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Max’s Cool Beans By Max the Magnificent Arduino Bootcamp – Part 23 (yet more on BCD!) I have received several emails from readers saying how much they’ve been enjoying our forays into the realm of digital logic design. They were talking about the 2:4 decoder we created out of AND and NOT gates, along with our discussions on ring and Johnson counters that can be implemented using D-type flipflops. A cunning conundrum On the software side of things, circa the late 1970s, monthly magazines like Byte often published programming competitions. First, they picked a simple microprocessor like an 8-bit 6502, 8080, or Z80. Next, they set a task like counting all the 1s in an 8-bit word using the smallest number of machine code instructions and/or the least amount of memory for both instructions and data. Similarly, on the hardware side of things, prior to the introduction of logic synthesis technology in the 1980s, digital logic designers delighted in setting and solving logic problems. One of my favorites is shown in Fig.1. We start with a black box (which means we don’t know what’s inside) that has three inputs (A, B, C) and three outputs (NotA, NotB, NotC). We want each output to reflect the logical negation of its corresponding input. That is, if A is 0 then NotA should be 1 and vice versa. Similarly for B and NotB, and for C and NotC. Our mission is to decide on a set of logic gates inside the box that will implement this function. If we could use any gates we wished, the solution would be easy peasy lemon squeezy: just three NOT gates would be sufficient to do the job. Unfortunately, for the purposes of A B C Black Box NotA NotB NotC Fig.1: A conundrum indeed. Practical Electronics | November | 2024 this puzzle, we are constrained to having only two NOT gates at our disposal. On the bright side, we can also use as many 2- and 3-input AND and OR gates as we wish. Even if you’ve seen this puzzle before (I may have mentioned this little brain-teaser years ago here in PE), try to solve it yourself from first principles. However impossible it may seem, there is a real-world solution (have I ever lied to you before?). I will present the solution in next month’s column. Lest we forget On the off-chance you are new to this series (or mayhap your memory isn’t what it used to be), we are in the process of creating a clock using four 7-segment light-emitting diode (LED) displays, a real-time clock (RTC) breakout board (BOB) and an Arduino Uno microcontroller. We are using this clock as a vehicle to learn interesting stuff while investigating different techniques for driving our displays. The current state of play is that we are employing a single binary-coded decimal (BCD) to 7-segment decoder in the form of a 7448 integrated circuit (IC) to drive all four of our 7-segment displays. We’ve also implemented a homegrown 2-to-4 decoder using a 7408 IC containing four 2-input AND gates and a 7404 IC containing six NOT gates (of which we are only using two). We are using our decoder to multiplex the displays. That means we are presenting different values to each display, switching them on and off so quickly that our eyes and brains are tricked into thinking that all the displays are lit all the time. As usual, you can download a PDF of our latest and greatest breadboard, component, and wiring ensemble (with a file name of CB-Nov24-brd-01. pdf). As is typical, all the files mentioned in this column are available from the November 2024 page of the PE website: https://pemag.au/ link/ac15 Data sheet induced disorientation A top view of our BCD decoder IC is shown in Fig.2. The BCD inputs ( ABCD ) are driven by the Arduino, while the decoded outputs (abcdefg) are used to drive the 7-segment displays (via current-limiting resistors). In our previous column (PE, October 2024), we showed the signal names associated with pins 3, 4, and 5 as ? (‘don’t know’) characters. We used a 10kΩ pull-up resistor to place them in their inactive states, and we omitted them from our truth table and discussions. Well, now it’s time to pull aside the curtains to reveal that the signals associated with pins 3, 4, and 5 are called LT , BI/RBO and RBI , respectively. An overscore, overline, or overbar, is a horizontal line immediately above the text. Digital logic designers see these overscores as indicating activelow signals (ie, they are ‘on’ when low and ‘off’ when high), which is why we used pull-up resistors to place these pins in their inactive-high states. That’s in contrast to the more intuitive ‘active-high’ scheme, where ‘on’ is represented by a high level, and ‘off’ a low level. To be honest, we aren’t going to use these pins ourselves because we have other poisson à frire (fish to fry). Having said that, it’s still worthwhile for us to take a moment to consider what they do and how they work their magic. If we revisit the data sheet for the 7448 (https://pemag.au/link/abzk), we find a cornucopia of confusing truth From Arduino See notes From Arduino B 1 16 VCC C 2 15 f LT 3 14 g BI / RBO 4 13 a RBI 5 12 b D 6 11 c A 7 10 d GND 8 9 e To Displays Fig.2: A top view of the 7448 BCD decoder. 19 Wow! Look at that! (they have weak internal pull-up LT RBI D C B A a b c d e f g resistors), we don’t 0 H H L L L L H H H H H H H L care what the RBI 1 H X L L L H H L H H L L L L signal is doing and 2 H X L L H L H H H L H H L H the outputs will be decoded versions 3 H X L L H H H H H H H L L H of the BCD inputs. 4 H X L H L L H L H H L L H H Actually, that’s 5 H X L H L H H H L H H L H H not 100% true. 6 H X L H H L H L L H H H H H Observe the first 7 H X L H H H H H H H L L L L row (0 in the lefthand column). In 8 H X H L L L H H H H H H H H this one case, the 9 H X H L L H H H H H L L H H RBI signal must 10 H X H L H L H L L L H H L H be pulled high or 11 H X H L H H H L L H H L L H left floating (aka 12 H X H H L L H L H L L L H H unconnected). In a moment, we 13 H X H H L H H H L L H L H H shall see why this 14 H X H H H L H L L L H H H H is the case. 15 H X H H H H H L L L L L L L Now let’s conBI X X X X X X L L L L L L L L sider the last three RBI H L L L L L L L L L L L L L rows (the “FuncLT L X X X X X H H H H H H H H tion” rows) in the table, starting with Fig.3: the complete 74LS48 BCD decoder truth table. the BI row. This tells us that if we tables. The one we are interested in are using the BI/RBO signal as an is Table T2, which I’ve replicated in input and we pull it down to its Fig.3 for your delight. active-low (0) state, we don’t care Remember that this is an old data what any of the other inputs are doing sheet that originated in the early because all the outputs driving the 1970s. The L (low) and H (high) values display will be set to 0. correspond to what, in today’s truth Since the 7448 is intended to tables, we would show as 0s and 1s, drive common-cathode displays, respectively. Also, the X characters this means all the segments on the indicate ‘don’t care’ values. (In the display will be off, resulting in a discussions below, I’m going to use ‘blank’ character, which is why this 0s and 1s instead of Ls and Hs.) incarnation of the signal is called Wrapping our minds around this the “blanking input”. truth table can make our brains Next, let’s turn our attention to wobble on their gimbals, so we shall the RBI row. In this case, we are considering the BI/RBO signal in take things step-by-step. Let’s start with the fact that the its role as an output. This row tells columns associated with the L T us that as long at the LT input is 1, (“lamp test”) and RBI (“ripple blank- then when the RBI input is 0 and the ABCD inputs are 0, this will cause the ing input”) signals are shown on the BI/RBO signal and all the outputs inputs side of the table. In contrast, the column associ- driving the display to be set to 0 (our ated with the BI/RBO (“blanking ‘blank’ character). input”/“ripple blanking output”) In turn, this explains why the RBI signal appears between the inputs signal had to be pulled high or left and outputs in the table, because floating in the first row of the table. it can act as either an input or an It’s only when this signal is 0 and the output. It manages that trick using ABCD inputs are 0 that the outputs will be set to 0; if RBI is 1 when the something called ‘wired-AND’ logic, but we don’t need to worry about ABCD inputs are 0, the outputs will cause the display to present a “0” that here. The reason LT stands for “lamp test” character. is that, in addition to LEDs, devices The final row returns to assuming of this ilk were originally used to that we are using the BI/RBO signal control displays that employed small as an input that is being pulled high or left floating. In this case, placing incandescent bulbs (aka lamps). For the first 16 rows of the table the LT signal in its active-low state (indicated as 0 to 15 in the left-hand will cause all the outputs driving the “Decimal or Function” column), as display to be set to 1, which means long as the LT and BI/RBO signals are all the segments on the display will pulled high or remain unconnected be switched on. I don’t know about you, but fighting my way through the discussions above made my head hurt. If you want to know how I currently feel, try explaining all this to someone else. So, why would the designers of the 7448 go to all this trouble? Well, let’s try to look at things from their point of view. The power, ground, four BCD inputs, and seven decoded outputs come to a total of 13 pins. If they had used a 14-pin package, which was very common at that time, this would have left them with only one free pin to play with. They could have used this pin to implement something like a LT (“lamp test”) or BI (“blanking input”) function but not both. Instead, they opted to use a 16-pin package, which was also very common, and which left them with three unused pins to play with. Part of the deliberations surrounding the creation of the 7448 would have involved looking at the problems design engineers (the users of these devices) were trying to solve at that time. In many cases, those engineers wanted to create pieces of equipment like frequency counters that boasted a bunch of 7-segment displays. For the purposes of these discussions, we will assume those displays had only five digits, but there could easily be 10 or more. Now suppose we have a value like 42. On a 5-digit display, if we didn’t blank the leading zeros, it would be presented as 00042. In addition to being aesthetically unpleasing, this degrades the user’s ability to quickly comprehend the value being presented. As a result, in many cases, we wish to blank any leading zeros. As we’ve seen with our own clock project, this is relatively easy to do if we are controlling our displays using a microcontroller. However, back in the early 1970s when the 7448 was introduced, microcontrollers and microprocessors were only just starting to appear on the scene, and very few engineers were using such components in their designs. Bearing all this in mind, consider a generic 5-digit display using five 7448s BCD decoders. If all the RBI inputs to the decoders are pulled up to their inactive-high states, the result will be no blanking of leading zeros (Fig.4a). By comparison, consider the case 20 Practical Electronics | November | 2024 Decimal or Function Inputs Outputs BI / RBO on the market but a very common type is the CD4511. This part number may be followed by a series of suffixes, but it’s any+ve +ve +ve +ve +ve one’s guess as to what they mean. I asked my friend Joe, who usually emulates a walking encyclopedia when it comes to this sort of thing, but Joe didn’t 7448 7448 7448 7448 7448 know, so he asked his friend ChatGPT, which responded RBI RBI RBI RBI RBI as follows (I’ve edited this for size): (010) 00002 (010) 00002 (010) 00002 (410) 01002 (210) 00102 ’B’ usually indicates a buffered version of the device, ‘N’ From processor or other electronics usually denotes a standard DIP (dual in-line package) part, ‘P’ (b) Automatic blanking of leading zeros could indicate a plastic DIP, ‘C’ might be connected to a commercial temperature range, and ‘I’ might indicate an industrial temperature range. I must say that these “usually”, “could”, and “might” qualifiers RBI RBI RBI RBI RBI make that information a little less helpful than I would have liked. 7448 7448 7448 7448 7448 What the heck. I found a pack RBO RBO RBO RBO of 20 CD4511BE devices on Amazon for around £14 (https:// (010) 00002 (010) 00002 (010) 00002 (410) 01002 (210) 00102 pemag.au/link/ac11), which seems like a good deal to me. From processor or other electronics I have no clue what the ‘E’ suffix might be trying to tell Fig.4: This shows how to use the 7448 feature that provides automatic blanking of leading zeros. us, but at least nothing has Exploded… yet. If we look at the data sheet where the RBI input to the most- Meet the CD4511 (https://pemag.au/link/ac12), we disThere are a variety of BCD decoders cover something very interesting. significant (left-most) BCD decoder is pulled down to its active-low state, after which the RBI inputs to any downstream decoders are fed from the RBO outputs from their upstream counterparts (Fig.4b). In this case, whenever a decoder sees a 0 on its RBI input and 0000 on its ABCD inputs, it will set its RBO output to 0 and disable the outputs driving its 7-segment display. However, any value other than 0000 on its ABCD inputs will ‘break the chain’. That is, this decoder’s RBO output will remain at 1, thereby resulting in any downstream decoders seeing 1s on their RBI inputs and displaying any values presented to them, including zeros. I don’t know about you, but I think this is incredibly clever, especially when we consider that all this functionality was realized using less than 40 primitive logic gates, as seen in the logic diagram provided in the data sheet and presented here as Fig.5. Suffice it to say that I’m constantly amazed by the ingenuity of the design engineers of yesteryear who managed to implement sophisticated functionFig.5: The logic gates used in the 74LS48 IC, from its data sheet. ality using minimal resources. (a) No blanking of leading zeros Practical Electronics | November | 2024 21 From Arduino See notes From Arduino B 1 16 VCC C 2 15 f LT 3 14 g BI 4 13 a LE 5 12 b D 6 11 c A 7 10 d GND 8 9 To Displays e Fig.6: A top view of the CD4511 IC. Consider the top view of this device, shown in Fig.6. Observe that, except for two of its three control pins, this little rascal is pin-compatible with the 7448 we’ve been using. As we see, pin 3 still provides the LT (“lamp test”) function to turn all the segments on, and pin 4 also still offers the BI (“blanking input”) function to turn all the segments off, thereby allowing us to show a blank character. However, pin 4 no longer implements the RBO (“ripple blanking output”) function. The most obvious difference is exhibited by pin 5. As opposed to performing the role of the 7448’s RBI (“ripple blanking input”), this pin now acts as the CD4511’s active-high LE (“latch enable”) input. What does this pin do? Well, let’s ponder the CD4511’s truth table (Fig.7), along with the segment identification, numerical designations and resultant displays (Fig.8), where all is revealed. From the first row in the table, we see that when the LT input is in its active-low state, we don’t care what values are on the other inputs; all the outputs will be on. Similarly, from the second row, we see that when the BI input is in its active-low state, so long as the LT input is in its inactive-high state, we don’t care what values are on the other inputs; all the outputs will be off. When both the LT and BI inputs are in their inactive-high states, and the LE input is set to 0, whatever BCD values are presented to the ABCD inputs will be decoded and the result presented on the abcdefg outputs. The BCD codes 0000 through 1001 result in displayed characters of ‘0’ through ‘9’, respectively. Unlike the designers of the 7448, the creators of the CD4511 took the time and effort to decode the unused Fig.8: Display segment identification and the possible displays from the CD4511, as per the data sheet. 22 input values of 1010 through 1111 into blank characters. It’s the table’s last row that calls for our attention. Once the LE input has transitioned from a 0 to a 1, whatever values were present on the outputs prior to this transition are latched (locked). After this, any future changes to the ABCD inputs will have no effect. The design of the CD4511 is clever on many levels, not least that, since it was developed after the 7448, its near pincompatibility with the 7448 meant that product developers could transition between devices with relative ease. Multiplex no more Inputs Outputs Display LE BI LT D C B A a b c d e f g X X 0 X X X X 1 1 1 1 1 1 1 All On X 0 1 X X X X 0 0 0 0 0 0 0 Blank 0 1 1 0 0 0 0 1 1 1 1 1 1 0 0 0 1 1 0 0 0 1 0 1 1 0 0 0 0 1 0 1 1 0 0 1 0 1 1 0 1 1 0 1 2 0 1 1 0 0 1 1 1 1 1 1 0 0 1 3 0 1 1 0 1 0 0 0 1 1 0 0 1 1 4 0 1 1 0 1 0 1 1 0 1 1 0 1 1 5 0 1 1 0 1 1 0 0 0 1 1 1 1 1 6 0 1 1 0 1 1 1 1 1 1 0 0 0 0 7 0 1 1 1 0 0 0 1 1 1 1 1 1 1 8 0 1 1 1 0 0 1 1 1 1 0 0 1 1 9 0 1 1 1 0 1 0 0 0 0 0 0 0 0 Blank 0 1 1 1 0 1 1 0 0 0 0 0 0 0 Blank 0 1 1 1 1 0 0 0 0 0 0 0 0 0 Blank 0 1 1 1 1 0 1 0 0 0 0 0 0 0 Blank 0 1 1 1 1 1 0 0 0 0 0 0 0 0 Blank 0 1 1 1 1 1 1 0 0 0 0 0 0 0 Blank 1 1 1 X X X X * * * * * * * * *Same as the BCD code applied prior to the 0-to-1 transition on LE Fig.7: The CD4511 truth table. We could replicate our existing 7448based clock implementation using a CD4511 with a single set of currentlimiting resistors. As before, we could multiplex our four 7-segment displays using a 2-to-4 decoder and four transistors. However, the fact that the CD4511 provides latching functionality allows us to opt for an alternative approach, as illustrated in Fig.9. In this case, each 7-segment display is provided with its own set of currentlimiting resistors and its own CD4511 BCD decoder. The ABCD inputs to all the CD4511s are connected in parallel. The LE input to each CD4511 is controlled individually. Our default state will be to have all the LE signals set to 1, which means the CD4511s won’t ‘see’ or respond to any activity on the ABCD inputs. We are no longer multiplexing our displays. All we need to do is place a BCD value on the ABCD signals and then apply a negative-going (1→0→1 ⍽) pulse to the LE input associated with whichever CD4511 we wish to load the BCD value into. Different approaches have their own advantages and disadvantages. The ad- vantages of the 7448-based multiplexing technique using a 2-to-4 decoder is that it requires one BCD decoder, one set of current-limiting resistors, and (in the case of four displays) only six of the Arduino’s pins. The disadvantage is that the more displays we add, the dimmer they get because each display is active for only part of the time. The advantage of the CD4511-based latching approach is that all the displays are on all the time, which means we can add more displays without any dimming. The disadvantages are that we need a separate BCD decoder with currentlimiting resistors for each display and, in the case of four displays, we’re using eight of the Arduino’s pins (although there are ways to reduce that…). As one final point of interest, observe that Fig.9 shows the LT and BI inputs being pulled up to their inactive states. Suppose that – in the fullness of time – we desire the ability to vary the brightness of our displays, perhaps to implement a dimmed nighttime mode. We could easily implement that function with our CD4511 devices by connecting all their BI inputs together and driving them using one of the Arduino’s pulse-width modulation a f g e b c d 0 1 2 3 4 5 6 7 8 9 Blank All On Practical Electronics | November | 2024 3 3 D3 3 D2 3 Connection D0 D1 No connection Ground (0V) 5 10 9 1 2 4 6 7 5 10 9 1 2 4 6 7 5 10 9 1 2 4 6 7 5 10 9 1 2 4 6 7 dp g f e d c b a dp g f e d c b a dp g f e d c b a dp g f e d c b a Display segments and pin numbers Resistor Packs g f e d c b a g f e d c b a g f e d c b a CD4511 CD4511 CD4511 CD4511 LE * * D C B A LE * * D C B A LE * * D C B A LE * * D C B A g f e d c b a BCD Decoders Fig.9: Our new circuit diagram using four CD4511 8421 BCD decoders. * LT and BI inputs pulled-up to inactive states 9 8 7 6 3 4 5 2 Arduino pin numbers Upper Breadboard (PWM)-equipped output pins (we introduced the concept of PWM in PE, March 2023). In this case, we could use the Arduino pin directly; that is, we wouldn’t have to employ any transistors (the use of which we introduced in PE, January 2024) because the CD4511s handle the current associated with driving their displays. If you have spares of everything, including breadboards, you may decide to keep your existing clock as-is and start a new version. Alternatively, if you’ve splashed your cash only on an as-needed basis, you will need to remove the components from your current breadboards before proceeding. The following instructions assume we’re building everything from the ground up. We’re going to go through this with our racing sneakers on because we’ve already done everything discussed here in one form or another. Let’s begin by setting up two breadboards as shown in Fig.10 (the full setup can be found in the file named CB-nov24-brd-02.pdf). I’ve decided to use current-limiting resistors values of 1kΩ in the green and blue power LED assemblies, thereby limiting each LED’s current Practical Electronics | November | 2024 Lower Breadboard Preparing to rock and roll GND 5V Jumpers 16V 100µF Electrolytic Capacitor From Arduino Fig.10: Our new breadboard setup. 23 D2 D1 D0 Upper Breadboard D3 From lower board Fig.11: Adding the four 7-segment displays. consumption to only 2mA. Once you’ve wired everything up, plug in the USB cable to power-up your Arduino, then use your multimeter to verify that you are seeing +5V (give or take) across all the power and ground rail pairs. The easiest way to do this is to apply your multimeter’s probes to the component leads soldered to the header pins on the green and blue LED/current-limiting resistor assemblies. Adding the 7-segment displays Power everything down and add four 7-segment displays on the lefthand side of the upper breadboard as illustrated in Fig.11. Make sure to add four wires connecting pin 3 on each display to the nearest ground rail (the full setup can be found in the file named CBnov24-brd-03.pdf). Before you do anything else, power it up and use your multimeter to reverify that you are seeing +5V (give or take) across all the power and ground rail pairs. Observe the temporary current- limiting resistor and flying lead. Remember that we’ve connected pin 3 of each display to the 0V rail. Also remember that pins 3 and 8 are connected inside each display. Use the end of the flying lead to verify that the segments associated with pins 1, 2, 4, 5, 6, 7, 9, and 10 of each display function as expected, after which you can power it down and remove the lead and resistor from your setup. As reflected in Fig.11, I used a 330Ω resistor for this test. Anything between 150Ω and 680Ω would be OK. The two competing criteria for the resistor’s value are (a) to be low enough to light the LED segments sufficiently without burning out the LED and (b) to be high enough to adequately limit the current should we accidentally probe pins 3 or 8, thereby shorting our 5V and 0V rails. tors (we will consider the rationale behind this value in a moment). One option is to use 4 × 8 = 32 individual resistors, but that would be a pain. Another possibility is to build four 8-resistor BOBs (we discussed how to build these in PE, July 2024). A third option – the one I went for – is to use four pre-built resistor packs, each presented in a 16-pin package containing eight isolated 270Ω resistors, as illustrated in Fig.12. These components are offered by various manufacturers, such as the Bourns 4100R Series (https://pemag. au/link/ac13). They are available from most major component vendors, like Digikey (https://pemag. au/link/ac14). Add wires connecting pin 1 on each resistor pack to the nearest ground rail (the full setup can be found in the file named CB-nov24-brd-04.pdf). We are doing this because the other side of these resistors (pin 16 on the packs) are going to be connected to the decimal point (“dp”) segments on the displays. Since we aren’t using these segments in this implementation, connecting these resistor pack pins to ground will ensure those segments remain off. Once again, power everything up and use your multimeter to re-verify that you are seeing +5V (give or take) across all the power and ground rail pairs. Cunning calculations Fig.12: Adding the four resistor networks. So, why did we decide to use 270Ω for our current-limiting resistors? From earlier columns we know that the red LED segments in our displays can support up to 20mA, which we could achieve using 150Ω resistors. Your first guess might be that it’s the current sourcing capability of the CD4511s that’s the problem. However, when we look at the CD4511 data sheet, we see it proudly proclaiming: “High current sourcing outputs (up to 25mA)”. The real reason is limitations in our power source. I’m assuming that you, like me, are using a USB cable to power your Arduino Uno, which is, in turn, powering your breadboards. I’m also assuming you are using a USB 2.0 port, which can deliver up to 500mA at 5V, so this is the limiting factor. I had a quick Google while no one was looking to discover that an Arduino Uno R3 has an average current consumption of 50mA. As was previously noted, with their 1kΩ current-limiting resistors, each of our four green and blue 24 Practical Electronics | November | 2024 RP2 270Ω Our next task is to add the currentlimiting resistors to the right-hand side of the upper breadboard. In this case, we are going to use 270Ω resis- RP1 270Ω RP0 270Ω Upper Breadboard RP3 270Ω Adding the current-limiting resistors Practical Electronics | November | 2024 CD4511 CD4511 CD4511 CD4511 Lower Breadboard power LED assemblies consumes only 2mA, which equates to 8mA in all. Our DS3231 real-time clock (RTC) consumes a negligible amount of current, but let’s assume 1mA to be on the safe side. “What about our four CD4511 devices?” I hear you cry. Well, the term “quiescent” means “calm”, “inactive” or “dormant”. In electronics, “quiescent current” refers to the amount of current being consumed by an IC when it is enabled but not doing much. If we return to our trusty data sheet, we see that the quiescent current for a CD4511 is 20µA at 25°C. This means that, even if we were using five of these devices, their total quiescent current would be only 100µA or 0.1mA. Again, let’s say 1mA to be on the safe side. So, excluding our 7-segment displays, our total current consumption is 60mA. Let’s also hold 90mA in reserve to power other functions – like a buzzer, perhaps – in the future, which means we have 500mA – 60mA – 90mA = 350mA remaining to power our displays. Since we aren’t using the “dp” segments on our displays, our worstcase scenario would be for all four displays to be presenting ‘8’ characters (ie, “88 88”), each of which lights seven segments, resulting in 4 × 7 = 28 segments lit. If we felt like being clever (or if we were desperate), we might note that, in a 24-hour mode, our hours digits will display values of “00” through “23”. Alternatively, in a 12-hour mode, they will display values of “01” through “12”. This means that the most-significant hours digit only ever displays “0”, “1”, or “2”, with 6, 2, and 5 segments lit, respectively (referring to Fig.8). Meanwhile, the least-significant hours digit may be used to display “0” through “9”, of which “8” lights all seven segments. Similarly, our minutes digits will display values of “00” through “59”. The worst case for the most-significant minutes digit is “0”, which lights six segments, while the worst case for the least-significant minutes digit is “8”, which lights all seven. Putting all this together, we could argue that, if we only ever used our displays to implement a clock, our worst-case scenario would occur at eight minutes past eight (“08 08”), which would light 6 + 7 + 6 + 7 = 26 segments. On the other hand, it’s possible to be too clever (“you’re so sharp you’ll cut yourself”, as my dear old grannie Fig.13: Adding the four BCD decoder chips. used to say). Suppose we decided to use our displays to present other information. In this case, “88 88”, lighting 28 segments could be a possibility, so let’s go with this. Based on our self-imposed limit of 350mA, this means each segment has 350mA/28 = 12.5mA at its disposal. Since the red segment LEDs have a forward voltage drop of 2V, we can calculate our ideal current-limiting resistor values as (5V – 2V) ÷ 0.0125A = 240Ω. The closest (and next highest) value in the Bourns series is 270Ω, so that’s what I used. A pert little poser a total allocation of 350mA to power all four of our 7-segment displays? We will contemplate the conclusion to this conundrum in next month’s column. For now, however, onwards and upwards… Adding the CD4511s Ensure that everything is powered down, then add the four CD4511 BCD decoders to the right-hand side of the lower breadboard, as shown in Fig.13. As part of this, add wires connecting pin 8 on each CD4511 to the nearest ground rail, along with wires connecting pin 16 on each IC to the nearest 5V rail. Also add a 0.1µF ceramic capacitor for each IC as shown, along with a 10kΩ pull-up resistor for each. Once side of each of these resistors should connect to 5V, while the other side connects pins 3 and 4 (the LT and BI inputs) of its respective IC. The latest incarnation of the complete setup can be found in the file named Why should I do all the work? I have a pert little poser for you to ponder. Assume our displays are only ever going to be used to implement a clock. Also assume that this clock can support both 12- and 24-hour modes. Now suppose that, in addition to everything we’ve discussed above, we decide to suppress any leading zeros in the most-significant hours digit (we introduced this concept in Online resources PE, August 2024). For the purposes of this series, I’m going to assume In this case, that you are already familiar with fundamental conwhat would be cepts like voltage, current and resistance. If not, you the maximum might want to start by perusing and pondering a short number of segseries of articles I penned on these very topics – see: ments we might https://pemag.au/link/ac16 see active at the Similarly, I’ll assume you are no stranger to soldersame time? less breadboards. Having said this, even if you’ve used And, based on these little scamps before, there are some aspects to this, what would them that can trap the unwary, so may I suggest you be the ideal value feast your orbs on a column I wrote just for you – see: for our currenthttps://pemag.au/link/ac17 limiting resistors Last, but not least, you will find a treasure trove of to obtain the maxresources at the Arduino.cc website, including examimum possible ple programs and reference documentation. segment current value, assuming Cool bean Max Maxfield (Hawaiian shirt, on the right) is emperor of all he surveys at CliveMaxfield.com – the go-to site for the latest and greatest in technological geekdom. Comments or questions? Email Max at: max<at>CliveMaxfield.com 25 Display pin numbers Display segments Upper Breadboard 7 a 6 b 4 c 2 d 1 e 9 f 10 g 5 dp D0 RP3 270Ω RP2 270Ω RP1 270Ω RP0 270Ω Probe DO NOT Probe Fig.14: Connecting the first 7-segment display. Connecting the Arduino Until now, we’ve been using only the 0V (GND) and +5V pins from the Arduino. Now it’s time to connect the digital outputs driving our clock as shown in Fig.15 (a full version of the setup is available in the file named CB-nov24-brd-07.pdf). Observe the BCD and LE probe points. As you may recall, we dis- Power everything down, then connect pins 9 through 16 of resistor pack RP0 to display D0 as shown in Fig.14. Also add a temporary current-limiting resistor and flying lead as shown. Power-up the Arduino, touch the end of the flying lead to pins CD4511 8, 7, 6, 5, 4, 3, and 2 of resistor pack RP0, 3 and observe segments a , b , c , d , e , f and g light up on display D0, respectively. The reason for the current-limiting resistor is to prevent problems if you accidentally touch pin 1 on BCD probe points RP0, thereby shorting the 0V and 5V rails. D C B A Once you’ve verified 3 2 1 0 the segments, you can 3 2 1 0 power everything down and remove the lead LE probe points and resistor from your 26 CD4511 CD4511 2 1 CD4511 2 1 0 0 3 Lower Breadboard Wiring up the first display cussed the concept of logic probes in a previous issue (PE, August 2024) If you haven’t already done so, use a half-size breadboard to create four such probes as illustrated in Fig.16. Connect probes 0, 1, 2, and 3 in Fig.16 to the corresponding ABCD BCD probe points shown in Fig.15. Remember to connect the 0V and 5V wires connecting the logic probe breadboard to the main clock breadboards. setup (you can see a full drawing in the file named CB-nov24-brd-06.pdf). Fig.15: Connecting the Arduino data pins to the circuit. AREF GND 13 12 ~11 ~10 ~9 8 7 ~6 ~5 4 ~3 2 TX-1 RX-0 CB-nov24-brd-05.pdf. As usual, power everything up and use your multimeter to re-verify that you are seeing +5V (give or take) across all the power and ground rail pairs. Also, check that you are seeing +5V between pins 8 and 16 on each IC. As well, verify that you are seeing +5V between pin 8 and both of pins 3 & 4 on each IC. Arduino DIGITAL IN/OUT (PWM ~) Practical Electronics | November | 2024 From other breadboards ValBcd Inputs to BCD D C B A 0 1 2 3 4 5 6 Mode/ Cycle LE Signals D3 D2 D1 D0 Setup 0 1 2 3 Fig.18: The BCD LE signals. 8 9 3 2 1 0 RP0 270Ω Fig.17: The expected BCD data values as shown on the logic probes when running the test program. If you get this sequence, everything is working correctly so far! Upper Breadboard 7 Stop! Before you power everything up, double-check that you’ve connected 0V to 0V and 5V to 5V. You’d be surprised how easy it is to slip up here. In the best case, reversing the supply polarity will mean it won’t work, but it sometimes is possible to do some damage this way. So it’s best to make a habit of double-checking this sort of thing before you proceed. Do you remember the simple program we created in our previous column (PE, October 2024)? I’m talking about the one than repeatedly counts from 0 to 9, pausing for a second between counts while it presents the current count value on the Arduino’s BCD outputs. Well, let’s take this program and tweak it a little to accommodate things like setting the CD4511’s LE input to logic 1. You can find a copy of this updated program in the file named CB-nov24-code-01.txt. When you power up your Arduino and load and run this program, you should see the BCD count sequence being displayed on your logic probe board as illustrated in Fig.17. Now let’s use this program as a starting point to create a new version that loops around applying a negative-going (1→0→1 ⍽ ) pulse to each of the CD4511’s LE inputs in turn. A copy of this new code is available in the file named CBnov24-code-02.txt. Power everything down, remove probes 0, 1, 2, and 3 in Fig.16 from the BCD probe points shown in Practical Electronics | November | 2024 Fig.15, then re-connect them to the LE probe points shown in Fig.15. When you power everything up and load and run the new program, you should be presented with the sequence illustrated in Fig.18 on your logic probe board. CD4511 Lower Breadboard Fig.16: Making our four basic logic probes. Finishing wiring up display 0 We are so close to finishing that I can taste it. Power everything down and connect the right-most CD4511 to resistor pack RP0 as illustrated in Fig.19. A full version of the latest breadboard setup is available to download in the file named CB-nov24-brd-08.pdf. Now, let’s create a new version of our program that loops around generating a random number between 0 and 9, presenting this number to the BCD inputs to our CD4511, pulsing the LE input low to load this value into the chip, and pausing for a second before doing it all again. We’ll also include a test to ensure that the new random number isn’t the same as the old one. A copy of this new program is available in the file named CB-nov24-code-03.txt. When you look at this program, you’ll see that, even though we currently have only one CD4511 decoder and one 7-segment display fully wired up, the program attempts to load random values into all four decoders. The reason for this is so that we don’t have to modify the program when we get around to connecting our remaining displays. Fig.19: Connect the decoder to the resistors. Power everything up, load and run our new program and observe the random values appearing on display D0 (I could watch this running for hours!). Connecting displays 1, 2, and 3 It’s time to add the remaining wires. Power everything down, then connect resistor packs RP1, RP2, and RP3 to displays D1, D2, and D3, respectively. Next, connect the resistor packs to their corresponding CD4511s. It’s easy to get excited at this point – I know I did – resulting in me forgetting to add the wires connecting the ABCD inputs on the right-most CD4511 to the ABCD inputs on the remaining CD4511s. I felt like a silly sausage when things failed to work as planned. It took me a while to realize what I’d done wrong (or neglected to do right), but I’m sure this won’t happen to you. If, unlike me, you manage to get the wiring right the first time, you will end up with a configuration like 27 To Display D2 To Display D1 To Display D0 RP3 270Ω RP2 270Ω RP1 270Ω RP0 270Ω CD4511 CD4511 CD4511 CD4511 Fig.20: Connecting the remaining displays to our four CD4511 BCD decoders. Vin Components for Part 19 Prototyping boards Kit of popular SN74LS00 chips Components for Part 20 16V 100µF electrolytic capacitors Ceramic capacitors (assorted values) Components for Part 22 SN74LS48N BCD Decoders 16-pin resistor pack (8 × 390Ω) Components for Part 24 4 × CD4511 BCD Decoders 16-pin resistor pack (8 × 390Ω) 28 https://bit.ly/3UMkcZ1 https://bit.ly/3wqgzyv https://bit.ly/44LzpNa https://bit.ly/4bEAUiv https://bit.ly/3zT18jx https://bit.ly/4d0ISDz https://bit.ly/3yzWl6k https://bit.ly/46HZ2PQ SCL BAT SQU GND SDA 32K RST To decoders AREF GND 13 12 ~11 ~10 ~9 8 7 ~6 ~5 4 ~3 2 TX-1 RX-0 This is it, the final step! Power Arduino Connecting the RTC everything down and add the DS3231 RTC BOB to the left-hand side of the lower breadboard as illustrated in Fig.21. A full drawing of our final setup can be found in the file named CB-nov24-brd-10.pdf. DS3231 Remember to add the power and ground wires as shown. Also, remember Lower Breadboard Lower Breadboard From Arduino that shown in Fig.20 (a full drawing can be found in the file named CB-nov24-brd-09.pdf). Remember that our current program is still loaded in the Arduino. This means that when we power everything up again, we should see different random values appearing on each of our displays. to add the I2C communications bus wires between the RTC and the Arduino as shown (SCL = clock, SDA = data). As we discussed deep in the mists of time, the two Arduino pins we’re using are connected in the board to its A4 and A5 analog inputs. That means we can’t use the A4 and A5 pins for anything else if we’re using them for the I2C protocol. All that remains is to modify our old 7448-based clock program to reflect our new CD4511-based implementation. I just did so. You can download and peruse my version of this code at your leisure, in the file named CB-nov24-code-04.txt. All I can say is I’m watching my clock perform its magic and I’m squealing with delight (well, I’m squealing inside my head because I don’t want to worry my wife). This should keep you busy until next time when… but no… let’s not let any spoilers ruin our excitement! As always, I welcome your insightful comments, penetrating PE questions and any suggestions. Upper Breadboard To Display D3 DIGITAL IN/OUT (PWM ~) Fig.21: Adding the real-time clock (RTC). Practical Electronics | November | 2024 The CD4511-based clock happily running. This is a good demonstration of why PCBs took over from point-to-point wiring in commercially-produced electronics! 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