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MITCHELECTRONICS Learn the basics of electronics with Robin Mitchell The 555 Timer IC – Part 1 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. M odern electronics has changed our world, providing us with all kinds of advanced technology, from smartphones and tablets to supercomputers, and even intelligent toasters that can be remotely operated from anywhere on the planet. Now, while such advances have been massively beneficial, it can also make electronics seem like a complex subject that only a few can understand. Worse, many modern projects that introduce makers to electronics often use complicated devices such as microcontrollers which use a ‘black box’ approach, meaning that the inner workings are hidden and not properly explained. However, when electronics was still young (pre-1970s), many would build their own circuits consisting of a handful of parts that could be easily understood. Even if the circuits used complex or sophisticated analogue theory, such as Wien bridge oscillators or advanced filters, they could still be built on stripboard, using discrete through-hole parts and wires. Recognising the challenges faced with modern electronic projects, MitchElectronics has developed a range of kits that only use simple components to build all kinds of projects, such as light flashers, oscillators, musical instruments, and even functional equipment. In this Practical Electronics series, we will cover a whole range of fundamental circuits that have played a critical role in the field of electronics, learning how they work, and how to build them. Project introduction In this article, we will cover the wellknown 555 timer IC, and show how it can be used to create two useful circuits: a 555-based astable and a monostable. These two circuits are fundamental to many electronic designs. What makes these circuits great for beginners is that they use just one chip, and they can easily be used with other circuits 54 What are resistors? Fig.1. MitchElectronics 555 Astable and 555 Monostable Kit bags, ready to be built. Resistors are one of the most common and basic electronic components. Their symbol and an example of a resistor are shown in Fig.2. As their name suggests, they ‘resist’ electrical current. Resistance is measured in ohms, indicated with the Greek upper-case symbol for omega (Ω), but because the ohm unit is quite a small value, resistors in electronics typically have resistances between 100 and 100,000 ohms. We would normally write this as ‘100Ω’ and ‘100,000Ω’, although the latter is usually shown as 100kΩ, where the ‘k’ (always lower case) means 1000 – kΩ is pronounced ‘kilohms’. The current flowing through a resistor is proportional to the voltage across it, meaning that as the voltage across a resistor is increased, the current also increases proportionally. This is known as Ohm’s law, first proposed by German physicist Georg Ohm, after whom the resistance unit is named. The formula that relates the current to the voltage is given as: for timing purposes. An astable provides a constant series of voltage pulses (think of a pendulum), switching between a high and low voltage, while the monostable provides a single pulse for a pre-set amount of time, just like turning a tap on and off. By reading this article, you will learn how to build these circuits, how to design your own timers, how to calculate LED currents, and the importance of various components in electronic circuits. Both the circuit kits (Fig.1) discussed The 555 Timer IC – Part 1 in this article can be purchased from MitchElectronics, and if you want to see 𝑉𝑉 this project in action, then you should 𝐼𝐼 = 𝑅𝑅 definitely check out the MitchElectronics For example, if a voltage of 1V (volt, Practical Electronics Video Series The 555 Timer IC – Part 1 symbol V, is the unit of electrical potential (coming out December 2023), just visit: voltage) is applied across a resistor www.mitchelectronics.co.uk 𝑉𝑉 or 1𝑉𝑉 𝑉𝑉 𝐼𝐼 = = with a =resistance of 10Ω, the resulting 𝐼𝐼 0.1𝐴𝐴 = 𝑅𝑅 10𝛺𝛺 𝑅𝑅 current flowing through that resistor will How the parts work be 0.1A (amp, symbol A, is the unit of Before we jump into the astable and electrical current): monostable circuits, we first need to learn about a few basic components, including 𝑉𝑉!"𝑉𝑉− 𝑉𝑉#1𝑉𝑉 = 0.1𝐴𝐴 what they do, and why they are important. 𝑅𝑅 =𝐼𝐼 = = 𝐼𝐼𝑅𝑅 $%& 10𝛺𝛺 Resistors can be made from all kinds of materials, but the ones used in the MitchElectronics kits are made from 𝑉𝑉!" − 𝑉𝑉# 5 − 2𝑉𝑉!" − 𝑉𝑉# carbon (specifically, quarter-watt 𝑅𝑅 = = 𝑅𝑅 =film = 600𝛺𝛺 𝐼𝐼$%& carbon-film 0.005 𝐼𝐼$%& axial resistors). Because these resistors are small and round, it’s difficult to print the resistor value across the body. Fig.2. Carbon film resistor (left) and Instead, manufacturers use colour bands to resistor circuit symbols (right). The box 5−2 𝜏𝜏𝑉𝑉!" =− 𝑅𝑅𝑅𝑅𝑉𝑉# their value. To learn more about and ‘sawtooth’ versions are both widely 𝑅𝑅 =indicate = = 600𝛺𝛺 𝐼𝐼 0.005 $%& this, visit: https://bit.ly/mitch-res used – PE uses the latter. 1.44 Electronics | December | 2023 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓Practical = (𝑅𝑅' + 2𝑅𝑅() )𝐶𝐶' 𝜏𝜏 = 𝑅𝑅𝑅𝑅 Fig.3. Potentiometer schematic symbol. The box and ‘sawtooth’ versions are both widely used – PE uses the latter. What are potentiometers? Potentiometers, often just called ‘pots’, are three-pin devices that can be used to produce a variable resistance, which is then used to produce a variable voltage or current. They almost always have a moveable mechanical component (such as a knob) that allows their setting to be altered. Potentiometers are commonly found on amplifier volume controls, and are also found on many other kinds of electronic equipment that need some kind of value varied, for example, measurement test gear and musical equipment. The symbol for a potentiometer is shown in Fig.3. Potentiometers used in MitchElectronics kits have three connection pins, and inside the potentiometer is a carbon track between the two outer pins with the third pin connected to a wiper that makes sliding contact with the carbon track. The resistance between the two outer pins will always be the same, but the resistance between one of these outer pins and the inner pin will depend on where the knob is positioned, and which outer pin is being measured. Fig.4 shows the type of potentiometer you will use in these projects. If one of the outer pins is connected to a positive supply, and the other to ground, the middle pin will produce a voltage whose value depends on the knobs position. Turning the knob towards the positive supply will increase the output voltage, while turning the knob towards ground will decrease the output voltage. What are capacitors? Capacitors are two-pin electronic components that can store electric charge, making them similar to batteries – but, unlike batteries, which can store massive amounts of energy and power circuits, capacitors are used to provide relatively small amounts of power for small amounts of time. This can be used to for timing Wiper – rotated with potentiometer knob Potentiometer resistive track Fig.4. Potentiometer constructon showing resistive track, wiper and terminals. Potentiometer connections Practical Electronics | December | 2023 + + Fig.5. Different capacitor symbols (left to right): plain unpolarised, electrolytic (polarised) and variable. Fig.6. (left) Electrolytic capacitor – note different length leads; shorter is negative connection, (right) ceramic capacitor. purposes, smoothing power lines, and (the negative terminal). This means that filtering signals. Several different symbols they must be inserted the correct way for capacitors are shown in Fig.5, and during soldering, otherwise they won’t light some examples of actual devices are up. Fig.7 shows the symbol for an LED. shown in Fig.6. Unlike resistors, which When using LEDs, it is critical to make tend to be pretty uniform in shape, sure that a series resistor is used, as, unlike capacitors come in all sorts of shapes, light bulbs, LEDs will try to conduct as 555 Timer IC – Part 1 sizes andThe colours. much current as possible. That means if The capacity of a capacitor is called its an LED is directly powered by a power 𝑉𝑉 ‘capacitance’, which is measured in farads, source, 𝐼𝐼 = they will fry themselves faster 𝑅𝑅 symbol F. One farad (1F) is a huge amount than you can blink. Thus, a resistor placed of capacitance, so most capacitors are in series with the LED limits the current measured in microfarads (1µF, a millionth that can flow through the LED, thereby of a farad), nanofarads (1nF, a billionth preventing it from being damaged, as 𝑉𝑉 1𝑉𝑉 of a farad), or even picofarads (1pF, a 𝐼𝐼 =shown Fig.8. = in = 0.1𝐴𝐴But what size resistor 𝑅𝑅 10𝛺𝛺 trillionth of a farad). In the case of our should you use? Well, there is a formula 555 astable and monostable circuits, we that can help us figure that out exactly, use two different capacitors, one with a and it is very simple too. value of 100µF, and one that is 100nF. 𝑉𝑉!" − 𝑉𝑉# Capacitors also come in two major 𝑅𝑅 = 𝐼𝐼$%& categories, termed polarised and non- polarised. Non-polarised capacitors can To use this formula, we have to decide on be used in either orientation, meaning that what current we want going through the both pins can handle positive and negative LED (ILED), what forward voltage the LED 𝑉𝑉!" − 𝑉𝑉# and 5 − 2what voltage we are applying voltages, a bit like a filament lightbulb. 𝑅𝑅 = (Vf) has, = = 600𝛺𝛺 0.005 (Vin). When deciding the Polarised capacitors, however, can only 𝐼𝐼to $%&our circuit be used in one orientation, with one pin current, we need to make sure that it is being the positive pin, and the other being well below the maximum forward current the negative pin – in this sense they are of the LED, otherwise it will burn out. But 𝜏𝜏 = 𝑅𝑅𝑅𝑅 of the LED depends on that similar to a battery, it matters which way the brightness you connect them. If a polarised capacitor forward current, so you need to decide how is inserted incorrectly, it can result in bright you want your LED. In the case of 1.44 kits, forward currents the breakdown of the capacitor, and in MitchElectronics 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 = (𝑅𝑅' + 2𝑅𝑅 some cases, they can burst! This is not are typically chosen () )𝐶𝐶' to be around 50% recommended – the capacitor is ruined of the maximum safe forward current. and the smell can be awful! The current through an LED is usually measured 𝑇𝑇 = 1.1 × in 𝑅𝑅𝑅𝑅thousandths of an amp, or ‘milliamps’, written as ‘mA’ (the ‘m’ must What are LEDs? be lower case). Light-emitting diodes, LEDs for short, The next step is to find out the forward are small semiconductor devices that, voltage of your LED, and this can be found when powered, produce light. Unlike oldin its datasheet. In the case of our 555 fashioned light bulbs which use a tungsten circuits, we will be using red LEDs which filament, LEDs do not rely on heating a have a forward voltage drop of Vf = 2V. small coil of wire to produce light, but instead take advantage of energy levels in semiconductor materials to directly Resistor ILED convert electricity into light. The finer Battery VR details of this are way beyond the scope + Vin of this article, but the important points – are they are compact, cheap, efficient and Vf can be made to produce different colours. LED LEDs are polarised, meaning that they Vin = VR + Vf only work when current flows from the The same current (ILED) flows through positive terminal (the anode) to the cathode the battery, resistor and LED beccause they are in series. Anode Cathode Fig.7. Diode schematic symbol. Fig.8. Series resistor and LED configuration. 55 Vcc Control pin 8 pin 5 Charge Battery Vin + Discharge R – VC Capacitor The 555 Timer IC – Part 1 C – Part 1 Reset pin 4 Threshold pin 6 Flip-flop R1 R Output pin 3 S Fig.9. RC charging/discharging circuit. Comparator 1 𝑉𝑉 𝑉𝑉 R 𝐼𝐼 = 𝐼𝐼 = 𝑅𝑅 𝑅𝑅 This figure means that when a voltage is applied across our LED circuit, the voltage Trigger pin 2 across the LED will always be equal to Comparator 2 this forward voltage drop. The crucial 𝑉𝑉 1𝑉𝑉 𝑉𝑉 1𝑉𝑉 R 𝐼𝐼 = = = 0.1𝐴𝐴 Pin 8 1 𝐼𝐼 = = that=the 0.1𝐴𝐴 insight here input voltage to 𝑅𝑅 Pin10𝛺𝛺 𝑅𝑅 is10𝛺𝛺 Pin 7 Pin 2 an LED circuit must be greater than this Discharge forward voltage. Pin 6 Pin 3 pin 7 Finally, plug these numbers into the Pin 5 Pin 4 𝑉𝑉!" − 𝑉𝑉# above equation, 𝑉𝑉!" and − 𝑉𝑉#you get the size of Gnd 𝑅𝑅 =need. For example, if a red 𝑅𝑅 = 𝐼𝐼 resistor you pin 1 𝐼𝐼$%& $%& LED (forward voltage of 2V) is used with a Fig.11. 555 timer: (left) classic 8-pin DIL through hole package and (right) internal 5V supply and needs a forward current of structure (right). 5mA, the resulting resistor would be 600Ω. 𝑉𝑉 − 𝑉𝑉# 5 − 2 𝑉𝑉!" − 𝑉𝑉# 5 − 2 !" 𝑅𝑅 = = in timing = 600𝛺𝛺 𝑅𝑅 = = = 600𝛺𝛺 useful applications. The equation the Output goes low and the Discharge 𝐼𝐼$%& 0.005 𝐼𝐼$%& 0.005 that relates the resistance and capacitance transistor is turned on. to the charging and discharging time The discharge transistor (pin 7) acts like What is an RC circuit? constant is simply: a switch, discharging the charge stored in A resistor-capacitor, or RC circuit, is one an external timing capacitor (C1) when it that consists of a resistor and capacitor 𝜏𝜏 = 𝑅𝑅𝑅𝑅 𝜏𝜏 = 𝑅𝑅𝑅𝑅 in series (Fig.9). A capacitor connected to is turned on. Important note on the time constant in a power source will instantly charge up Power is supplied via pin 8 for VCC this equation: it actually refers to the time to the battery voltage, but if a resistor is and pin 1 for 0V or ground. For now, we 1.44 1.44 = needed to charge the capacitor to around placed in series, time, can ignore the effect of the Reset (pin 4) 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 = this charging takes𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 (𝑅𝑅 + 2𝑅𝑅() )𝐶𝐶' 2𝑅𝑅() )𝐶𝐶 ' +across ' capacitor 62% of' the input voltage. Why this is the and the voltage(𝑅𝑅 rise the – we will cover it in later articles – just case is to do with mathematics involving follows a very specific curve (see the graph connect it to VCC. exponential functions, which is beyond in Fig.10). Likewise, a capacitor that is Interestingly, many believe that the 555 𝑇𝑇 the = 1.1 × 𝑅𝑅𝑅𝑅of this article. A good rule of 𝑇𝑇 = 1.1will × 𝑅𝑅𝑅𝑅 scope short-circuited instantly discharge, timer gets its name from the three internal thumb is that a capacitor will charge or but discharging a capacitor through that 5kΩ resistors, but in fact, the name was discharge completely (or to around 99%) same resistor will see this voltage fall with chosen arbitrarily. in about four to five time constants (5×RC). the same (but inverted) curve. In this article, we are going to use the 555 The time it takes for the voltage to rise timer to create an astable and monostable and fall for a given RC value is called the timer. Both of these circuits will take What is the 555 timer? time constant (τ, Greek lower-case ‘t’). For advantage of RC circuits to control timing, The 555 timer is one of the most famous a particular RC value, it is always the same potentiometers to allow for adjusting integrated circuits, and there is good time, and this makes RC circuits extremely the timing without needing to replace reason for this. In fact, the 555 timer is components, and LEDs to show the output. so popular that, despite having been first sold in 1972, it is still in production, with RC charging of a capacitor millions produced each year. How does a 555 astable work? VC 100% Vin The 555 timer is a chip that consists Before we dive into the circuit function, of two internal comparators, a flip-flop, we first need to study the circuit diagram 63% Vin discharge transistor, and three resistors for the 555 astable circuit (see Fig.12). The (see Fig.11) which, when used together, RC circuit is made up of R1, RV1, and C1, Time τ = RC can make a whole range of different timing while the LED (D1) is connected to the VC is the voltage across the capacitor. circuits. Don’t worry if you are not sure output of the 555 timer via resistor R2. You τ is called the time constant and equals RC. VC rises to 63% of Vin (the battery charging how flip-flops or transistors work – we will also notice that the Control pin of the voltage) in one time constant, and reaches will cover them in later articles. For now, 555 timer (pin 5) is connected to a small near 100% after about 4 to 5 time constants. the point is we have an IC with a box of capacitor (C2), this is needed to provide tools that enable us to make timing circuits. stability to the internal components of RC discharging of a capacitor While we don’t need a thorough working the 555 timer. VC 100% Vin knowledge of the inside of a 555 to use it Assuming that the circuit has just been effectively, an overview of its essential powered by a 9V battery, the first thing 37% Vin characteristics is helpful. The Output (pin 3) that happens is that the voltage across C1 goes high (close to the supply voltage, V is 0V and the output of the 555 is high. ) τ = RC Time CC VC falls by 63% of Vin (to 37%) in one time This also puts the 555 into its set state, when the Trigger input (pin 2) falls below constant, and reaches near 0% after about whereby the discharge pin is shut off, and 1/3 of VCC. The Output stays high until the 4 to 5 time constants. current can flow through R1 and RV1, and Threshold input (pin 6) is activated by a voltage above 2/3 of V into capacitor C1. , at which point CC Fig.10. RC charge/discharge graph. 56 Practical Electronics | December | 2023 𝜏𝜏 = 𝑅𝑅𝑅𝑅 V+ can be from 4.5 to 16V 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 = V+ R1 1kΩ 7 RV1 100kΩ 4 8 Reset Vcc IC1 NE555 Discharge Output 6 2 Threshold 3 R2 1kΩ Trigger Ground Control + 1 5 C1 10µF C2 100nF D1 LED 0V Fig.12. 555 Astable Circuit. This current flow charges up C 1 , resulting in the voltage across C1 rising. The speed at which the capacitor C 1 charges up depends on the resistance of R1 plus the potentiometer RV1, because resistance resists current flow. Thus, if R1 plus RV1 has a low resistance, the capacitor C1 will charge quickly, and if RV1 has a large resistance, then capacitor C1 will charge slowly. However, this voltage across C1 will eventually go beyond the trigger level of the 555 timer, and when this happens, the output of the 555 timer will go low. At this point, the internal discharge circuit will activate, and this will see capacitor C1 begin to discharge through RV1 and into pin 7. This discharging causes the voltage across C1 to fall. Eventually, the voltage across C1 will fall below the threshold point of the 555 timer IC, and this causes the 555 timer to turn off its discharge circuit (pin 7) and turn off its output (pin 3). Thus, the capacitor C 1 will begin to charge again, and the entire process of charging and discharging continues to repeat forever. This is called an astable oscillator because it has two states that are both unstable. The capacitor voltage compared to the output voltage can be seen in Fig.13, and formula that describes the frequency of a 555 Astable circuit is given by: 1.44 (𝑅𝑅' + 2𝑅𝑅() )𝐶𝐶' If you want to ‘cheat’ and use 𝑅𝑅 a = calculator, then you can instead visit: 𝑇𝑇 = 1.1 × 𝑅𝑅𝑅𝑅 https://bit.ly/mitch-resources How does a 555 monostable work? 𝑉𝑉!" − 𝑉𝑉# 𝐼𝐼$%& V+ can be from 4.5 to 16V V+ 𝑉𝑉!" −R1 𝑉𝑉# R2 5−2 1kΩ 1kΩ = = 600𝛺𝛺4 𝐼𝐼$%& 0.005 Reset 8 Vcc RV1 100kΩ 7 6 𝜏𝜏 = 𝑅𝑅𝑅𝑅 2 Discharge Output Threshold Trigger IC1 NE555 3 R3 1kΩ Ground Control + D1 1 5 C1 The 555 monostable circuit, just like Switch LED 100µF C2 the astable, is made up of an RC circuit 100nF 1.44 0V (R2, RV1 and C1) and an output LED𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 = (𝑅𝑅' + 2𝑅𝑅() )𝐶𝐶' – see Fig.14. However, unlike the astable, only pin 6 and 7 (threshold Fig.14. 555 Monostable Circuit. and discharge) are connected to our 𝑇𝑇 = 1.1 × 𝑅𝑅𝑅𝑅 RC circuit, and pin 2 (the trigger) is connected to a switch and resistor the on period (T), use the following: combination. If the switch isn’t being pushed, then pin 2 is pulled up to 9V Where:  T, measured in seconds is the through the resistor R1, and this deactivates width of a monostable pulse the trigger in the 555 as the trigger is  R, measured in ohms is equal to active low (meaning that it needs to be R2 + RV1 (vary RV1 to vary T) 0V to trigger).  C, measured in farads is equal Under normal conditions, the 555 IC to C1 is in its discharge mode, meaning that the capacitor C1 cannot charge (as the Again, if you want to use a calculator, go current flows thorough R2 and RV1 into to: https://bit.ly/mitch-resources the discharge pin instead of going into the capacitor C1). However, when the switch Build advice is pushed, the voltage at pin 2 (the trigger) When building the MitchElectronics of the 555 causes it to go into its set mode, 555 Astable and Monostable kits, it is which turns off the discharge pin. Now, important to triple check the position current starts to flow into C1, causing the of components and their orientation. All MitchElectronics boards are doublevoltage to rise across it. In this set mode, sided and through-hole plated, so while the output of the 555 also switches on, this means that they are extremely strong providing 9V and turning on LED D1. and pads don’t break off, it does mean Eventually, the voltage across C1 will that soldering mistakes can be a little reach 2/3 of the supply voltage, and tricky to undo (not impossible, but when this happens, the 555 will be certainly challenging). triggered into its reset mode, where the It is highly recommended that the first capacitor C1 is drained rapidly through parts soldered to the PCB are the smallest pin 7 (discharge). This also turns off components, not by width and length, the output of the 555 timer, thereby but by height. So, in this case, it is best to turning off the LED. See Fig.15 for a start with the resistors, followed by the visual explanation of the monostable. IC socket. Then proceed with capacitors, This circuit is called a monostable LEDs and finally, the potentiometer. If because it is only stable in one state (hence, you have trouble with getting solder to mono or one). When triggered into its set stick to components and pads, then a bit state, it can only remain there until the of cleaning with IPA followed by some capacitor has charged up significantly, and flux can go a long way. this second state is unstable. To calculate Fig.13. Oscilloscope output comparing capacitor voltage (yellow) to output voltage (blue). Practical Electronics | December | 2023 𝑅𝑅 = Fig.15. Oscilloscope output showing capacitor voltage (yellow), trigger voltage (blue), and output voltage (purple). 57 Testing the project a) b) c) d) e) f) To test the boards, you don’t require any specialist equipment; eyeballs are more than enough to get the job done. If all goes well, you should see the LED blink when powering the astable circuit, and if it appears that the LED just stays on, try adjusting the potentiometer until you see it blink. With the monostable circuit, you should see the LED turn on after pushing the button, and if you don’t, try adjusting the potentiometer and pushing the button again. When operating the astable and monostable at high frequencies / short periods, an oscilloscope can be used to observe signals in the circuit – see Fig.18. A scope can also help to show the nature of charging circuits, as well as diagnosing any potential problems. If you are exceptionally worried about damaging your circuit, you can opt to use an external power supply with a current limiter, as shown in Fig.19. Start by setting the current limiter to a very low value (such as 50mA), and then gradually raise the voltage. If you see that the current limiter quickly maxes out, then you may well have a short circuit and you need to check your soldering. Fig.16. Construction stages of the kit: a) start with the smallest flattest components – resistors, b) next, the IC holder, c) now add capacitors – check polarity of the electrolytic one!, d) now the (polarised) LED, e) the potentiometer... and last, f) add the power leads. Be extra careful when inserting the LED and electrolytic capacitors, as these are polarised components. It should also be noted that MitchElectronics kits follow the standard KiCad PCB design software format for pads, meaning that with LEDs, the positive pin is the circular pad, and not the square pad. However, for electrolytics, the positive pin is the square one, and not the round one! Fig.16 provides a step-by-step overview of circuit assembly. When inserting the 555 IC into the socket, ensure that it is oriented the correct way, otherwise you may damage the chip when you power it up. Finally, make sure to discharge any static electricity which may be dancing around your body before 58 constructing the kit, as this can damage the chip. The fastest way to do this is to touch something grounded, such as an exposed radiator pipe. If you want to build an SMD version of these kits, you can also explore the MitchElectronics 555 Astable and Monostable SMD Trainers on our website. Powering the project Powering the 555 circuits is dead easy thanks to the inclusion of a PP3 battery connector. However, this doesn’t have to be used, and can be replaced with wires connecting to an external power source. You must make sure the voltage range is between 4.5V and 16V and applied with the correct polarity! Fig.17. Scan these QR codes to see additional kit instructions. Practical Electronics | December | 2023 Fig.19. You can use the current limiter on a power supply to prevent damage. Parts list Fig.18. A scope can be handy when testing the boards, but you can build these kits successfully without one. Troubleshooting Not much can go wrong with these circuits, but that isn’t to say that mistakes can’t happen. If you find that the LED doesn’t blink, then the first thing you need to check is the orientation of the LED. Using a continuity probe on the LED can tell you if you have the orientation of the LED correctly, and a multimeter can be used to measure the output voltage of the 555 chip (pin 3). If the LED seems to be fine, check the orientation of the IC, as it is quite easy to insert these the wrong way. If the IC is correctly positioned, then the next logical step is to check the orientation of the electrolytic capacitor. Assuming all this checks out, then it is possible that the IC may have been damaged, but this is unlikely as the 555 timers used in MitchElectronics kits are BJT variations, which are far less susceptible to static damage. Simulation Simulations for both kits are available on the MitchElectronics website (see the website’s resource page or each kit’s packaging for a QR code – and also Fig.17). These simulations are powered by the free circuit simulator Falstad Circuit Simulator, and provide an excellent visual reference of how these kits work. Furthermore, due to the simplicity of the 555 circuits, the simulation is a near-perfect representation of the real circuit! An example simulation is shown in Fig.20. The parts needed to build the 555 Astable and Monostable circuits are shown in Fig.21 and Fig.22; but for those who want to build the project from their own parts, use the following list. 555 Astable 1 x 8 DIP socket 1 x 555 IC 1 x 100nF capacitor 1 x 10µF capacitor 2 x 1kΩ resistor 1 x 100kΩ potentiometer 1 x 3mm red LED 1 x PP3 connector 1 x PCB 555 Monostable 1 x 8 DIP socket 1 x 555 IC 1 x 100nF capacitor 1 x 10uF capacitor 3 x 1kΩ resistor 1 x tactile switch 1 x 100kΩ potentiometer 1 x 3mm red LED 1 x PP3 connector 1 x PCB Taking the project further Fig.20. Watch and control simulations of the circuits on the MitchElectronics website. Practical Electronics | December | 2023 At this point, you may be wondering now that you have a circuit which blinks and one that stays on for a set period of time, what can you do with it? Well, these circuits have plenty of use in the real world, so you won’t struggle to find practical applications for the astable and monostable. The astable is great as a clock source for digital circuits which require a regular pulse to function correctly. Of course, such a clock pulse will often need to be much faster than what is provided in the 59 The monostable can be useful for applications involving one-shot actions, such as a water valve which needs to deliver a specific amount of water before turning itself off. This action can also be useful in digital circuits which ignore accidental repeated presses of a button, such as keyboards and interfaces. The set on period is also useful for keeping car lights on for a minute after parking, so you don’t need to try and find your way to your front door in the dark. If you want to see these projects in action, then consider checking out the upcoming MitchElectronics and Practical Electronics Video Series, which will cover this project and more, showing live oscilloscope traces, in-depth building guides, and more. Partnership with PE Fig.21. MitchElectronics 555 Astable kit and completed PCB. Fig.22. MitchElectronics 555 Monostable kit and completed PCB. MitchElectronics kit, so you can reduce the value of C1 to make the 555 oscillate much quicker. The slow blinking nature of the 555 astable can also be useful for optical projects, such as lights placed on the back of a bike to alert other users, or as a model railway warning signal indicating oncoming trains. MitchElectronics Ltd is an independent UK company. These articles are not ‘advertorials’, PE does not pay for the articles and MitchElectronics does not pay for their publication. All the kits/parts described in the series are available from: https://mitchelectronics.co.uk ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. Your best bet since MAPLIN Chock-a-Block with Stock Visit: www.cricklewoodelectronics.com Or phone our friendly knowledgeable staff on 020 8452 0161 Components • Audio • Video • Connectors • Cables Arduino • Test Equipment etc, etc 3D Printing • Cable • CCTV • Connectors • Components • Enclosures • Fans • Fuses • Hardware • Lamps • LED’s • Leads • Loudspeakers • Panel Meters • PCB Production • Power Supplies • Relays • Resistors • Semiconductors • Soldering Irons • Switches • Test Equipment • Transformers and so much more… Monday to Friday 08:30 - 17.00, Saturday 08:30 - 15:30 Station Road Cullercoats North Shields Tyne & Wear NE30 4PQ Tel: 0191 2514363 sales<at>esr.co.uk www.esr.co.uk 60 Visit our Shop, Call or Buy online at: www.cricklewoodelectronics.com 020 8452 0161 Visit our shop at: 40-42 Cricklewood Broadway London NW2 3ET Practical Electronics | December | 2023