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The easy way into electronics Pt.3 This month we feature a few more circuits based on the 555 timer. As we shall see, this chip can be used in more than just timer and oscillator circuits. You can even use as an audio amplifier and in pulse width modulation circuits. By LEO SIMPSON B UT FIRST, AS THEY say in the news programs, we have some corrections to make on last month’s circuit. Red faces all around here because the circuits published on page 61 both had the same mistake: the 1kΩ and 10kΩ resistors were swapped for both IC1 and IC2. Both circuits will still work but the negative pulse widths will be about 10 times narrower than those shown on the oscillo­ scope waveforms on page 63. That error was bad enough but we also made a mistake with the Proto­ board wiring layout shown on page 62. In this case the connection of potentiometers VR1 and VR2 is different from the circuits on page 61 and to compound the misery, the pot values were actually 50kΩ instead of 100kΩ. Again, the circuits still work but the pulse width is variable instead of being fixed and the range of frequencies is not as large with 50kΩ as it would be with 100kΩ. Why did these mistakes happen? Put it down to old age, poor eyesight, Olympic Games’ distractions or straight out incompe­tence – we’ll own up to all of those and will now try to make amends. In fact, these mistakes demonstrate how easy it is easy to make changes to circuits when you are using Proto­ boards and at the same time, how easy it is to make mistakes. You have to keep your wits about you and carefully check that what you think you’ve done is actually what you should have done! It is also very easy to push a wire into the row or column next to the one you really want. So look before you jab! OK. Fig.1 shows the high frequency part of the Siren cir­cuit as it should have been on Fig.1, page 61, of last month’s article. Fig.2 shows the circuit actually depicted in the Proto­board layout on page 62 of last month’s issue. If you wired up your version along the same lines, you will find that the This photo features all the components shown in the diagram of Fig.3. You can use this layout to demonstrate how a 555 timer can be used as an amplifier for the signal from a CD player. 86  Silicon Chip fre­quency and negative pulse width are variable. If you haven’t tried the circuit, wire it up now on your Protoboard. The wiring layout is shown in Fig.3. This has extra parts associated with pin 5 and these should be omitted for the time being. The scope waveforms of Figs.4 & 5 demonstrate the circuit’s performance. Fig.1: this 555 oscillator circuit has a fixed resistor between pin 6 & 7 and this results in a fixed negative pulse width as the frequency is varied over a wider range. Square waves not possible Two things are demonstrated by the waveforms of Figs.4 & Fig.5. First, when the resistance between pins 7 and 6,2 is variable, as it is in Fig.2, the negative pulse width is also variable; when the resistance between pins 7 & 6,2 is fixed, as shown in Fig.1, the negative pulse width is fixed. This is be­ cause the aforementioned resistance determines the discharge time for the capacitor at pin 6. But the other consequence of this is that this 555 circuit cannot ever deliver a perfect square wave; ie, a waveform with 50% duty cycle or to put it another way, where the positive and negative pulse widths are equal. You might get close to 50% at one particular frequency (as shown in the waveform of Fig.5) but as soon as you change the frequency, the duty cycle goes far off that for a square wave. So is it impossible to get a square wave from a 555? No. It can be done Fig.2: this version of the 555 circuit uses exactly the same components but now the resistance between pins 6 & 7 is variable and this results in variable negative pulse width over the entire frequency range. Fig.3: use this diagram to wire up the circuit of Fig.2 but leave out VR2 and the components associated with pin 5 for the time being. November 2000  87 Fig.4: this waveform demonstrates the fixed negative pulse width produced by the circuit of Fig.1. This is determined by the time constant of the 1kΩ resistor and 0.1µF capacitor. but the circuit has to be changed so that the inter­nal transistor at pin 7 no longer does the discharging of the capacitor. The circuit of Fig.6 shows how it can be done. Instead of charging the capacitor from the positive supply and then dis­charging via pin 7, the charging and discharging of the capacitor at pins 2 & 6 is done from pin 3. So pin 7 has no connection in this circuit. Square wave circuit To change your Protoboard circuit from that shown in Fig.2 to that of Fig.6, remove the 10kΩ resistor connecting pin 7 to the +12V line and move the pot lead that connects to pin 7 so that it now goes to pin 3. And leave the speaker disconnected for the moment. Fig.5: this waveform looks much the same as in Fig.4 but now the negative pulse width is variable as well as the frequency, as per the circuit of Fig.2. The scope waveforms of Figs.7 & 8 show that the output waveform at pin 3 (Ch2 – lower trace) now has a duty cycle of close to 50%. Fig.7 shows the circuit oscillating at around 138Hz with pot VR1 set for maximum resistance while Fig.8 shows it running at around 6.8kHz, with VR1 set for minimum resistance. Two things can be noted about the “square” waves of Figs.7 & 8. First, the duty cycle is not exactly 50%, in spite of what we said above. Second, in Fig.8 the tops of the square wave are sloping rather than dead square. Both of these effects are caused by the output stage of the 555. If we had a “perfect” output stage in the 555, it would switch between the full supply voltage (12V nominal) and 0V. But it doesn’t. Depending on the current it has to “source” or “sink”, Fig.6: by charging and discharging the capacitor at pin 6 from a variable resistance connected to pin 3, the 555 can be made to deliver a square wave regardless of its frequency of operation. 88  Silicon Chip it typically won’t quite get to 0V and it will do worse in switching up to the positive supply. In our circuit for example, it will switch down to about 0.1V but will only switch up to within about 0.4V of the positive supply rail. Furthermore, if we make the 555 drive the speaker via a 68Ω resistor and 100µF capacitor, it will have to source and sink substantially more current (around 110mA) and so it will do considerably worse. In fact, Fig.9 shows how bad it is. The output waveform is considerably reduced in amplitude, with the negative excursion now being about 1V (instead of close to 0V) but the positive excursion is only about +8.5V. Clearly, the output stage of the 555 is far from perfect and nor is it symmetrical. The result of this is that the output waveform from pin 3 is now nothing like “square” as the positive excursions of the waveform are now more than double the negative excursions. Be­cause the output at pin 3 is not switching as high as it should, it is taking that much longer to charge the capacitor at pins 2 & 6. OK. So if we want a near perfect square wave from a 555 we can use the circuit of Fig.6 but we have to maintain the minimum possible loading on the output at pin 3. In other words, don’t connect the speaker. You might ask why most 555 circuits do not use the configu­ ration of Fig.6 since it gives a more ideal square wave. The answer is that the conventional circuits of Fig.1 & Fig.2 are normally preferred because they give much better frequency stability. Fig.7: this shows that the 555 can produce a near ideal square wave, using the circuit of Fig.6. In this case, the circuit is set to oscillate at 138Hz and the loading on pin 3 is minimal. The frequency is less affected by circuit loading at pin 3 and is almost entirely independent of supply voltage varia­tions. So, for example, for a given setting of VR1 in Fig.2 and with the speaker disconnected, the frequency will be substantial­ly the same, regardless of whether the supply voltage is 3V or 15V. That’s a pretty good result for an oscillator. Fig.8: when set for the maximum frequency, the circuit of Fig.6 still delivers a duty cycle of close to 50% but the higher load­ing on pin 3 means that the tops of the pulse waveform are no longer square. Fig.9: with the speaker connected, there is high loading on pin 3 and so the output is much reduced and no longer can be called a square wave. Frequency modulation While most oscillator circuits using 555s tend to be along the lines we have discussed so far, few make any use of pin 5 which is usually referred to as the CV or Control Voltage input. In most circuits, it is not connected at all or it might be connected to the 0V line via a capacitor. However, it can be used to produce pulse width modulation or looking at it another way, frequency modulation. To demonstrate this effect, let’s change the circuit to that of Fig.10. The Protoboard can be mounted on a simple folded aluminium baseplate, with the pots and DC power socket mounted on the front panel. November 2000  89 Fig.10: used to demonstrate pulse width modulation, this circuit is similar to that of Fig.2 except that we have another 50kΩ pot, VR2, connected between the positive and negative supply and its wiper goes to pin 5 via a 10kΩ resistor. Ignore the components shown in red for the moment. Note that the capacitor value at pins 2 & 6 is now .01µF instead of 0.1µF. Fig.11: these wave­­ forms demon­strate pulse width modulat­­ion with the 555. The top trace is the 500Hz sinewave applied to pin 5 while the lower trace is the pulse width modulat­ed wave­form which is running at around 5kHz. This is similar to that of Fig.2 except that we have another 50kΩ pot, VR2, connected between the positive and negative supply and its wiper goes to pin 5 via a 10kΩ resistor. Note that the capacitor value at pins 2 & 6 is now .01µF instead of 0.1µF. Now by leaving the setting of VR1 constant and varying VR2, we can vary the frequency and pulse width over a very wide range. To demonstrate the effect, connect the speaker (if not already connected) and wind VR2 over its full range. If VR1 is already set for a reasonably high frequency (say 3kHz) you will find that VR2 will vary the frequency over a range from 3kHz to above 20kHz (ie, supersonic). But not only do we vary the frequency, we are varying the pulse width. This can be seen on a scope if you have one but if you don’t you can still demonstrate that the pulse width is varying. How? By using your multimeter to measure the average DC 90  Silicon Chip voltage at pin 3. If you go through the same exercise in varying VR2, you will find that the DC voltage at pin 3 varies from about 2V to 10V. This principle is widely used in pulse width modulation circuits to vary the average DC or power level to a load. PWM amplifier Finally, we can use this pulse width modulation principle to make the 555 function as an audio amplifier. To do this, we connect the positive electrode of a 10µF electrolytic capacitor to the wiper of VR2 and the negative lead of the capacitor is connected to 0V via a 4.7kΩ resistor. These extra components are shown in red on Fig.10. VR2 and these extra components are included in the Protoboard layout of Fig.3 and you can plug them in now. We now connect an audio signal to the 4.7kΩ resistor. In our case, we applied a 500Hz sinewave signal of about 2V RMS and the result can be seen in the scope waveforms of Fig.11. The top trace is the 500Hz sinewave while the lower trace is the pulse width modulated waveform which is running at around 5kHz or thereabouts. Note that the wide pulses correspond to the positive peaks of the sinewave modulation signal and the narrow pulse correspond to the negative peaks. If you listen to the speaker it won’t sound too pleasant but if you wind up VR1 or VR2 so that the “carrier” frequency becomes supersonic, you will then hear a clear 500Hz tone. You can play around with the settings of VR1, VR2 and the input signal level to get the loudest and clearest signal from the speaker. So there you are – it works as an amplifier. If you don’t have an audio oscillator, don’t worry. You can feed in the signal from a standard CD player. Go ahead and try it. It won’t be high fidelity but you can listen to it – a 555 does work as an audio amplifier. What is happening here is that we are pulse width modulat­ing a carrier frequency of say 30kHz with an audio signal. The speaker cannot respond to the 30kHz signal but it can respond to the average DC level and this is the audio signal we feed in from the CD player. Feedback wanted Finally, we’d like some feedback about these Protoboard articles. Do you like them? Do they explain enough? And would you like a particular circuit demonstrated and explained? Please email your comments to leo<at>siliconchip.com.au SC