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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 RCL Box gives you not only 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, it appears to have been discontinued. That is perhaps not surprising when you consider that Dick Smith Electronics were advertising the same product in SILICON CHIP in the late 80s! More recently, we published a Resistor-Capacitor Decade Substitution Box in August 2014 (siliconchip.com. au/Article/7961). This was designed by Altronics, who have it available as a kit (Cat K7520; siliconchip.com.au/link/ ab0z). In 2012, we also published designs for separate capacitor and resistor boxes (siliconchip.com.au/Article/617 and siliconchip.com.au/Article/707 The LCR box can individually select inductance, capacitance and resistance. 24 Silicon Chip Australia’s Australia’s electronics electronics magazine magazine siliconchip.com.au Features The inspiration behind this project: a resistance substitution wheel. We still use one! 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 resistors and E6/E12 for capacitors) are of limited use. Also, they are all fairly large units, fitting into boxes measuring 195 x 145 x 65mm (2014 design) and two 157 x 95 x 53mm boxes (2012 designs). By contrast, this do-it-all RCL box measures just 130 x 67 x 44mm; considerable smaller than either of the earlier designs, while offering more capabilities and being 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. Providing various resistances The Programmable 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 prosiliconchip.com.au • 43 E6 resistance values (1W to 10MW, ±2%, 1/4W) • 19 E3 capacitance values (10p F to 10µF, ±10%, 50V) • 10 E2 inductor values (100nH to 3.3mH, ±20%) • Independent control of R, C and L values via a touchscreen interfac e • Compact design (fits into UB3 Jiffy Box) • Powered from USB 5V • Automatically sweep through valu e ranges • Frequency display based on RC, LC and RL combinations • Based on Micromite V3 BackPa ck with 3.5in LCD touchscreen • Programmed in BASIC vide 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 with the use of 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. Capacitances and inductances Similarly, 19 capacitor values (from the E3 series) from 10pF to 10µF are 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 Desired Paralleled resistor(s) value Desired Paralleled resistor(s) value 1Ω 1.5Ω, 3.3Ω, 33Ω 1.5Ω 1.5Ω 2.2Ω 3.3Ω, 6.8Ω, 330Ω, 680Ω 3.3Ω 3.3Ω 4.7Ω 6.8Ω, 15Ω 6.8Ω 6.8Ω 10Ω 15Ω, 33Ω, 330Ω 15Ω 15Ω 22Ω 33Ω, 68Ω, 3.3kΩ, 6.8kΩ 33Ω 33Ω 47Ω 150Ω, 68Ω 68Ω 68Ω 100Ω 150Ω, 330Ω, 3.3kΩ 150Ω 150Ω 220Ω 330Ω, 680Ω 330Ω 330Ω 470Ω 1.5kΩ, 680Ω 680Ω 680Ω 1kΩ 1kΩ 1.5kΩ 1.5kΩ 2.2kΩ 2.2kΩ 3.3kΩ 3.3kΩ Table 1 – Available resistance values 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 Australia’s electronics magazine 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 June 2020  25 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 valMICROMITE V3 BACKPACK RESET GPIO3 GPIO4 GPIO5 GPIO9 GPIO10 GPIO14 GPIO16 GPIO17 GPIO18 GPIO21 GPIO22 GPIO24 GPIO25 GPIO26 +3.3V +5V GND 1 2 ues could be provided if the software were modified. The complete circuit The front end display and inter- +5V +5V 10k 1 7 3 2 10 5 15 CLR DR7 SDIN DR6 RCK DR5 SCK DR3 8 9 DR2 DR1 G/EN DR0 SDOUT 10 12 TX RX 7 16 2 17 10 18 15 GND 22 3 RLY3 VCC DR7 SDIN DR6 RCK DR5 SCK 8 a RLY4 9 DR2 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 G/EN 15k RLY5 14 3.3 RLY3 100nF CLR 19 21 RLY1 RLY2 b RLY2 RLY14 5 1 14 6.8k a RLY2 11 +5V 13 20 12 16 DR3 5V 13 RLY6 4 1.5 RLY1 GND 11 15 14 RLY8 IC1 DR4 TPIC6 C595 6 7 b RLY1 VCC 6 9 a 100nF 4 8 face is simply the Micromite LCD BackPack V3 described in our August 2019 issue (siliconchip.com.au/ Article/11764). We have mounted two PCBs behind 33 150k RLY5 RLY8 RLY4 b 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 1 a RESISTANCE 2 a b BLACK BAR MARKS RELAY COIL END b b a SC 26 MICROMITE CONTROLLED R-C-L BOX Silicon Chip b 2.2k 6.8M RLY12 a RLY13 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 3.3M RLY10 a 2020 680 1.5M RLY9 RLY14 CON1 b b 3.3k 10M RESISTANCE BOARD Australia’s electronics magazine siliconchip.com.au it to provide the RCL Box functions. The circuit implemented by these boards is shown in Figs.1 & 2. Fig.1 shows the resistor switching functions, while Fig.2 shows the capaci+5V MICROMITE V3 BACKPACK GPIO3 GPIO4 GPIO5 GPIO9 2 7 3 2 4 10 5 15 VCC CLR DR7 SDIN DR6 RCK DR5 DR3 8 8 GPIO16 9 9 GPIO17 DR2 G/EN 14 RLY24 RLY16 RLY17 RLY30 a 5 RLY18 4 RLY19 3 RLY17 RLY19 +3.3V +5V GND TX 16 7 17 2 18 10 DR7 100nF RLY17 SDIN DR6 RCK DR5 9 DR2 G/EN DR1 SDOUT DR0 220pF b 220nF RLY22 12 RLY29 5 RLY22 8 a RLY18 14 RLY26 20 DR3 RLY21 13 RLY21 19 22 GND VCC CLR 11 RLY23 IC4 DR4 SCK TPIC6 C595 TPIC6C595 6 RLY28 21 RX 100nF 1 15 5V 91pF b +5V 15 GPIO26 47nF a RLY20 14 GPIO25 36pF b RLY16 RLY18 16 13 GPIO24 22nF RLY15 RLY25 12 GPIO22 b 12 RLY16 GND 11 GPIO21 DR0 SDOUT 10 GPIO18 DR1 12pF a 13 RLY15 11 IC3 DR4 SCK TPIC6 C595 TPIC6C595 6 7 GPIO14 5.6pF RLY15 1 6 GPIO10 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 +5V 100nF 10k 1 RESET tor 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 a RLY23 470pF b 470nF RLY19 RLY24 4 RLY27 3 RLY20 a RLY25 GND 1nF b 1 F RLY20 16 RLY26 L1 100nH a a b RLY27 L6 33 H RLY25 2.2nF b 2.2 F RLY21 RLY28 L2 330nH a a b RLY29 L7 100 H RLY26 a +5V b L4 3.3 H a CON2 b 1 L9 1mH RLY28 10nF b 10 F RLY23 L8 330 H RLY27 4.7 F RLY22 RLY30 L3 1 H a 4.7nF b RLY24 a CAPACITANCE 2 b L5 10 H a b L10 3.3mH RLY29 CON3 1 RLY30 a INDUCTANCE 2 SC 2020 b 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. MICROMITE CONTROLLED R-c-l BOX siliconchip.com.au CAPACITANCE & INDUCTANCE BOARD Australia’s electronics magazine June 2020  27 The larger 3.5in 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. 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 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 & 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 28 Silicon Chip 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. 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 & IC2 are driven from Micromite GPIO21 (pin 11). This has a 10kΩ pullup resistor to 5V, so when the Micromite is not driving this pin, all those outputs are off and so 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 & IC2. 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, RLY15RLY23 connect This photo shows how the two PCBs are piggybacked inside the case. We’ll look at construction details next month. 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 and IC4 is essentially the same as for IC1 & IC2 in Fig.1, except that this time, the latch (RCK) pins are brought back to the Micromite GPIO21 output (pin 11). Thus, with both boards attached, the Micromite can control them independently. 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 Australia’s electronics magazine siliconchip.com.au 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. some number of capacitors in parallel to the 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, but when 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. 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 later, in the component selection section. As with the resistors, the software doesn’t provide for all the possible capacitance options. Instead, we limit the choice to the E3 range to keep things simple. If we could have combined capacitors to provide the E6 range, we would siliconchip.com.au 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. 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. Inductors The inductors are switched by RLY25-RLY30, with RLY30 switching between two banks of five inductors. The pairs of inductors are toggled in or out of circuit by RLY25-RLY29. As for 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 reasonablysized 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 doubleAustralia’s electronics magazine 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 fitted. 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, Figs.3 and 4. In essence, the two PCBs are mounted back to back, forming a sort-of-fourlayer 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, incorporating all of the features. 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 x 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 June 2020  29 TPIC6C595 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 COIL RLY8 RLY6 RLY4 RLY2 COIL IC2 IC1 TPIC6C595 CON1 COIL 4.7M 330 1.5M 1k 68 680k 15 RLY10 1.5k 680 3.3M 1M 150k 150 330k 33 3.3 33k RLY13 68k 1.5 15k 6.8k 6.8M 6.8 RLY11 RLY9 RLY7 RLY5 RLY3 COIL COIL COIL COIL COIL RLY1 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 RLY19 3 470nF RLY21 1 F 220nF 47nF RST 4 9 5 10 14 16 17 GPIO21 GPIO22 24 25 26 5V 3.3 GND TX RX 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 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 RLY30 L8 330 H L7 100 H CON3 L1 100nH L2 330nH RLY28 L4 3.3 H L6 33 H COIL Silicon Chip 2.2k COIL 30 10M COIL 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 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 10k RLY14 3.3k COIL Component selection 100nF RLY12 COIL 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 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 Box 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. 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. the RCL Box and your test circuit could easily be more than this. 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 (nomAustralia’s electronics magazine inal) 22pF, 47pF and 100pF values. If you have an accurate picofarad meter, leave the 5.6pF part off and 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 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 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. siliconchip.com.au Parts list – RCL Box 1 Micromite BackPack V3 module with 3.5in LCD touchscreen [eg, built from an SC5082 kit] 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 Resistor module 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. 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 accompanying photo 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 RCL Box. SC siliconchip.com.au 1 double-sided PCB coded 04104201, 115x58mm 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) 10MW 106 or 1005 6.8MW 685 or 6804 4.7MW 475 or 4704 3.3MW 335 or 3304 1.5MW 155 or 1504 1MW 105 or 1004 680kW 684 or 6803 330kW 334 or 3303 150kW 154 or 1503 68kW 683 or 6802 33kW 333 or 3302 15kW 153 or 1502 10kW 103 or 1002 6.8kW 682 or 6801 3.3kW 332 or 3301 2.2kW 222 or 2201 1.5kW 152 or 1501 1kW 102 or 1001 680W 681 or 680R 330W 331 or 330R 150W 151 or 150R 68W 680 or 68R0 33W 330 or 33R0 15W 150 or 15R0 6.8W 6R8 or 6R80 3.3W 3R3 or 3R30 1.5W 1R5 or 1R50 Inductance/Capacitance module 1 double-sided PCB coded 04104202, 115x58mm 16 SMD low-profile miniature signal relays with 5V coil (eg, Panasonic TQ2SA-5V) 2 TPIC6C595 high-current shift register ICs, SOIC-16 1 10kW 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 1nF 100nF (3 required) 4.7µF 470pF 47nF 2.2µF 220pF 22nF 1µF 91pF 10nF 470nF 36pF 4.7nF 220nF     12pF 2.2nF 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 Australia’s electronics magazine June 2020  31