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Max’s Cool Beans By Max the Magnificent Arduino Bootcamp – Part 20 let’s start this column with what they call a ‘pop quiz’ in the US (this means an unexpected test; it’s nothing to do with pop music, thank goodness). The way we currently have our clock working, we always display the most-significant (left-hand) digit, even if it’s a 0 (Fig.1a). Suppose we decided to suppress this leading 0 and instead display a blank (Fig.1b). Can you think of a way to implement this in our latest and greatest program from our previous column (PE, July 2024) using the minimum number of modifications? Dazed and confused? With respect to the 7404 and 7408 integrated circuits (ICs) we introduced in our previous column, I received several emails from dazed and confused readers asking about the extra letters marked on the packages, which should read something like SN74LS04N and SN74LS08N. For one stalwart fellow, it was the ‘N’ that threw him into a tizwoz. ‘Have I ended up with the wrong ones?,’ he pleaded, plaintively. Let’s start with the first two letters, which are shown as ‘SN’ here. These refer to the manufacturer, which is Texas Instruments (TI) in this case – under TI’s part numbering scheme, ‘SN’ stands for ‘Semiconductor Network’. The ‘74’ portion of the moniker indicates commercial components that operate over a temperature range of 0°C to +70°C. If this were 64, it would indicate an industrial component (–40°C to +85°C), while 54 would signify a military component (–55°C to +125°C). (a) Display leading 0 (b) Blank leading 0 Fig.1. Do we want a leading 0? 52 There are many different families of these components. Top view I suggested the LS (‘LowSide Power Schottky’) family To other views because they are cheap and rails and breadboard cheerful and do the job. My a k k second choice would have 16V 100µF a = anode (+ve) From Electrolytic been the HC (‘High-Speed k = cathode (-ve) Capacitor Arduino CMOS’) family because these are more tolerant with respect to the voltages on their input Fig.2. Adding a 16V 100µF electrolytic capacitor. signals. On the other hand, ohms (kiloohms or kΩ, meaning 103), they are less tolerant of static, and they may be a tad more expensive. Some or millions of ohms (megaohms or MΩ, hobbyist kits offer both LS and HC types meaning 106). – see: https://bit.ly/44JMZRa By comparison, in the case of The 04 and 08 reflect the types of logic capacitors, their values (capacitances) are functions (the 04 contains six NOT gates; measured in units of farads (F). The term the 08 boasts four 2-input AND gates). ‘farad’ was originally coined by Latimer You can peruse and ponder the entire list Clark and Charles Bright in honour of of available functions on the interweb the English scientist Michael Faraday. at: https://bit.ly/3wON1L3 We typically deal with capacitors having Last, but certainly not least, the final small values measured in millifarads character (or characters) indicates the (mF, meaning 10 –3), microfarads (µF, package type, where N tells us that our meaning 10–6), nanofarads (nF, meaning devices are presented in plastic dual 10–9), and picofarads (pF, meaning 10–12). in-line packages (PDIPs). Why am I waffling on about this now? If you wish to delve deeper, a handyWell… dandy Interpreting TI Logic Part Numbers document is available for your perusing Smoothing things out pleasure at: https://bit.ly/3ylkYn3 As I mentioned at the end of our last column, I ran into a WTW (‘what Little and large the what?’) problem with one of my There used to be a British comedy duo breadboards. During the process of called Little and Large. Straight man diagnosing this problem, I did what I Syd Little (born Cyril John Mead) and should have done much earlier, which is comic Eddie Large (born Edward Hugh to add a 16V 100µF electrolytic capacitor McGinnis) became household names straddling the power and ground rails in the late 1970s and the 1980s. The on the breadboard as close as possible to reason they just popped into my mind the point where the power and ground is that we tend to deal with very small wires arrive from the Arduino (Fig.2). and very large values with respect to This type of capacitor looks like a electronic components (just call me the small soda can with two legs. It’s a ‘Sovereign of Segues’). polarised component (ie, a part with Take resistors, for example. The values polarity), which means it can be correctly (resistances) of these little scamps are connected into the circuit in only one measured in units of ohms (symbol orientation. The longer lead is the anode, Ω). These are named after the German which is connected to the more positive physicist Georg Simon Ohm, who studied rail (the +5V rail, in our case). The shorter the relationship between voltage, current lead is the cathode, which is connected and resistance (and often forgotten, to the more negative rail (the 0V rail, in temperature). We typically deal with our case). The cathode side of the can is resistors having large values measured also marked with a minus sign to give in hundreds of ohms, thousands of us a clue. You can obtain these devices 16v100uF J ust for giggles and grins, Practical Electronics | August | 2024 100nF Ceramic Capacitor Top view Side view 104 7404 Fig.3. Adding an 0.1µF (100nF) ceramic capacitor to the breadboard. from any electronic component supplier, including Amazon – see, for example: https://bit.ly/44LzpNa The 100µF is the device’s capacitive value read as ‘100 microfarads’. The 16V is its rated value, which means it isn’t capable of handling more than 16V. I picked 16V because (a) it provides nice headroom over our 5V supply and (b) it’s a commonly available value. It would be perfectly OK to use a higher rated component like 25V or 50V if that’s what you have available. However, a higher voltage rating typically means a physically larger or more expensive part, often both. This capacitor is used to filter out any electrical noise from the power source, which is the Arduino in our case. It can also help to smooth out any power dips caused by components switching and drawing extra current. This isn’t a mission-critical component for our existing circuit, but I’ve decided to add one anyway. No coupling allowed As you may recall, the terms ‘currentlimiting,’ ‘pull-up,’ and ‘pull-down’ are simply qualifiers we add to resistors to remind ourselves as to the functions we’re using them to perform (the physical devices we use are the same). Similarly, ceramic capacitors can be employed for a wide variety of tasks, including acting in ‘bypass’ and ‘decoupling’ roles. Although these serve different purposes, many people use the terms ‘bypass capacitor’ and ‘decoupling capacitor’ interchangeably. A ‘bypass capacitor’ is usually applied between the power and ground pins of an IC. It provides a low-impedance path to steer voltage spikes and high-frequency noise to ground, thereby ‘bypassing’ the IC. (This noise can come from the power supply or other parts of the circuit or external sources). The term ‘coupling’ refers to the undesired transfer of electrical energy between subsystems. For example, when Practical Electronics | August | 2024 digital logic devices – like our 7404 and 7408 chips – are switching, they can pull the supply rail voltage down by introducing a momentary (transient) current load. This can affect nearby devices, which are ‘coupled’ via their power and ground pins. To prevent this, a ‘decoupling capacitor’ is usually applied between the power and ground pins of the IC (the same as the bypass capacitor). In this case, we can think of the capacitor as acting like a small, localised energy reservoir (a miniature battery, if you will), holding the voltage steady long enough for the main supply to catch up. It’s a good idea to add decoupling/ bypass capacitors in the form of ceramic components to our circuits. If you are unsure as to the number(s) and value(s) of these capacitors, you should check the data sheets for the specific manufacturers’ parts you are using. Having said this, a good rule of thumb is one 0.1µF (100nF) capacitor per IC. This should be positioned as close to the IC’s power pin(s) as possible, connecting this pin to the ground plane in the case of a printed circuit board (PCB), or to the ground rail in the case of a breadboard (Fig.3). Ceramic capacitors aren’t polarised, so we can connect them either way round. Why haven’t I mentioned these before? Well, our current circuit involves only two chips containing combinatorial functions that are less susceptible to noise and voltage dips on the power rail. Things will become more ‘interesting’ when we start to use chips containing sequential (clocked) elements, such as shift registers, for example. Having said this, it’s good to get into the habit of adding these capacitors as a matter of course, so that’s what we are going to do. You can get ceramic capacitor kits containing multiple values – see: https://bit.ly/4bEAUiv One thing that can trip beginners up is that the values printed on these components are not particularly intuitive, so do your best to not get them mixed up. What we are looking for is components that say ‘104’ (if we are lucky) as illustrated in Fig.3. What does this mean? I was hoping you wouldn’t ask. It means 10 x 10 to the fourth power (10 x 104) = 100,000 measured in picofarads (pF), which equates to 100nF or 0.1µF (now, you try explaining this to someone). I’ve added all the capacitors mentioned here into the latest and greatest incarnation drawing of our prototyping platform (file CB-Aug2401.pdf). And, as usual, all the files mentioned in this column are available from the August 2024 page of the PE website: https://bit.ly/pe-downloads Mea culpa I have something to share with you – mea culpa – which means ‘through my fault’ in Latin, and is used to mean ‘it was my fault’ or ‘I apologise.’ So, why am I apologising? Well, I came to realise that I sort of threw you in at the ‘deep end’ in our previous column. I presented you with the schematic and layout for our new 2:4 decoder without any accompanying assembly and test instructions... yup, mea culpa Throughout this series, I’ve enjoined you to take things one step at a time. I should have followed my own advice when it came to the addition of the decoder. ‘It’s just two jellybean logic chips,’ I thought. ‘I can do this in my sleep,’ I thought. ‘What could possibly go wrong?’ I thought. So, you can only imagine my surprise when I first ran the program with the circuit featuring our new decoder and… it didn’t work. Even worse, it didn’t work in a very unintuitive way. That’s when I started to worry about how you were getting on at your end. After a lot of messing around trying to diagnose the issue, eventually resorting to having a video call with my chum, Joe Farr, it turned out there was a problem with the internals of my breadboard in the area where I’d added my 7404 and 7408 chips (sometimes one is tempted to wonder if Gremlins truly are naught but a myth). When I eventually come to remove all the components from this breadboard at some stage in the future, I’m going to pry the back off and look inside to locate the problem. However, since I was pushed for time, and since I knew that everything apart from the decoder was operating as expected, I simply reconstructed the decoder portion of the circuit on a separate half-size breadboard (Fig.4). As you can imagine, I was just a little less cocky this second time around, causing me to implement the decoder in a more staged way, as discussed later in this column. That’s shocking! Before we plunge into the fray with gusto and abandon (and aplomb, of course), we should remind ourselves that semiconductor devices like lightemitting diodes (LEDs), transistors and ICs can be fatally affected by static electricity in the form of electrostatic discharge (ESD). If you happen to build up a static charge (by walking across a carpet in your stocking feet, for example) and you generate a spark, although there’s not much current, that spark can be as high as 35,000V, which can give any semiconductor device with which it comes into contact a very ‘bad hair’ day indeed. 53 Fig.4. Working system with the 2:4 decoder on the half-size breadboard at the bottom. As I said deep in the mists of time when we started Part 1 of this Arduino Bootcamp series (PE, January 2023): ‘To prevent any such unfortunate occurrence, may I make so bold as to suggest you invest in an anti-static mat (https://amzn.to/3g1YH4A) and an anti-static wrist strap (https://amzn. to/3WYZ9Bu). Both typically come with crocodile clips (a.k.a. alligator clips in the US), which you can pull off to reveal banana plugs. My preferred modus operandi is to plug these banana plugs into an anti-static grounding plug (https://amzn.to/3hDIcML), which is – in turn – plugged into a wall socket or a power strip.’ With respect to the anti-static mat, get the largest one you can that fits your workspace (and your budget). Also, and this bit is very important, if you are working with static-sensitive components (diodes, transistors, ICs…), then do not waste your money on a blue or grey ‘Heat-Resistant Silicone Work Mat’ (like this one: https://bit. ly/4bMwYfw). I know the description includes the words ‘anti-static,’ but this 54 is naught but ‘click bait.’ Silicone is an electrical insulator, but anti-static mats must be able to conduct static build-up away to ground. A giveaway clue is that there is no way to connect these mats to ground! Testing times If you don’t already own a digital multimeter, now would be a really good time to invest in one. You have two main options in the form of hand-ranging or auto-ranging products. There are more pros and cons to this decision than you might suppose, but the accuracy will be the same in both cases. For the purposes of this column, we will simply note that a hand-ranging multimeter takes a little longer to set up because you first need to select the desired range. This can be a little tricky for beginners to wrap their brains around. Suppose our hand-ranging multimeter indicates support for the 2V, 20V and 200V DC voltage ranges, for example (I can no longer remember why multimeter manufacturers use these arcane values). The 2V setting will measure voltages up to 2V, the 20V setting will measure voltages up to 20V, and so on. Now suppose we wish to measure a signal that we know can vary only between 0V and 5V, like the signals generated by our Arduino Uno, for example. In this case, we would select the 20V range because this is the closest to the values we want to measure while still being higher than the values we want to measure. If we were to select the 200V range, this would still measure our signal, but with less precision. By comparison, in the case of an autoranging multimeter, the setup is faster because all we need to do is select the DC voltage option. However, this type of multimeter will take longer to make the reading because it first needs to decide on the most appropriate range. For what we are doing here, which will include verifying our power connections and monitoring the states on our logic signals, I would suggest buying a cheapand-cheerful hand-ranging device, such as the ULTRICS Digital Multimeter, which is available for only £9.99 from Amazon at the time of writing: https://bit.ly/4dMDu7V Practical Electronics | August | 2024 One very important point +5V when you opt for a costa conscious offering like this is k to use it only for low-voltage 680Ω and low-current work, say up Probe to 40V and a couple of amps. 680Ω Don’t use it for mains or highera = anode a voltage applications. I’m in no k = cathode way impugning the purveyors k 0V (GND) of these products, but why take any chances? Fig.5. Logic tester. If you decide to delve deeper into electronics, then at some stage in the future you may decide to invest in more sophisticated equipment, for example, an oscilloscope. I was just about to say that, since you are reading this Arduino Bootcamp column, you probably don’t have a tool like this in your collection. But then it struck me that you could be an analogue hero dipping your toes in the digital waters for the first time, in which case (a) you may indeed have an oscilloscope and (b) welcome to the digital world; come on in, the water’s fine. That’s logical One very useful tool for the sort of work we’re currently doing is a logic probe, which can be used to detect and reflect our logic 0 (0V) and logic 1 (5V) states. These are cheap, cheerful and extremely useful. You can buy one already built, like the Laser 5263 for £16.44 (https://bit.ly/4aqwo6h) or the JDXFENG for £3.99 (https://bit.ly/3wSHovd), or you might even opt for a DIY soldering kit version for £8.75 (https://bit.ly/3yv5D3e). I decided a logic probe would be helpful in tracking down my own problem, but I can’t remember where I put mine, and I didn’t want to wait for one to be delivered (or spend any money on one, for that matter), so I decided to whip up a little something with what I had to hand. In a crunchy nutshell, we are talking about using a small breadboard (or part of our existing breadboard), two LEDs, two resistors, and a multicore jumper wire. We will use items from the various kits we purchased in Part 1 of this epic saga (PE, January 2023). A schematic is shown in Fig.5. The reason I selected red and yellow LEDs is that the ones in my kit both claim forward voltage drops of 2V. I would Top View a a Side View k k k a 10 15 k From other breadboards 1 5 P1 P2 P3 20 P4 Fig.6. Six logic probes. Practical Electronics | August | 2024 J I H G F J I H G F E D C B E D C B A a = anode a k = cathode 25 P5 30 P6 Components from Part 1 LEDs (assorted colours) https://amzn.to/3E7VAQE Resistors (assorted values) https://amzn.to/3O4RvBt Solderless breadboard https://amzn.to/3O2L3e8 Multicore jumper wires (male-male) https://amzn.to/3O4hnxk Components from Part 2 7-segment display(s) https://amzn.to/3Afm8yu Components from Part 5 Momentary pushbutton switches https://amzn.to/3Tk7Q87 Components from Part 6 Passive piezoelectric buzzer https://amzn.to/3KmxjcX Components for Part 9 SW-18010P vibration switch https://bit.ly/46SfDA4 Components for Part 10 Breadboard mounting trimpots https://bit.ly/3QAuz04 Components for Part 12 Light-Dependent Resistor https://bit.ly/3S2430m Components for Part 13 BC337 NPN Transistor https://bit.ly/40xAgyS Components for Part 14 HC-SR04 Ultrasonic Sensor https://bit.ly/49AMBq4 Components for Part 15 Real-Time Clock (RTC) https://bit.ly/3S9OjHl Components for Part 18 Long tailed (0.1-inch pitch) header pins https://bit.ly/3U1Vp2z Components for Part 19 Prototyping boards Kit of popular SN74LS00 chips https://bit.ly/3UMkcZ1 https://bit.ly/3wqgzyv Components for Part 20 16V 100µF electrolytic capacitors https://bit.ly/44LzpNa Ceramic capacitors (assorted values) https://bit.ly/4bEAUiv have preferred to use red and green devices, but my green LEDs are noted as having a forward voltage drop of 3V. Let’s assume we leave the end of our probe waving in the air. In this case, once we’ve accounted for the two 2V voltage drops, we are left with 5V – 2V – 2V = 1V to power both LEDs. I’ve decided to use 680Ω resistors (with blue-grey-brown bands). Since there are two of them in series (one after the other), the total resistance is 1,360Ω. Using Ohm’s law of V = I × R, from which we derive I = V / R, we know our current (I) will be 1V / 1,360Ω = ~0.7mA. What this means is that both of our LEDs will glow very dimly indeed. Now suppose we connect the flying end of our logic probe to +5V. This could be our power supply rail or an output from one of our digital ICs that’s currently driving a logic 1 value. In this case, we will have 5V on both sides of our yellow LED, which therefore won’t do anything at all. By comparison, our red LED now has the full 5V across it (and one 680Ω resistor). Removing this LED’s 2V voltage drop gives 5V – 2V = 3V. Since all the current is now flowing through a single resistor, this current will be 3V / 680Ω = ~4.4mA, which means the red LED will glow with a respectable brightness without hurting our eyes. Similarly, if we connect the flying end of our logic probe to +0V (this could be the ground rail or an output from one of our digital ICs that’s currently driving a logic 0 value, our red LED will be disabled and our yellow LED (which we are pretending is green) will glow like a champion. As I previously mentioned, we could implement our logic probe on an unused portion of one of our existing 55 4 Connection 5 No connection & s0 s0 s0 → U1.5 → U2.13 → U2.5 y2 s1 s1 s1 → U1.1 → U2.1 → U2.4 To Transistors U1.6 U1.6 → U2.9 → U2.2 11 U1.2 U1.2 → U2.10 → U2.12 6 y3 U2_2 1 From Arduino & U2_1 5 s0 Pin 10 1 2 U1_1 s1 Pin 11 2 6 U1_3 12 13 & 3 y1 U2_4 10 U1 = 7404 U2 = 7408 9 (a) Schematic & U2_3 8 y0 U2.8 → U2.11 → U2.3 → U2.6 → y0 y1 y2 y3 (b) Wiring List Fig.7. Schematic and wiring list for 2:4 decoder. breadboards. However, if we have a spare board available – preferably a half-size board – then it might be better to employ this because it will be useful for other projects in the future. You can lay this out however you wish, but one possibility is shown in Fig.6. Yes, I know, I got a bit carried away (LEDs… what can I say?). In the discussions below, you will be able to get by with a single logic probe, but more will be better, and they’ll always come in handy in the future. Note that, although I show the power and ground wires feeding our probe board as coming from our other breadboards, we could use a separate +5V supply if we wished. The important point here is that all the breadboards and power supplies must share (be connected to) the same ground (I’ll explain the rationale behind this next month). One point to go along with our philosophy of taking things step-bystep is to start by building only probe P1. Once you’ve added the components as shown, power things up and observe that both LEDs associated with P1 glow faintly. Next, plug the probe into the lower power rail and check that the red LED glows brightly while the yellow LED the breadboard (along with all their wiring), and we’ll do it all again from the beginning. Even if your decoder worked from the get-go (pat yourself on the back), you may still find these discussions to be instructive. We start by returning to our 2:4 decoder schematic (Fig.7a) and using it to create a wiring list (Fig.7b). If you have a photocopier or a scanner handy, it would be a good idea to make a couple of copies of Fig.7. The first check is to make sure our wiring list matches our schematic. Take a colored pencil and highlight a section of wire (not the whole wire) in the schematic, like the bit that goes from s0 to U1.5 (IC1, pin 5), and then highlight the corresponding line in the wiring list. Do this for all the wire segments in the schematic. When you’ve finished, all the wires in the schematic and all the lines in the list should be highlighted. If anything remains unhighlighted, we have a problem that needs to be resolved before we proceed further. Before we do anything else, we want to deactivate our 7-segment displays to (a) make sure they don’t flicker annoyingly and (b) make sure we don’t have multiple is completely off. Finally, plug the probe into the lower ground rail and check that the yellow LED glows brightly while the red LED is completely off. If any of this doesn’t work as stated, start by using your multimeter to verify that you have 5V between the anode (a) of the yellow LED and the cathode (k) of the red LED. Next, check that you have both LEDs plugged in the right way round (compare the schematic to your breadboard layout). If all else fails, try swapping out the LEDs (they could be bad… 100nF Ceramic remember ESD). Capacitors Once you have your first probe working (bravo!), build as many as you wish, one at a time, testing each one as you add it to your breadboard, and then proceed to the next section. 7408 14 (5V) 8 7404 What we should have done 1 7 So, finally we come to what (0V) we should have done the U1 = 7404 first-time round. If you are U2 = 7408 having problems with your From Arduino 11 (s1) 10 (s0) 2:4 decoder, then I know this is going to hurt, but let’s power everything down, remove the Fig.9. Add the ICs along with their power and ground 7404 and 7408 chips from connections and ceramic capacitors. To Displays D3 y3 D2 D1 y2 y1 D0 y0 From 2:4 Decoder (eventually) Fig.8. Disable the displays. 56 Listing 2a. Simple test of 2:4 decoder. Practical Electronics | August | 2024 Time (a) Order of display (b) All off Fig.10. Test values. displays active simultaneously because we know that’s a bad idea. So, connect the ends of the y0, y1, y2 and y3 wires into the lower ground rail (the blue 0V rail) as illustrated in Fig.8. These are the wires connected to the resistors that drive the bases of our transistors. Connecting them to 0V will turn the transistors off, thereby disabling the displays. Next, power everything up and verify that the green and blue LEDs straddling the power and ground rails at the top of the breadboard light up. Now, set your multimeter to the 20V DC range, and then use its probes to check the voltage across the power and ground rails. As a ‘rule-of-thumb,’ I typically apply the black probe first. Start with the pair of rails at the bottom of the board and then the pair at the top of the board. Ideally, you should see a 5V (meaning +5V) value, but the Arduino’s supply isn’t ferociously robust, so you may have a slightly lower reading. In my case, I’m seeing 4.87V, as shown in Fig.4. If your reading is substantially lower than this – say 4.5V, for example – then you have a problem, possibly even a bad breadboard, which is what happened to me. If you get a negative reading like –5V, then this probably means you’ve applied your probes the wrong way round. Your red probe should be on the power rail and your black probe should be on the ground rail. If the reading continues to show negative, then check the probe connections at the multimeter end. The black wire should be plugged into the ‘COM’ (common) port and the red wire should be plugged into the ‘VΩmA’ port. Now, power everything down again, add the 7404 and 7408 ICs to the breadboard, and connect their power and ground pins as shown in Fig.9. Remember that the ‘dimple’ at one end of the package is used to determine which end is which. In this case, both of our chips use pin 14 for power and pin 7 for ground, but every IC is different, so you should always consult the data sheets for any chips you are using. While we’re at it, let’s also add the two 0.1µF ceramic capacitors we discussed earlier. We’ll mount these as close to each IC’s power pin as possible. Feel the power! Re-apply the power, and then check the voltage across pins 7 and 14 of the 7404. I mean this literally – apply the black probe to pin 7 and the red probe to pin 14. Once again, this should reflect a voltage of ~5V. Next, verify that the voltage across pins 7 and 14 of the 7408 is the same ~5V. Testing, testing This is where things get interesting. All we are going to do is repeatedly write the values 0, 1, 2 and 3 to the displays D0, D1, D2 and D3, respectively. Remember that we can have only one display active at a time. Also, since we want to be able to see what’s happening, we’ll set things up so each value will be on for one second and off for three seconds. Let’s start by creating a simple test program (file CB-Aug24-02.txt). I’ve based this on the latest and greatest program from the previous issue, but I’ve stripped out anything extraneous to what we’re trying to do here. As an example, our loop() function is now as shown in Listing 2a. With respect to the f o r ( ) loop established on Line 63, the iDisp control variable repeatedly counts from 0 to 3. We start by turning all our display segments off (Line 63). Then we set the values on our s0 and s1 signals (Lines 66 and 67). We derive these signals from the current value of iDisp (we discussed this last month). Next, we activate the segments associated with the current digit we wish to display (Line 68). Once again, we use the current value of Fig.11. Using logic probes to make sure everything works. Practical Electronics | August | 2024 57 Online resources For the purposes of this series, I’m going to assume that you are already familiar with fundamental concepts like voltage, current and resistance. If not, you might want to start by perusing and pondering a short series of articles I penned on these very topics – see: https://bit.ly/3EguiJh Similarly, I’ll assume you are no stranger to solderless breadboards. Having said this, even if you’ve used these little scamps before, there are some aspects to them that can trap the unwary, so may I suggest you feast your orbs on a column I wrote just for you – see: https://bit.ly/3NZ70uF Last, but not least, you will find a treasure trove of resources at the Arduino.cc website, including example programs and reference documentation. Listing 4a. Modifying the DigitSegs[] array. To be honest, digital multimeters aren’t great for monitoring logic signals, but we can use them ‘at a pinch.’ You might be surprised to learn that analogue multimeters (or analogue voltmeters) are much more useful here. Given a choice, however, we will be better off using the simple logic probes we created earlier. iDisp to select the segments. Finally, we pause for a delay of ON_OFF_TIME, which I’ve set to 1000 milliseconds (1 second). All of this means that when we run our program, the sequence presented on our displays will appear as shown in Fig.10a. Well, the sequence would appear as shown in Fig.10a except for two small points (I’m reminded of Black Adder telling Baldrick, ‘It’s a cunning plan with one tiny flaw’). Can you spot the tiny flaws in our case? You’re correct: (a) we haven’t wired up our 2:4 decoder yet and (b) we’ve forcibly deactivated all our displays as per Fig.8. So, when we first run our program, the output will be as illustrated in Fig.10b. Check that the two purple s1 and s0 wires are connected as shown in Fig.9. That is, one end of the s1 wire should be connected to the Arduino’s digital pin 11, while the other end is connected to U1.1. Similarly, one end of the s0 wire should be connected to the Arduino’s digital pin 19, while the other end is connected to U1.5. We know that our s1 and s0 signals should be repeatedly going through the sequence 00, 01, 10 and 11. If we think about this another way, s0 will be going through the sequence 0, 1, 0, 1 (off for a second, on for a second, ...) while s1 will be going through the sequence 0, 0, 1, 1 (off for two seconds, on for two seconds, …). Assuming your multimeter is still set to 20V in the DC range, place the black probe on U1.7 (that’s pin 7 of the 7404) and the red probe on U1.5. Since pin 5 is connected to signal s0 from the Arduino, you should see your multimeter display showing 0V, 5V, 0V, 5V. If not, re-check that the other end of the purple wire is plugged into pin 10 on the Arduino. If it is, try swapping the wire for a new one. Once you’ve confirmed that you are seeing what you expect, highlight the s0 → U1.5 track segment in a new copy of Fig.7 (or use the old copy and mark it with a different colour). Do this both in the schematic (Fig.7a) and the wiring list (Fig.7b). Keep on doing this with every new segment as it’s verified. Leave the black probe on pin 7 and move the red probe to pin 1, which is connected to signal s1 from the Arduino. Now the multimeter display should show 0V, 0V, 5V, 5V. If not, re-check that the other end of the purple wire is plugged into pin 11 on the Arduino. If it is, try swapping the wire. Listing 4b. Modifying the loop() function. 58 Practical Electronics | August | 2024 If you created only one probe, connect it to U1.5 and watch do is modify our program to replace a 0 with a blank on the LEDs flash yellow, red, yellow, red, then connect it to U1.1 display D3. As always, there are numerous ways to do and watch the LEDs flash yellow, yellow, red, red. If you created this. The approach I opted to use works as follows (file two probes, you can monitor both signals simultaneously. CB-Aug24-04.txt). Now, let’s perform our first test on U2 (the 7408). Add a First, as illustrated in Listing 4a, I modified the instantiation wire to connect U1.5 (s0) to U2.5 and use one of your logic of our DigitSegs[] array on Line 31 by increasing its size probes to verify that you are still seeing the yellow, red, yellow, from NUM_DIGITS (ten elements that are indexed from 0 red pattern on U2.5 (if not, check both ends are plugged into to 9) to NUM_DIGITS+1 (eleven elements that are indexed the right holes in the breadboard and/or swap out the wire). from 0 to 10). This new element, which corresponds to all Add another wire to connect U1.1 (s1) to U2.4 and use one our segments being off, appears on Line 44. We can access of your logic probes to verify that you are still seeing the this using DigitSegs[10] in our code. We could have yellow, yellow, red, red pattern on U2.4. specified this directly as B00000000, but I decided to use U2.4 and U2.5 are inputs to one of the AND gates in U2. If our existing ALL_SEGS_OFF definition. everything looks good, connect a probe to the output of this Next, we need to add a simple test just before we display the AND gate, which is U2.6. In this case, you should be seeing digits. This test is shown on Lines 93 and 94 of Listing 4b. If the yellow, yellow, yellow, red. digit to be displayed on display D3 is 0, we replace this with This is the good part. Disconnect the end of the green y3 NUM_DIGITS, which is defined as being 10. This will cause wire from the ground rail (Fig.8) and connect it to U2.6. the call to DisplaySegs() on Line 102 to display our blank. Display D3 should present blank, blank, blank, 3. That is, it should be off for three seconds and then display the number Next time 3 for one second and then repeat. I know you’re as tired of hearing me say this as I am of OK, now you know the drill. Use a similar process to bring saying it, but in our next column we honest-to-goodness up the remaining displays, one at a time. I only hope you are going to start using a BCD-to7-segment decoder in our will be as happy as I was once everything starts to work as clock. As always, until that frabjous time comes (next planned (Fig.11) month, I pomise!), I welcome your comments, questions All that remains now is to reload the program that uses our and suggestions. 2:4 decoder to control the displays and show the time (file CB-Aug24-03.txt). Cool bean Max Maxfield (Hawaiian shirt, on the right) is emperor of all he And finally… surveys at CliveMaxfield.com – the go-to site for the latest and greatest I hope you haven’t forgotten our in technological geekdom. pop quiz from the beginning of Comments or questions? Email Max at: max<at>CliveMaxfield.com this column. 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