreen
[see PE August 2020 for details]
1 Resistor module (see below)
1 Inductance/capacitance module (see below)
1 UB3 Jiffy Box
6 banana sockets (CON1, CON2, CON3)
30cm of medium-duty hookup wire
4 M3 x 9mm tapped or untapped insulating spacers
(eg, nylon)
4 M3 x 32mm panhead machine screws
4 M3 hex nuts (nylon or steel)
1 18-way female header
1 4-way female header
1 18-way male header strip
1 4-way male header strip
Kapton (polyimide) or other insulating tape
Here’s a trick we even seen 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 10µF part. It’s not hard to do as long as you
don’t apply too much heat.
for hand soldering) and will accommodate slightly larger
parts if necessary. You might even be able to use a small
leaded part in one or two places, if required.
We also tried a trick which the part manufacturers sometimes pull off too. Instead of ordering a 10µF capacitor part,
we stacked a pair of 4.7µF capacitors.
If you have to buy your parts in sets of 10, this will save you
some money, although the nominal value will be slightly off.
We soldered the two capacitors together, then fitted them
as though they were a single part. This works fine unless you
apply too much heat and the two parts fall apart. In the past,
we’ve also had success in soldering one SMD component
to the board, then soldering another one on top. The photo
above shows how the result looks.
Inductors
You will have to pick and choose some inductors that match
our specifications. There’s a wide range of nominal frequencies, maximum currents and resistances to choose from, apart
from actually having the correct inductance value.
You may have to compromise on some specifications to
get parts that will fit. We suspect that this variation is why
there aren’t as many inductor boxes around.
As for the capacitors, the PCB footprints suit parts larger
than 3216/1206 size. Many inductors come in in 3226/1210
size (more square than 3216/1206 at 3.2 x 2.6mm); that is
what we used for most of our parts.
You can also stack inductors to get different values, but
remember that their value is reduced when connected in parallel, just like resistors (the current rating increases, though).
But beware that two inductors in close proximity could
interact, giving a different value to that expected.
Construction
Next month, we’ll have the full construction and usage
details for the Touchscreen Wide-range RCL Box.
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
22
Resistor module
1 double-sided PCB coded 04104201, 115x58mm – available
from the PE PCB Service
14 SMD low-profile miniature signal relays with 5V coil (eg,
Panasonic TQ2SA-5V)
2 TPIC6C595 high-current shift register ICs, SOIC-16
2 100nF 50V X7R 3216/1206 size ceramic capacitors
Resistors (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
Inductance/Capacitance module
1 double-sided PCB coded 04104202, 115x58mm – available
from the PE PCB Service
16 SMD low-profile miniature signal relays with 5V coil (eg,
Panasonic TQ2SA-5V)
2 TPIC6C595 high-current shift register ICs, SOIC-16
1 10kΩ 1% 3216/1206 size chip resistor (code 103 or 1002)
Capacitors (all 1 of each, SMD 3216/1206 size X7R 100V if
possible; see text)
10µF
100nF (3 required)
1nF
4.7µF
47nF
470pF
2.2µF
22nF
220pF
1µF
10nF
91pF
470nF
4.7nF
36pF
220nF
2.2nF
12pF
5.6pF (or vary based on stray capacitance; see text)
Inductors (all SMD 3226/1210 or 3216/1206 size except
where noted)
3.3mH (5mm x 5mm footprint)
1mH 330µH
100µH
33µH
10µH
3.3µH 1µH
330nH
100nH
Practical Electronics | June | 2021
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