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the easy way into electronics This month we feature two Protoboard circuits based on the 555 timer. This chip must be one of the most popular devices ever developed but when it came out in around 1973, people won­dered what to do with it! The two circuits are quite similar to look at but they produce different functions and you can change from one circuit to the other in just a few moments. Most electronics enthusiasts know that the 555 can be used in timer and oscillator circuits and these are what we are pre­senting here. However, in each case we use two 555s and one is used to control the other. The first circuit (Fig.1) is a siren with one 555 being used to frequency modulate the second. The second circuit could be used as part of an alarm. It consists of a 555 oscillator which runs for a time determined by the second 555. Fig.3 shows the wiring layout for the siren circuit on a protoboard. As we did last month, the two ICs straddle the cen­tral channel of the Proto­board and the eight pins of each IC connect 60  Silicon Chip to individual conducting strips. The other components and jumper leads are then inserted to make up the circuit. Note that the three electrolytic capacitors must be con­nected the right way around (ie, positive voltage to their posi­ tive terminal) otherwise they will be damaged although initially, the circuit would probably work. You will need two potentiometers, both 100kΩ, but other values from 50kΩ-250kΩ will work just as well although produc­ing a different range of frequencies and times, as we shall see. The two pots are mounted on the vertical panel of our Protoboard’s metal chassis and then wired to the circuit. You will also need a momentary contact pushbutton switch and this is also mounted on the panel. The diode (D1) is wired in series with the DC socket for the 9V plugpack but it should also be included if you are using a 6V or 9V battery because without D1, a reversed DC supply will mean instant death for the two ICs. Note that supply from a 9V DC plugpack is likely to be closer to 12V because this circuit does not have a heavy current drain. Hook up all the components for the circuit of Fig.1, as shown in the diagram of Fig.3. Don’t hurry the job as you might make a tragic mistake. Even Fig.1: IC1 runs at a slow rate (about 0.5Hz to 2.8Hz) and it is used to frequency modulate IC2 which drives a loud­speaker. so, it will only take a few minutes to wire it up. On the other hand, if you are not confident with the task, just wire up the circuit involving IC2 first. This will produce an oscillator with a frequency which can range from around 200Hz to 700Hz, depending on how you set pot VR2. Scope waveforms The scope waveforms of Fig.4 and Fig.5 show the frequency extremes which can be expected at pin 3 of IC2. Fig.4 shows a waveform with negative pulses which are 700µs wide at a repeti­tion rate of 200Hz while Fig.5 shows the same 700µs wide negative pulses but at a repetition rate of 675Hz. The negative pulse width of 700µs (or 0.7 milliseconds) is set by the 0.1µF capacitor at pins 2 & 6, combined with the 1kΩ resistor to pin 7. The variable part of the waveform, the positive section, is determined by the 10kΩ resistor and 100kΩ pot connected to pin 7, the 1kΩ resistor to pins 2 & 6 and the aforementioned 0.1µF capacitor. So by varying the pot setting, we vary the overall frequency. By the way, both those waveforms were taken with the loud­ speaker disconnected (the noise gets to you after a while!) and this means that the amplitude of over 11V peak-to- peak is larger than it would be. With the speaker connected the amplitude drops to between 6V and 9V peakto-peak, depending on the frequency setting. OK, now let’s have look at IC1. This is almost identical to the circuit for IC2 except that the capacitor at pins 2 & 6 is 47µF instead of 0.1µF. This means that the frequency range will be about 500 times lower. Fig.6 shows the waveforms at pin 2 and pin 3 of IC1. The upper trace shows the sawtooth or triangle waveform at pin 2. The frequency can be made to vary between about 0.5Hz and 2.8Hz, depending on the setting of pot VR1. Actually, the waveform at pin 2 looks more “sawtoothy” at the low Fig.2: in this circuit, IC1 is used as monostable and it allows IC2 to produce a burst of oscillation lasting for about one second. If you want it longer, increase the 47µF capacitor at pin 6. October 2000  61 Fig.3: this shows the Protoboard component layout for the siren circuit of Fig.1. Note that you can build the circuit in stages, starting with the parts for IC2 (see text). frequency setting of VR1, with a long upslope and then a relatively sudden down-slope. We use the sawtooth waveform from pin 2 of IC1 to modulate the frequency of IC2. Hence, pin 2 of IC1 is connect­ed via a 10kΩ resistor to the CV (control voltage) input, pin 5, of IC2. This causes the internal thresholds of the 555 window comparator to be moved up and down. The result is that IC2’s frequency jumps up with each downslope of the sawtooth and then falls away, just like a wailing siren. Fig.7 is an attempt to show this modulation effect but the digital scope can’t really give a true picture because its time­base speed of 100ms/ The completed Siren circuit, viewed from the front. The two potentiometers (VR1 & VR2) are fitted to holes in the front panel at top left. 62  Silicon Chip div (to display the sawtooth) is far too slow to show the siren frequency (lower trace). OK, we have just about covered all the waveforms that you can check on this modulated siren circuit but there are still a few wrinkles to uncover. First, disconnect the 10kΩ resistor from pin 5 of IC2; just plug the unused end into one of the unused Protoboard boles. Now IC2 is unmodulated and produces a steady tone. Want to make it louder? There are two ways. First, you can reduce the value of the 68Ω resistor in series with the loudspeaker. This will allow more current through the speaker but is not the most efficient way. Second, try mounting an empty toilet roll over the speaker and then “tune” VR2 for maximum loudness. You can have quite a lot of fun with this effect. Finally, you might wonder why we bothered putting a 100µF coupling capacitor in series with the speaker. Why have it there when the circuit will work without it? If you don’t believe us, try shorting the capacitor out. Still works, doesn’t it? But notice that Fig.4: this is the waveform at pin 3 of IC2 in the circuit of Fig.1. It has negative pulses which are 700µs wide at a repeti­tion rate of 200Hz. Fig.5: this shows the same circuit conditions as at Fig.4 but VR2 has been set to increase the pulse repetition rate to 675Hz. Fig.6: these are the waveforms at pin 2 and pin 3 of IC1 in the circuit of Fig.1. The upper trace shows the sawtooth or triangle waveform at pin 2. The frequency can be made to vary between about 0.5Hz and 2.8Hz, depending on the setting of pot VR1. The lower trace is the negative pulse train; the pulse width is about 38ms. Fig.7: these waveforms attempt to show the modulation effect of the circuit of Fig.1. The digital scope can’t really give a true picture because its timebase speed of 100ms/div (to display the sawtooth) is far too slow to show the siren frequency (lower trace). Fig.8: this scope shot shows the current waveform through the 68Ω resistor and loudspeaker when the 100µF capacitor is shorted out. The ampli­tude is 3.76V peak-to-peak. Fig.9: this shot shows the current waveform through the loudspeaker and 68Ω resistor when the 100µF capacitor is in circuit. Note that the peak-to-peak and RMS current values are substantially higher than in Fig.8. October 2000  63 of Fig.2, the timed alarm. Pin 4 of IC2 now goes to pin 3 of IC1 instead of to the +9V line. The 10kΩ resistor bet­ween pin 2 of IC1 and pin 5 of IC2 is removed. Also, pin 2 of IC1 now goes to a pushbutton instead of to pins 6. This converts IC1 from a low frequency oscillator (ie, an astable) to a pulse generator (ie, a monostable). Bursts of sound This rear view shows how the completed is connected to the DC socket, the pots (VR1 & VR2) and the loudspeaker using flying leads. it is not as loud as with the capacitor doing its job? Now why is that? We’ll give you a hint. With the capacitor out of circuit, the 555 only “sources” current into the loud­speaker when its output at pin 3 is high. So we get a train of positive pulses through the speaker. But when the 100µF capacitor is in series with the speaker, current can flow in two directions: first, when pin 3 is high, it charges up the 100µF capacitor via the 68Ω resistor and speaker and second, when pin 3 is low, it then discharges the 100µF capacitor via the resistor and speaker. So we get more drive to the speaker. To prove it, we used the scope again. Fig.8 shows the cur­ rent waveform through the 68Ω resistor when the 100µF capacitor is shorted out. The amplitude is 3.76V peak-to-peak. Then, when the capacitor is back in circuit, the frequency remains the same, as you would expect, since the output loading has no effect on the frequency of a 555 oscillator. But note that the amplitude is now almost doubled, in Fig.9. It sounds quite a lot louder too. Timed alarm Now that you’ve had fun with the siren, let’s change the circuit to that Fig.10: this shows the burst effect that can be obtained with the circuit of Fig.2 (see text). 64  Silicon Chip Actually, as part of the change, you can leave the pushbut­ton out of circuit for the moment and just connect pin 4 of IC2 to pin 3 of IC1 (leave pin 2 connected to pin 6). Now if you apply power, IC1 will cause IC2 to produce bursts of sound from the speaker at a rate which can be varied by VR1. Fig.10 shows the effect. If you want the timed burst to be longer, you can increase the value of the 47µF capacitor at pin 6 of IC1 and VR1 can be increased in value. How the circuits work After all that, you probably have nutted out how each of the circuits work but we will go through it briefly for the sake of completeness. Apart from the timed alarm circuit of Fig.2 where IC1 works as a mono­ stable, all the circuits work as astable multivibrators. The resistors and pot­entiometer at pins 2 & 6 charge the capaci­ tor (0.1µF or 47µF) up to two-thirds of the supply voltage wher­e­upon the internal transistor at pin 7 discharges the capacitor down to one-third of the supply voltage. The one-third and two-thirds voltage thresholds are set internally for the 555’s window comparator. While the capacitor at pin 6 is charging, the output at pin 3 is high and while it is being discharged, the output is low. You can check this effect on the various scope waveforms in this article. Pin 4 (RESet) is normally tied high and if it is low it inhibits oscillation. We use this fact in the timed alarm of Fig.2. Pin 2 is normally tied to pin 6 in the astable oscillator circuit but is used in the timed alarm as the trigger input for the monostable circuit. Pulling pin 2 suddenly low starts the monostable; pin 3 goes high while the capacitor at pin 6 charges and then goes low when it reaches two-thirds SC of the supply vol­tage.