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MITCHELECTRONICS Learn the basics of electronics with Robin Mitchell The 555 Timer IC – Part 2: Enter Logic MitchElectronics is a series of projects by Robin Mitchell that introduces beginners to useful, simple, easy-to-understand circuit designs. Each month, he will introduce fundamental components, theory and ideas used in electronics. The series will cover both analogue and digital electronics. I n last month’s article – The 555 Timer IC Part 1 – we looked at how the iconic 555 timer IC can be used as an astable and monostable, as well as learning about a number of fundamental circuit components, including resistors, capacitors and LEDs. This month, we will learn how to use the astable and monostable circuits in practical applications, including the MitchElectronics 4017 Light Chaser, Traffic Light and Electronic Dice kits. Plus we will introduce a number of new circuit concepts, including a start on how logic chips work. Fundamentals Before we can jump into the practical applications of the 555 astable and monostable circuits, we first need to discuss several new fundamental components and circuit ideas: diodes, potential dividers, a deeper dive into the 555 IC, logic, and an introduction to the cheap and easy-to-use CMOS 4000 series of logic ICs. What are diodes? Diodes are one of the most fundamental and important components (next to resistors and capacitors) and are used to control the flow of current. Like capacitors and resistors, diodes are passive components meaning that they are unable to control a current flow (whereas devices like transistors, op amps and logic chips are active components). Diodes are made of two layers of subtly different kinds of semiconductors, and just like the LEDs (light-emitting diodes) we met last month, diodes only conduct current in one direction. You can think of them as the electronic equivalent of a one-way valve in plumbing. Diodes have two pins: the ‘anode’ which must be positive compared to the other, called the ‘cathode’ if current is to flow – see Fig.1. This is a vital point for diodes, current can only flow through 54 I (milliamps) Anode (+) Cathode (–) V (volts) Fig. 1. Diode schematic symbol and 1N5817 diode. a diode from the anode to the cathode. This makes diodes extremely useful for ‘rectification’, where alternating current (AC) is converted into a direct current (DC), as well as for circuit protection and preventing reversed power supplies from damaging circuits (such as inserting batteries in the wrong direction). Diodes are cheap, widely used and have many uses. You will encounter them in all shapes and sizes as you build and study electronic circuits. Diodes can be made from various semiconductors, but the most common material for diodes is silicon. (Older devices were often germanium.) To make a diode, two differently doped pieces of semiconductor, called ‘n-type’, and ‘p-type’ are fused together – see Fig.2. n-type semiconductors are doped with materials such as phosphorus, arsenic or antimony, which give a semiconductor material an excess number of electrons (hence, N-type for negatively doped). p-type semiconductors are doped with materials such as boron or gallium, which give a semiconductor material a deficit of electrons resulting in net positive doping (hence, p-type). Let’s look at the electrical characteristics of diodes – how its current flow for a given applied voltage varies. Anode (+) p-type silicon Cathode (–) n-type silicon Fig. 2. Diode semiconductor structure. I-V curve of an ideal diode I (milliamps) 200 V (volts) –10 0.5 1.0 I-V curve of an real diode (eg, 1N914) Fig. 3. Voltage -current characteristics of an ideal diode vs real diode. An ideal or ‘perfect’ diode conducts current in one direction only, with no voltage drop across it; zero resistance when conducting, and infinite resistance when not conducting. In reality, diodes are far from perfect, and have a few important characteristics, including a forward voltage drop, and a non-linear current behavior. Looking at the graph shown in Fig.3, barely any current flows through a diode until the voltage across the diode goes beyond its ‘forward voltage’, often abbreviated to Vf. This can be thought of as the voltage needed to turn on the diode and make it function. When a diode has sufficient voltage across it to make it conduct, it is said to be forward biased. For silicon diodes, this forward voltage is typically around 0.7V, but it can be as low as 0.5V and as high as 1V (and remember it is typically 1-2V for LEDs). One neat feature of the forward voltages is that because the voltage across a diode cannot exceed the forward voltage of the diode (when used within the diode’s safe operating parameters), diodes can be used to clamp voltages. If the input voltage shown in Fig.4 exceeds the forward Practical Electronics | January | 2024 𝐼𝐼 = 𝐼𝐼 = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 𝑉𝑉 𝑅𝑅 𝑉𝑉 𝑉𝑉!" = 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉!" × 𝑅𝑅$ = 𝑉𝑉%&' 𝑅𝑅# + 𝑅𝑅$ Fig. 4. Diode-resistor circuit comparing the voltage across the diode and resistor in series. 𝑉𝑉 = 𝐼𝐼𝐼𝐼 voltage, then the remaining voltage is At this stage there 𝑉𝑉 𝑅𝑅$ are three things to notice: 𝑉𝑉 = 𝐼𝐼𝐼𝐼 𝐼𝐼 = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 n  ‘dumped’ across the series resistor. This R is much smaller than R1, which agrees 𝑉𝑉 = 𝑉𝑉 × %&'2 !" 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉 = 𝐼𝐼𝐼𝐼 can be handy for protecting circuits with our explanation that the bigger or 𝑉𝑉 and devices that may be damaged by smaller R2 is compared to R1 the bigger Since the 𝐼𝐼 =resistors are in series we know 𝑅𝑅 excessively large input voltages, such the total resistance seen by Vin is just R1 or smaller will be its proportion of the 𝑉𝑉 𝑉𝑉 𝑉𝑉R , and 𝑉𝑉𝐼𝐼!"thus 𝐼𝐼 = as most integrated circuits. voltage + the current though R and = 2 1 𝑅𝑅$ drop. 1 𝑉𝑉!" 𝐼𝐼 = = 𝑅𝑅 𝑉𝑉%&' = 𝑉𝑉!" ×he equation = 𝑉𝑉!" × the = ratio of resistor n The two diodes that MitchElectronics R𝑅𝑅2 is:𝑅𝑅# + 𝑅𝑅$𝑅𝑅 𝐼𝐼 = 𝑉𝑉 T gives 𝑅𝑅 + 𝑅𝑅 2 2 $ $ 𝑅𝑅 kits use are the 1N4148 signal diode and values, not the actual values. You could 𝑉𝑉 𝑉𝑉!" the 1N5817 Schottky diode. These are use 1kΩ and 7kΩ, or 100kΩ and 700kΩ 𝐼𝐼 = = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ very common in electronic circuits, with and the result would be the same. This 𝑉𝑉 𝑉𝑉!" 𝑉𝑉!" 𝑉𝑉 𝑉𝑉𝐼𝐼!"= 𝐼𝐼 = = =to calculate the voltage the 1N4148 being great for low-voltage, flexibility can be useful. we need 𝑅𝑅 4700 𝑅𝑅 𝑅𝑅 + 𝑅𝑅 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = Now × 𝑅𝑅 = 𝑉𝑉 $ 𝑉𝑉 𝑉𝑉 𝑅𝑅# + %&' 𝑅𝑅 $ #$ $ !" 𝑉𝑉%&'we = 𝑉𝑉use = 5.1 ×we wanted a voltage = 4.2𝑉𝑉drop to 𝑅𝑅# + 𝑅𝑅𝑉𝑉$𝑅𝑅 !" × 𝐼𝐼which = =is n  low-current signals, while the 1N5817 is across R=2,𝐼𝐼𝐼𝐼 V ; again, A lthough out 𝑅𝑅 + 𝑅𝑅 1000 + 4700 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ # $ great for rectifying power, thanks to its Ohm’s 𝑉𝑉 law: 1/8 of V 𝑉𝑉 , the resistor ratio is 1/7 – this in !" 𝐼𝐼 = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = × 𝑅𝑅$ = 𝑉𝑉%&' maximum forward current of 1A. is because the resistor calculation uses 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉!" 𝑉𝑉𝑅𝑅 R /(R +R ), and not R /R – you always !" 2 1 2 1 2 𝑉𝑉𝑉𝑉== = 𝑉𝑉%&' $ 𝑉𝑉 ==𝐼𝐼𝐼𝐼 × 𝑅𝑅$ ×=𝑅𝑅𝑉𝑉$%&' =×𝐼𝐼𝐼𝐼 𝐼𝐼𝐼𝐼=𝑉𝑉𝑅𝑅 𝐼𝐼𝑅𝑅 need𝑅𝑅$to do 1the actual calculation to What are potential dividers? 𝑉𝑉%&' 𝑉𝑉!"𝑉𝑉 𝑉𝑉!" == +𝑅𝑅 𝑅𝑅𝑅𝑅 #$+ 𝑅𝑅 $ # 𝐼𝐼𝐼𝐼 = 𝑅𝑅 + 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = × 𝑅𝑅 # $ $ = 𝑉𝑉%&' 𝑉𝑉 = 𝐼𝐼𝐼𝐼 find the A potential divider is a special resistor 𝑅𝑅# + 𝑅𝑅$correct 8 values and don’t just 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉 𝑉𝑉 !" assume it’s ‘obvious’. Rearranging a 𝑅𝑅 little, we finally get: combination that, as the name suggests, $𝐼𝐼 = = 𝑉𝑉%&' = 𝑉𝑉!" × n can be used to divide a potential, ie, a Resistors come in ‘funny’ values and 𝑅𝑅# + 𝑅𝑅$ 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉 𝑅𝑅$ 𝑉𝑉𝑅𝑅$ 𝐼𝐼 𝑉𝑉 =𝑉𝑉 𝑉𝑉 =𝑉𝑉!" voltage. (Remember, ‘potential’ is just not always the most convenient steps 𝑅𝑅 1 𝑉𝑉 × $ 𝐼𝐼= = %&' 𝑅𝑅 𝑉𝑉𝑅𝑅!"𝑉𝑉 ××!" =𝑅𝑅 !" %&' = + 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ while there is a 1kΩ or 𝑉𝑉%&' = 𝑉𝑉!" × 𝐼𝐼𝑉𝑉= = 𝑅𝑅 𝑅𝑅 + 𝑅𝑅 # $ $ = a slightly archaic term for ‘voltage’.) –8𝑅𝑅 for example, !" $ #2$ # +=𝑅𝑅 𝑅𝑅$ + 𝐼𝐼𝑅𝑅 𝑅𝑅= 𝑉𝑉𝑅𝑅%&' 𝑉𝑉!" ×$2 $ 𝑅𝑅 Whenever you hear the term ‘potential 100kΩ value, there is no such thing as 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉!"𝑉𝑉 divider’ it simply means to divide/reduce a 7kΩ or 700kΩ resistor. Often you have Notice 𝑅𝑅that R1 ==R12 then $ 𝑉𝑉 if !" the equation = 𝑉𝑉%&' = 𝑉𝑉!" × = 𝐼𝐼𝐼𝐼 𝑉𝑉!" × 𝑅𝑅#=+ 𝑅𝑅$ × 𝑅𝑅$ = 𝑉𝑉%&' 𝑉𝑉 𝑅𝑅$ +𝑉𝑉𝑅𝑅 a voltage by a controlled amount. to play around with available values becomes: 2 2 !"$ 𝐼𝐼 = 𝑉𝑉 = 𝑉𝑉!"𝑅𝑅$𝑉𝑉!"𝑅𝑅$ 𝑅𝑅# the = 7𝑅𝑅 1 𝑉𝑉1!" 𝑉𝑉!" $ So how does it work? When two (or to get ratio you want. For the above 𝑅𝑅 4700 𝐼𝐼 = = =×𝑉𝑉𝑅𝑅 =×𝑉𝑉!" = × = 𝑅𝑅 +𝑉𝑉!"𝑅𝑅×$ 𝑅𝑅 $ 𝑉𝑉 %&' !"# × 𝑉𝑉𝑉𝑉%&'= = 𝑉𝑉= =$𝑅𝑅𝑉𝑉= 𝐼𝐼𝐼𝐼 = !"𝑉𝑉 !" 𝑉𝑉 %&' 𝑅𝑅 𝑅𝑅 + 𝑅𝑅 𝑅𝑅 + 2 2 𝑉𝑉 = 𝑉𝑉 × 5.1 × = 4.2𝑉𝑉 1 𝑉𝑉 # $ 𝑅𝑅 + 𝑅𝑅 2 2 $ $ 𝐼𝐼 = = %&'in series !" $ !" 𝑅𝑅 + 𝑅𝑅 $ $ more) resistors are placed (in example, a good choice would be 1.3kΩ # $ 𝑅𝑅# + 𝑅𝑅$ 𝑉𝑉%&' = 𝑅𝑅 =𝑅𝑅𝑉𝑉#1000 !"+×𝑅𝑅$+ 4700= 𝑉𝑉!" × 𝑅𝑅$ + 𝑅𝑅$ 2 2 line, as opposed to in parallel), the voltage and 9.1kΩ, since: 𝑅𝑅$ 𝑅𝑅$ 4700 across each resistor will be proportional 1300 1 𝑉𝑉%&' = 𝑉𝑉!" × = 5.1 × 𝑉𝑉%&' = 𝑉𝑉!" × = 4.2𝑉𝑉 𝑅𝑅 + 𝑅𝑅$ # +other 𝑅𝑅𝑉𝑉!" words,1000 + 4700 = to its resistance, such that the larger a 𝑅𝑅#In input voltage is halved, 𝑅𝑅$ × 𝑅𝑅$ 𝑅𝑅=the 4700 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = 𝑅𝑅$$ 𝑉𝑉!" 𝑉𝑉 1300 + 9100 8 4700 %&' 15.1 𝑉𝑉𝑉𝑉 ==× 𝐼𝐼𝐼𝐼 = ×=𝑅𝑅 𝑉𝑉𝑉𝑉!" ×𝑉𝑉%&'would = = 4.2𝑉𝑉 𝑅𝑅𝑅𝑅× +𝑉𝑉!" 𝑅𝑅is resistor is, the larger the proportion of!" which what you intuitively $$= 𝑉𝑉%&' 𝑉𝑉 =%&' = ×5.1 4.2𝑉𝑉 #$ $ = 𝑉𝑉 × 𝑅𝑅 + 𝑅𝑅 %&' !" 𝑅𝑅 + 𝑅𝑅 1000 + 4700 = 𝑅𝑅 4700 #𝑅𝑅#$ $ $$𝑅𝑅𝑅𝑅$+= 1000 + 4700 𝑉𝑉 = 𝐼𝐼𝐼𝐼𝑅𝑅𝑅𝑅expect = +𝑅𝑅 𝑉𝑉 #two $ %&' the input voltage across it. resistors are identical. 8× 𝑉𝑉%&' =##𝑉𝑉+ =𝑅𝑅5.1 × = 4.2𝑉𝑉 It’s worth noting that one or both of the 𝑅𝑅 + 𝑅𝑅$the $ if !"# × 1000 + 4700 # + 𝑅𝑅particular $ The most basic and most common If we𝑅𝑅have values of V1in, R𝑉𝑉 𝑅𝑅 1 !" elements in a potential divider can be a $ 𝑅𝑅𝑉𝑉$%&' = 𝑉𝑉1!" × = 𝑉𝑉 × = !" potential divider consists of just two and R2 and want to calculate the resulting = + 𝑅𝑅$ 2 2 potentiometer – a variable resistor, which 𝑅𝑅# +voltage 𝑅𝑅$𝑅𝑅$𝑅𝑅8 (V 𝑅𝑅)$ then resistors, as shown in Fig.5: R1, R2, an𝑉𝑉 output means you can vary the output. Also, you we just plug 𝑅𝑅 1 ×$𝑅𝑅$ $$ 1 out %&' = 𝑉𝑉!" 𝑅𝑅 𝑉𝑉!" 𝑉𝑉 = =1use 𝑅𝑅𝑅𝑅 𝑅𝑅 = 𝑅𝑅𝑉𝑉#!"values. +× 𝑅𝑅+ = #𝑅𝑅 $!" $× $+ can have as many resistors and outputs input voltage and an output voltage. in the Let’s V = 5.1V, R = $ 𝑉𝑉%&' =𝑉𝑉8𝑅𝑅 𝑉𝑉%&' = 𝑉𝑉 × = in 1 𝑅𝑅 !" 𝑅𝑅𝑅𝑅## + 𝑅𝑅8$$ 28 12 𝑅𝑅+ = 𝑉𝑉$!" 𝑅𝑅R #×+ = as you want, although the analysis can The voltage across R1 and R2 is the input %&'1kΩ𝑅𝑅and 𝑅𝑅$2# $=+4.7kΩ: 𝑅𝑅$ 𝑅𝑅# + 𝑅𝑅$ 8 𝑅𝑅$ 4700 get a bit longwinded. Finally, and this voltage, referred to as Vin, and the voltage 8𝑅𝑅= 𝑉𝑉%&' 𝑉𝑉!"𝑅𝑅#×+ 𝑅𝑅$ = 5.1 × = 4.2𝑉𝑉 $ = is beyond the scope of this article, you across R2 is the output voltage, Vout. 𝑅𝑅 + 𝑅𝑅 1000 + 4700 # 1 $ 𝑉𝑉 𝑅𝑅 𝑅𝑅 =$𝑅𝑅7𝑅𝑅$ !" 1 𝑉𝑉 can use other types of components in a So how do we calculate Vout? First $ = !" 8𝑅𝑅 = 𝑅𝑅 + 𝑅𝑅 𝑉𝑉%&' let’s = 𝑉𝑉!" ×𝑅𝑅$# 8𝑅𝑅 = 𝑉𝑉 × = $ # $ !" 𝑅𝑅 + 𝑅𝑅 4700 $ #𝑉𝑉 × $ 𝑉𝑉%&' = 𝑅𝑅$ +𝑅𝑅=𝑅𝑅 !" 21 =2𝑉𝑉!" = 4.2𝑉𝑉 $ $5.1 𝑉𝑉%&'through = 𝑉𝑉!"== ×𝑉𝑉𝑉𝑉!"×× ×8𝑅𝑅 potential divider, for example a capacitor, calculate the current running 𝑅𝑅 + 𝑅𝑅 2 2 $ $ 𝑉𝑉%&' = 𝑉𝑉 × = = 𝑅𝑅 + 𝑅𝑅 𝑅𝑅!"# + 𝑅𝑅 𝑅𝑅$ + 𝑅𝑅 1000 !"$ 2+# 4700 2$ which would let you produce a simple both resistors using Ohm’s law: $ 𝑅𝑅 = $ 7𝑅𝑅 # $ 𝑅𝑅$a bigger1 resistance filter. All in all, the potential divider is a Notice that since R has 2 1300 1 = 𝑉𝑉 = 𝐼𝐼𝐼𝐼 𝑅𝑅7𝑅𝑅 = 7𝑅𝑅across = 𝑅𝑅 + 𝑅𝑅 8 very powerful and useful circuit element than R the voltage it (V ) is larger. # $ 𝑅𝑅 # $ # $ 1 out 𝑅𝑅$ + 9100 8 4700 1300 𝑅𝑅$ Finally, 4700 = get: 𝑉𝑉!" × = 5.1 × how = 4.2𝑉𝑉a potential that is well worth mastering. do you create Or dividing both sides by𝑉𝑉R %&'we 𝑅𝑅 = 7𝑅𝑅 𝑅𝑅 1 # $ $ 𝑉𝑉%&' = 𝑉𝑉!" × = 5.1 × = 4.2𝑉𝑉 𝑅𝑅# +𝑅𝑅𝑅𝑅 1000 + 4700 4700 $ $𝑅𝑅 = 𝑅𝑅 + 1000 + 4700 1300 1 If you want to save a little time, you divider with a particular output for a given # $ 𝑉𝑉%&' = 𝑉𝑉!" × =# 5.1 𝑅𝑅 + 𝑅𝑅× 8= + 4700 = 4.2𝑉𝑉 $ 1000 𝑅𝑅# + input? 𝑅𝑅1300 $ can access the online MitchElectronics Let’s say you + 9100 8 want a divider that 𝑉𝑉 1300 8𝑅𝑅 1300 1 $ of =1𝑅𝑅# +input 𝑅𝑅$ (×1/8, 𝐼𝐼 = I potential divider calculator – see: gives you one eighth = the = 𝑅𝑅 R1 1300 + 9100 8 1300 1 1300 + 9100 8 https://bit.ly/pe-jan24-pdcalc – but in the or ×0.125). What this means is that: Battery 𝑅𝑅$ 1 + 𝑅𝑅$ = 1300 1 + 9100 = 8 long term you really need to be able to do 8𝑅𝑅 = 𝑅𝑅 + 𝑅𝑅 Vin = 𝑅𝑅# +𝑅𝑅$𝑅𝑅 $ $𝑅𝑅 #818 $ – 𝑅𝑅# + $= this calculation yourself! R2 Vout 𝑅𝑅# + 𝑅𝑅$ 8 𝑅𝑅# = 7𝑅𝑅$ 𝑉𝑉 𝑉𝑉!" A great use for potential dividers is 𝐼𝐼 = = 𝑅𝑅 𝑅𝑅# + 𝑅𝑅$ in sensor circuits, where one of the If we cross multiply each side we get: Vin resistors is replaced with a resistive # =+7𝑅𝑅 8𝑅𝑅$ =𝑅𝑅𝑅𝑅 𝑅𝑅 $ I= R1 + R2 8𝑅𝑅$ =#𝑅𝑅# +$𝑅𝑅$ sensor element. For example, if R 2 is 1300 1 8𝑅𝑅$ = 𝑅𝑅# + 𝑅𝑅$ Vin × R2 = Vout = I × R2 = Leading to: replaced with a PTC (positive temperature R1 + R2 1300 + 9100 8 𝑉𝑉!" coefficient) thermistor whose resistance 𝑉𝑉 = 𝐼𝐼𝐼𝐼 = × 𝑅𝑅$ = 𝑉𝑉%&' 1 𝑅𝑅# +5.𝑅𝑅Potential 𝑅𝑅# 1300 = 7𝑅𝑅$ $ goes up with an increase in temperature, Fig. divider circuit. 𝑅𝑅# + =9100 7𝑅𝑅$ = 8 1300 𝑅𝑅# = 7𝑅𝑅$ Practical Electronics | January | 2024 55 𝑉𝑉%&' = 𝑉𝑉!" × 𝑅𝑅$ 1300 1300 = 1 1 Vcc Control pin 8 pin 5 Reset pin 4 Flip-flop R R1 Threshold pin 6 R Output pin 3 S R Comparator 1 Trigger pin 2 Comparator 2 Fig. 6. Schematic of a 555 IC, its RC voltage and output (see last month for more 555 operation details). R having a value of 5kΩ. These three resistors form the potential divider shown in Fig.7, and as we just learned about potentiometers, the voltage drop across each one is proportional to the input voltage. As each resistor is identical, each has a voltage drop across it of 1/3 of the voltage supply. But, as the sum of these voltages must be equal to the supply voltage (this is a very important rule in electronics), this also means that the voltage at the first resistor is the voltage supply itself, the voltage at the second resistor is 2/3 of the voltage supply, and the last is 1/3 of the voltage supply. Returning to the 555 internal schematic, the 1/3 and 2/3 voltages are connected to the comparators, this means that our trigger and threshold voltages are 1/3 and 2/3 of the voltage supply. These are the values that the capacitor charges and discharges to. (We haven’t met comparators yet, but in essence these are circuit elements that ‘compare’ two voltages and their output goes high or low depending on which of their two inputs is bigger.) Discharge pin 7 Gnd pin 1 then the voltage across the thermistor will increase as its temperature increases. The same could also be done with a light-dependent resistor (LDR), whose resistance decreases as the intensity of light falling on it increases. In this case, the voltage across the LDR decreases as the light intensity increases. Note that most LDRs on the market are based on cadmium, which is a toxic (carcinogenic) material. We recommend avoiding LDRs for light sensors, and instead use a phototransistor and/or photodiode. All MitchElectronics kits that use light sensors use photodiodes, which are safe to use and RoHS compliant. 555 in more detail Last month we looked at how the 555 IC works, with the capacitor charging and discharging to control the state of the 555. It’s important to understand that the the voltage across the capacitor (Fig.6, right), doesn’t go right to the power supply and back down to ground, but instead, rises and falls between two trigger points (otherwise known as the trigger and threshold voltages). But how exactly are these voltages defined? If we look at the inside of the 555 IC (Fig.6, left), you will notice that there are three resistors in series, each Vin + – R1 1V 3 in R2 1V 3 in R3 Vin 2V 3 in 1V 3 in Fig. 7. Voltages across the three internal 5kΩ resistors of a 555 IC. 56 Introduction to logic you can see it only take two values and is hence binary in nature. In our 555 astable and monostable circuits, the voltage across the timing capacitor varies throughout time, and it is this continuous range of possible values that makes this capacitor voltage analogue. However, the output of the 555 IC is either high (VCC) or low (0V), and hence, we call the output digital. Because the 555 timer IC has both analogue and digital components, it is referred to as being a ‘mixed-signal’ IC. What are CMOS ICs and the 4000 Series? So far, we have only looked at one IC, the 555 timer, a mixed-signal device dealing with both analogue and digital voltages. However, many ICs only deal with digital signals. These range from very simple devices up to the most sophisticated microprocessors. At the ‘simple’ end we have logic devices that process digital signals with simple functions, often called ‘gates’, or sub-systems built up from gates, such as counters. Arguably, the two most famous families of logic devices are the 7400 and 4000 series. The older of the two, the 7400, was initially brought out in 1966 by Texas Instruments to help engineers reduce the So far, we have looked at circuits Magnitude whose voltages and currents have (volts) been continuous, meaning that over 20 a range, they could be any value: 1V, 0 5V, 2.384V, or for current 1A, 0.659A… and so on. In the field of electronics, –20 such continuous values are thought of as ‘analogue’, which is how analogue electronics gets its name. Fig.8 shows Fig. 8. Example of an analogue signal. a continuous, analogue signal. However, in digital electronics, Magnitude voltages only have one of two discrete (volts) states – high or low – also called on/off, 5 1/0 or true/false respectively. As these values can only be one of two different 0 states, they are said to be ‘binary’, and this is why binary numbers (base 2) and binary arithmetic are so easily used in electronics. Fig.9 shows a digital signal; Fig. 9. Example of a digital signal. Time (secs) Time (secs) Practical Electronics | January | 2024 +5V Pin 14 Pin 8 Pin 1 Pin 7 7400 TTL quad NAND gate 0V Fig. 10. Example of a 7400N and its internal gates. Fig. 11. Most computing systems from the 1980s used ICs from the 7400 and 4000 series of logic devices. This photo shows the motherboard of a Sinclair ZX Spectrum. number of components on circuit boards by integrating logic circuits into silicon chips. The popularity of these chips was so massive that the 7400 quickly accounted for over 50% of the logic market shortly after being released to the public. Fig,10 shows a typical 14-pin DIL 7400-series IC; in this case a quad NAND gate chip. The 7400 series used energy-hungry design techniques (called TTL), which where fast but consumed a large amount of current. Recognising this problem, RCA developed the 4000 series of logic chips which used the much more energyefficient CMOS technology. While the first CMOS devices were much slower compared to their TTL counterparts, the fact that they consumed far less power made them ideal for lowpower environments – for example, battery-powered applications. Eventually, as CMOS technologies improved, not only did CMOS logic devices come to match the speed of TTL, but rapidly surpassed it and became the dominant logic technology that is now used throughout electronics. In fact, CMOS technology was so beneficial to engineers that many 7400 series devices now have CMOS variants. Despite the intense battle between the 7400 series and the 4000 series, both have proven to be extremely capable, and can even be mixed and used in the same circuit. While some chips in both families have been discontinued, the most important ones are still in active production. The 4000 series of logic chips consist of a large range of ICs that cover the most essential logic devices, including logic gates, counters, shift registers, and multiplexers. These components can be combined to build more complex circuits, with early computers being made almost out entirely of 4000 series devices. Even as late as the 1980s it was common to see plenty of these handy ICs supporting microprocessor-based PCs. However, considering that most electronic designs have now moved towards complex microcontrollers and microprocessors, nowadays it is rare to see circuits using more than one or two 4000 series devices. What makes the 4000 series especially handy to makers is that they are all 16 14 13 15 VDD CLK Q0 CKEN Q1 Q2 Q3 Reset Q4 4017 Q5 Q6 Q7 Q8 Q9 VSS Cout 3 2 4 7 10 1 5 6 9 11 12 8 Fig. 12. 4017 counter and its schematic circuit symbol. Practical Electronics | January | 2024 Fig. 13. 4017 output count graph. 57 SW1 +VIN R1 1kΩ 7 RV1 10kΩ 8 Vcc 16 Output Discharge IC1 NE555 6 2 Fig. 14. 4017 Light Chaser kit schematic. 4 Reset 14 3 13 15 Threshold VDD CLK Q0 CKEN Q1 Q2 Q3 Reset Q4 Trigger Ground Control 1 5 Q5 4017 Q6 Q7 C1 100µF + Q8 C2 100nF Q9 VSS Cout 3 D1-D10 D1 2 4 7 10 C3 100nF 1 5 C4 100nF 6 9 11 12 8 D10 R2 1kΩ 0V piece of pipe on a radiator, or better still, investing in an inexpensive grounded antistatic work mat and wristband. What is the 4017 IC? Fig. 15. Assembled 4017 Light Chaser kit. available in through-hole DIP packages, which can be used with breadboards, stripboards and simple PCBs. Thus, not only can they be used in prototyping, but also they can be reused in future circuits/projects. Special note on using 4000 series devices – it is important to keep in mind that the 4000 series is based on CMOS technology, which is extremely sensitive to static electricity. Therefore, it is vital when using these chips that static electricity is removed from your body, your project and workstation. This can be done by touching a grounded The first 4000 series IC that we will be introduced to is the 4017 10-stage Johnson counter. It’s schematic representation is shown in Fig.12. This IC is used to create all kinds of lighting effects – for example a light chaser, where an illuminated LED appears to move across a series of LEDs. The 4017 10-stage Johnson counter is a counter with ten stages, a clock input pin, a clock disable pin, and a reset pin. It is built with a 5-stage binary counter connected to an output decoder to produce the 10-stage output. The 4017 can also be described as a ‘decade’ counter, which means it counts to ten using the numbers 0 to 9. The counter increases with one for every rising clock pulse. After the counter has reached 9, it starts again from 0 with the next clock pulse. Fig.13 shows how each rising (low-tohigh) edge of the clock input (where the signal goes from low to high), results in the counter incrementing by one, and the next output stage turning on. Once the counter has reached its maximum count of 9, a final clock signal will reset the counter to 0. The reset pin to the 4017 IC is used to reset the current state of the counter to 0 if the reset pin is set to a high state. The 4017’s disable pin set to a high state prevents the clock from incrementing the counter. Logic ICs need a power supply, usually referred to as VDD and VSS, where VDD is connected to a positive supply (such as 9V for CMOS), and VSS is connected to the negative supply, typically 0V. The 4017 Light Chaser The 4017 Light Chaser kit from MitchElectronics is our most basic 4017 circuit, and not only demonstrates how the 4017 IC works, but also how to use the 555 astable as a clock source. Its schematic is show in Fig.14 and a completed kit in Fig.15. The speed of oscillation of the 555 astable is Fig. 16. 4017 Light Chaser simulation. 58 Practical Electronics | January | 2024 R1 10kΩ 7 + B1 9V R2 22kΩ 8 Vcc 16 Output Discharge IC1 NE555 6 2 C1 100nF 4 Reset 14 3 13 15 Threshold VDD CLK Q0 CKEN Q1 Q2 Q4 Trigger Ground Control – Q3 Reset 1 5 4017 Q5 Q6 Q7 C2 100µF + Q8 C3 100nF Q9 VSS Cout 3 D1 2 D2 4 7 To amber LED To green LED D4 To 0V D5 1 5 D6 6 D7 11 To red LED D3 10 9 J1 Lights D8 D9 12 8 Fig. 17. (Above) Traffic Light schematic and (below) completed project kit. counter, resulting in the next LED in the chain to shine. After ten clock pulses, the 4017 IC resets its count, shining the first LED in the chain, and repeating the cycle forever. For those who want to see a working simulation of this kit (as shown in Fig.16) head over to the 4017 Light Chaser Instruction page and use the in-browser Falstad Circuit Simulation, which allows you to adjust the 4017 Light Chaser frequency in real-time: https://mitchelectronics.co.uk/resources/ simulator/ SMD version determined by the timing capacitor C1, the resistor R1 and the potentiometer RV1. If the value of RV1 is low, then the 555 astable will oscillate quickly, and if the value of RV1 is high, then the 555 astable will oscillate more slowly. The output of the 555 astable is connected to the clock input of the 4017 IC, and both the clock disable and reset pin are connected to 0V, meaning that they are not used / never operating in this circuit. Each output of the 4017 is connected to its own LED, and each LED shares a single resistor, R2. Only one output from the 4017 will ever be high/on, so only one LED will ever be illuminated, thus each LED takes turn in using resistor R2. Each clock pulse from the 555 astable makes the 4017 IC increments its internal Fig. 18. LED sequence of the Traffic Light kit. Practical Electronics | January | 2024 The 4017 Light Chaser uses through-hole components, which are easy to solder for beginners, but for those who want to practise their skills at soldering, then the 4017 Light Chaser SMD Trainer kit offers the same 4017 Light Chaser circuit but using only SMD parts. The kit uses 0805-sized resistors and capacitors, a small potentiometer, and a 555 and 4017 in SOIC SMD packages. Traffic Light The Traffic Light kit is very similar to the 4017 Light Chaser in that it uses a 555 astable connected to a 4017 IC. However, there are a few differences that make it behave differently: specifically, the astable itself and the output stage of the 4017. Unlike the 4017 Light Chaser, the Traffic Light doesn’t have a potentiometer to change the speed of the astable, and the use of larger timing resistors (R1 and R2) results in a rather slow frequency (less than 0.5Hz). On the output side of the 4017 IC, outputs 0, 1 and 2 are connected to the red LED of the traffic light, output 3 is connected to both the red and amber LED, outputs 4, 5 and 6 are connected to the green LED, and output 7 is connected to the amber LED. Finally, output 8 is connected to the reset pin, so that when the counter reaches the ninth count, it automatically resets back to output 0. Now, you may have noticed from the schematic in Fig.17 that each output of the 4017 IC is connected to a diode, and there is a very important reason for this. CMOS logic devices have outputs that are either connected to the positive power supply or the negative supply. If a CMOS output is connected directly to one of the power rails, then it becomes possible for a large current to flow either in or out of the CMOS output, which will damage or destroy the device. The purpose of the diodes is to allow multiple outputs to be connected without risking current flowing back into the 4017. For example, in the case of the first state (where output 0 is high), the diode D1 becomes forward biased, and therefore can conduct electricity. However, because outputs 1 and 2 are low, their associated diodes D2 and D3 are not forward biased, and therefore do not conduct electricity. This prevents electricity from output 0 traveling back into outputs 1 and 2, which would damage the 4017 IC. Simply put, current can flow out of the outputs and into the LEDs, but current cannot flow back into the 4017 IC. The resulting pattern that the Traffic Light exhibits is the standard UK traffic light sequence, with red being followed by red plus amber, then green, then amber alone, and finally back to red – see Fig.18. 59 R1 SW1 10kΩ R2 10kΩ + 7 B1 9V R3 1kΩ 4 8 Reset Vcc Output Discharge IC1 NE555 – 6 2 R9 10kΩ 6 Threshold 2 Trigger 1 Q2 2N3904 8 Vcc 16 Output Discharge R4 680kΩ Ground Control R10 10kΩ Q1 2N3904 7 3 4 Reset IC2 NE555 14 3 13 15 Threshold VDD CLK Q0 CKEN Q1 Q2 Q4 Ground Control 5 Q3 Reset Trigger 1 IC3 4017 5 Q5 Q6 Q7 Q8 + C1 100µF C2 100nF C3 100µF Q9 C4 100nF VSS Cout 3 2 4 7 10 1 5 6 9 D1 Fig. 19. Electronic Dice schematic and kit. Electronic Dice The Electronic Dice kit combines one 555 astable, one 555 monostable and a 4017 IC to create an electronic dice that simulates a dice roll – see schematic in Fig.19. (Revisit Part 1 last month for a refresher on how the 555 astable and monostable operate.) Upon pushing the roll button, the dice begins a rolling animation, and after a predetermined length of time, will stop on a value between 1 and 6, with the LED pattern showing the dice face. In order for this kit to work, the first stage in the circuit is a 555 monostable, 1 2 3 4 5 6 Fig. 20. Image sequence of the Electronic Dice: 1 to 6 (top left to bottom right) 60 D4 D3 D5 D6 11 12 8 which is triggered upon pressing the roll button. The high time of this monostable is determined by resistor R2 and capacitor C1, and as there are no potentiometers, this time length is fixed. However, a transistor Q2 is also connected to the roll button, which, when pushed, keeps the capacitor C1 discharged. This is useful for allowing the user to maintain the roll action for as long as is needed by holding onto the button (similar to keeping the dice rolling in one’s palm).(We haven’t discussed transistors in any detail in this series yet, but for now you can think of the transistor here as simply an electronically controlled switch that applies a short-circuit across the capacitor.) The second stage of the Electronic Dice is a 555 astable, whose reset input is connected to the output of the 555 monostable. Before the roll button is pushed, the output of the 555 monostable is low, meaning that the 555 astable is D2 R5 R6 470Ω 330Ω R7 330Ω R8 330Ω D8 TR D9 R D10 TL D11 BL D12 L D13 BR D7 A kept in reset, and thus, doesn’t oscillate. When the roll button is pushed, the output of the 555 monostable goes high, and this reslults in the 555 astable starting to oscillae. The output of the 555 astable is connected to the 4017 IC clock input, so the 4017 begins to count while the output of the 555 monostable remains high. The output of the 4017 IC is connected to a complex arrangement of diodes and resistors that generate the six different faces of a dice (Fig.20). Determining the logic pattern of each dice face is beyond the scope of this article but may be revisited in future articles when we cover logic and truth tables. Eventually, the 555 monostable’s output goes low, and this not only stops the 555 astable oscillator, but also prevents further counting of the 4017 IC. Thus, the dice face is fixed, and this indicates the face the electronic dice shows. Build advice For a full explanation and example of building a MitchElectronics kit, see the December 2023 issue of Practical Electronics, where we cover the challenges involved with soldering and what order parts need to be soldered in. A quick build and assembly recap is demonstrated in Fig.21: it is always good to solder small parts first, with the most bulky and awkward components being soldered in last. It is essential that the polarisation/orientation of parts is checked, including ICs, electrolytic capacitors and diodes. In MitchElectronics kits, the anode of a diode is indicated by the circular pad, while for electrolytic capacitors, it is a square pad – see Fig.22. For a full guide on how to solder both through-hole and SMD parts, you can check out the MitchElectronics soldering guide, which can be found at: https://mitchelectronics.co.uk/resources Practical Electronics | January | 2024 a) b) Fig. 23. Oscilloscope showing voltage across the capacitor (top) in an astable circuit (bottom) While these kits can in theory operate down to 3V, the 555 can be somewhat temperamental at this voltage, so it is recommended that the minimum power supply voltage applied is 4.5V. Furthermore, it should also be noted that the maximum voltage is around 16V; going beyond this value could easily damage capacitors and the 555. c) d) Testing the projects e) Fig. 21. Construct your 4017 Light Chaser kit using stages a) to e). Another handy feature of these kits is that they do not require any specialist equipment to test – good old eyeballs can easily see if LEDs are flashing or not. Most of the kits operate at frequencies low enough that a multimeter can be used to check voltage levels, but in the case of the Electronic Dice, an oscilloscope can Powering The Projects One of the great advantages of the kits presented in this article is that they all use PP3 battery connectors, making them easy to power. However, that doesn’t mean that they have to be powered by a PP3 battery – they can just as easily be powered using smaller batteries, dedicated PSUs, or even a solar panel. Part Lists for the kits Fig. 22. Check the polarity of the LEDs and capacitors to make sure they are correct. 4017 Light Chaser Kit 1 x 16 DIP socket 1 x 8 DIP socket 1 x 4017 IC 1 x 555 IC 2 x 1kΩ resistors 3 x 100nF capacitors 1 x 100uF capacitor 1 x 10K potentiometer 1 x small slide switch 10 x red LEDs 1 x PP3 connector 1 x PCB 1 x 4017 IC 2 x 555 ICs 2 x 2N3904 NPN trans 3 x 330Ω resistors 1 x 470Ω resistor 1 x 1kΩ resistor 4 x 10kΩ resistor 1 x 680kΩ resistor 6 x 100nF capacitors 1 x 100uF capacitor 1 x tactile switch 7 x red LEDs 6 x 1N4148 diodes 1 x PP3 connector 1 x Dice PCB Electronic Dice Kit 1 x 16 DIP socket 2 x 8 DIP socket Traffic Light Kit 1 x 16 DIP socket 1 x 8 DIP socket Practical Electronics | January | 2024 1 x 4017 IC 1 x 555 IC 1 x 10kΩ resistor 1 x 22kΩ resistor 2 x 100nF capacitors 1 x 100uF capacitor 9 x 1N4148 diodes 1 x 4-way pin header 1 x PP3 connector 1 x Controller PCB 3 x 680Ω resistors 1 x red LED 1 x yellow LED 1 x green LED 1 x 4-way pin header 1 x Light PCB Scan these QR codes to see additional kit instructions. 61