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By JOHN CLARKE V8 Is the sound of a V8 music to your ears? Does the roar and deep rumble of a V8 engine raise your pulse rate? With the SILICON CHIP V8 doorbell you can impress your friends and neighbours. You can have the sound of a V8 without even starting your car. In fact, you don’t even need a car. H AVE YOU SEEN the Repco V8 Doorbell advertised on TV? Does the sound of a powerful V8 instead of a boring 2-tone doorbell appeal to you? Then forget the Repco doorbell – it sounds puny. The SILICON CHIP V8 doorbell really does sound like a V8 and it is loud as well, with an inbuilt 5-watt amplifier. Not only does it sound like a proper V8, it also lights up an 8-LED “V” display, each time you press the doorbell pushbutton. In fact, you can have a V8-LED display at your front door and another on the doorbell case. The V8 Doorbell is housed in a plastic case and is powered by a plugpack. 24  Silicon Chip For normal use, the internal 100mm loudspeaker can be used and this provides a good simulation of the V8 sound, particularly if the loudspeaker is tuned using a length of PVC pipe – more on this later. For more volume, you can use a larger loudspeaker or if you want to go the whole hog, connect a bigger power amplifier and loudspeaker. If the V8 Doorbell does not sound quite how you like it, you can easily tailor the circuit to make small changes to the way the V8 sounds. Altering the software can make even greater changes. That way, you may be able to reproduce the Cleveland engine, Holden Monaro, Corvette or something else. We even allow for simulation of V6 engines. Well, grudgingly, and there are some restrictions on the settings that can be used. But enough of pony engines; let’s get back to V8s. That “luvverly” V8 burble V8s have a characteristic sound that makes them stand out from smaller engines. Each marque has its own “sound” that distinguishes it from the others and much effort is made by the manufacturers to ensure that their V8 has the most appealing “signature”. The characteristic V8 sound is mainly determined by the way the exhaust system is configured. In a typical V8, each cylinder of the engine is connected to an exhaust outlet pipe with four pipes merging into one, on each side of the engine. Some V8s have completely separate left and right exhaust systems (twin exhausts). The lengths of pipe between the engine and muffler affect the way the sound is mixed from the two sides of the engine. In a single exhaust system, one exhaust pipe must be longer siliconchip.com.au Fig.1: the block diagram of the V8 Doorbell. Most of the action takes place inside the microcontroller but there’s also some fancy filtering and mixing to get the V8 “burble” sound we want. than the other, to reach to the one side of the car body. In a twin exhaust, the mixing happens in the air and at our ears. Block diagram We have simulated the sound of a V8 with the above principles in mind. The block diagram of Fig.1 shows how it is done. When the doorbell is pressed, the microcontroller begins to produce signals from eight ports to simulate the firing of the eight cylinders. They produce tones in a sequence similar to the firing in a real engine. Typically, there can be an overlap between when one cylinder fires and the next so in effect there can be two sources of signal at any one time. In this design, you can select several overlap options and the degree of overlap between the cylinders will affect the sound of the simulated engine. The overlaps that can be selected are a 60° overlap, a 30° overlap, zero overlap or a 30° gap between cylinder firing. Cylinder outputs 1-4 produce their tones with different phasing to that of cylinder outputs 5-8. This is to simulate mixing of the left and right side siliconchip.com.au exhausts of the engine. You can alter the phase from its initial 180° setting to any other value in steps of about 20°. Cylinder outputs are mixed together in IC2b and then fed through a low-pass filter. This filter acts like a muffler in that it attenuates high frequency noise but allows through some low frequency noise. The accelerate filter control (Q1) and the snarl filter control (Q2) alter the way the filter works. These make changes to the filter characteristics to allow more high frequency signals to pass during acceleration and at high RPM simulation. The microcontroller’s RA2 port functions as a gated noise source, generating random noise only during part of each cylinder’s firing cycle. This simulates valve, tappet, drive train and air inlet sounds. This is fed to bandpass filter IC3a and then mixed with the cylinder signal by IC3b. Q3 provides for an increase in volume level at higher RPM, under control of a pulse width modulated (PWM) signal from port RB3 of the micro. After filtering, the PWM signal becomes a DC voltage to drive Q3. This DC voltage also controls a voltage controlled oscillator (VCO) which alters its frequency depending on the input voltage. The VCO’s output is fed to port RA4 of IC1 and it therefore determines the effective engine RPM. The final signal is fed to the volume control pot and amplifier IC5a. This is Where To Buy The Parts Jaycar Electronics has sponsored the development of this project and they own the design copyright. A full kit of parts will be available from Jaycar – Cat. KC-5405. This kit includes a screen-printed and solder-masked PC board; all on-board parts; and a case with pre-punched front and rear panels and screened lettering. Alternatively, you can purchase a short-form kit with just the PC board and all on-board parts (does not include loudspeaker or pushbutton switch) – Cat. KC-5405. The 12V DC plugpack is available separately. January 2005  25 26  Silicon Chip siliconchip.com.au Fig.2 (left): the complete circuit of the V8 Doorbell. A PIC16F628 microcontroller (IC1) produces the simulated V8 engine signals. These signals are then processed and fed to audio amplifier stage IC6. muted so that there is no signal until the doorbell is pressed. Power amplifier IC6 drives the loudspeaker. Circuit details Fig.2 shows the complete circuit. IC1 is a PIC16F628 microcontroller that produces the simulated V8 engine signals. IC1 operates at 20MHz, as set by crystal X1. The doorbell input at RA5 is normally low (0V) when the switch is open since the 1kΩ resistor pulls it to ground. When the switch is closed, the input is pulled to +5V. A 100nF capacitor across the resistor removes noise picked up by the doorbell wiring while the 2.2kΩ resistor acts to restrict current to the RA5 input if there is a transient voltage spike. The closed switch is detected by IC1 and so it begins to produce the engine sound sequence. The port outputs at RB0 to RB2, RA3 and RB4-RB7 are applied via 2.2kΩ resistors to op amp IC2b, connected as a mixer with its gain set by trimpot VR1. These ports also drive LEDs 1-8 via 560Ω resistors to give the V8 display. Op amp IC2a is the low-pass filter stage. In its normal state, this filter acts to sharply roll off the signal above 600Hz when Mosfets Q1 and Q2 are both switched on. When Q1 is switched off, its associated 220nF capacitor is effectively switched out of circuit and this reduces the filter’s effectiveness at rolling off signal level above 185Hz. Similarly, when Q2 is switched off, the 100nF capacitor is out of circuit and the filter action is further reduced. The 1MΩ resistors tying the capacitors to ground are included to maintain the DC voltage across these capacitors so that there is no DC shift in signal when they are switched in or out. Q1 and Q2 are controlled by the RA1 and RA0 outputs of IC1 respectively. The 10kΩ resistor and 1µF capacitor on the gate of each Mosfet slow down the switch-on and switchoff rates of the Mosfets to eliminate switching noise. In practice, both Mosfets are switchsiliconchip.com.au Parts List – V8 Doorbell 1 main PC board, code 05101051, 171 x 105mm 1 display PC board, code 05101052, 56 x 48mm 1 plastic utility box, 197 x 113 x 63mm 1 12VDC 1A plugpack 1 4Ω 100mm loudspeaker 1 130mm length of 100mm diameter PVC tubing1 doorbell switch (S1) 1 20MHz crystal (X1) 1 8-way right-angle pin header 1 8-way pin header 2 8-way pin header sockets 1 2-way PC-mount screw terminal block 1 2.5mm DC socket 1 panel-mount RCA socket 1 knob to suit potentiometer 1 80mm length of 8-way rainbow cable 1 150mm length of 3-way rainbow cable 1 80mm length of hookup wire 1 80mm length of single core shielded cable 1 80mm length of figure-8 light duty wire 1 suitable length of figure-8 doorbell wire 1 150mm length of 0.8mm tinned copper wire 4 12mm M3 tapped spacers 13 M3 x 10mm screws 5 M3 nuts 9 PC stakes Semiconductors 1 PIC16F628 microcontroller programmed with engine3.hex (IC1) 3 LM358 dual op amps (IC2, IC3, IC5) 1 7555 CMOS timer (IC4) ed on during idle to provide the full effect of the filter. When the “engine” speed is increased, Q1 is switched off to produce the noise of acceleration and as RPM rises further, Q2 is switched off for the “snarl” effect at high RPM. The low-pass filter output at pin 1 of IC2a is fed to op amp IC3b, another mixer, via a 2.2kΩ resistor. Gated noise Gated noise from the RA2 output of IC1 is attenuated via a voltage 1 TDA1905 5W amplifier (IC6) 3 2N7000 Mosfets (Q1-Q3) 1 BC547 NPN transistor (Q4) 1 7805 5V regulator (REG1) 1 1N4004 1A diode (D1) 1 1N4148 switching diode (D2) 8 5mm red high intensity LEDs (LED1-LED8) Capacitors 2 1000µF 16V PC electrolytic 1 470µF 16V PC electrolytic 3 100µF 16V PC electrolytic 1 47µF 16V PC electrolytic 10 10µF 16V PC electrolytic 2 2.2µF 16V PC electrolytic 3 1µF 16V PC electrolytic 3 220nF MKT polyester 5 100nF MKT polyester 2 10nF MKT polyester 1 5.6nF MKT polyester 2 2.2nF MKT polyester 1 1nF MKT polyester 1 100pF ceramic 3 22pF ceramic Resistors (0.25W 1%) 4 1MΩ 2 4.7kΩ 5 100kΩ 13 2.2kΩ 2 47kΩ 1 1.2kΩ 1 33kΩ 5 1kΩ 1 22kΩ 8 560Ω 11 10kΩ 1 100Ω 1 8.2kΩ 1 1Ω Potentiometers 1 1kΩ multi-turn side adjust screw trimpot (code 102) (VR1) 1 500kΩ horizontal trimpot (code 504) (VR2) 1 10kΩ log 16mm potentiometer (VR3) 1 10kΩ horizontal trimpot (code 103) (VR4) divider comprising a 1MΩ resistor and a 10kΩ resistor in series with a 10µF capacitor. The 10nF capacitors and 1.2kΩ resistor form a half-T filter that allows a relatively narrow band of frequencies centred on about 6.6kHz to pass through. The 100kΩ resistor between pin 2 and pin 1 broadens the bandwidth of the filter to allow a wider range of frequencies to pass than if the resistor was not present. The output of IC3a is fed to mixer IC3b via a 1MΩ resistor. January 2005  27 Table 1: Capacitor Codes Value 220nF 100nF 10nF 5.6nF 2.2nF 1nF 100pF 22pF μF Code 0.22µF 0.1µF .01µF .0056µF .0022µF .001µF   NA   NA IEC Code 220n 100n   10n   5n6   2n2   1n0 100p   22p EIA Code   224   104   103   563   222   102   100    22 The output from IC3b is passed through a 2.2kΩ resistor and 10µF DC blocking capacitor. Mosfet Q3 shunts this signal to ground when conducting but has no effect on the signal throughput when it is switched off. Q3 is controlled via the filtered PWM signal from pin 9 of IC1. The 1kΩ resistor and 10µF capacitor filter this 19kHz signal and the filtered DC voltage is applied via trimpot VR2 to the gate of Q3. Op amp IC5a is a non-inverting amplifier with a gain of 11. It amplifies the signal taken from the wiper of VR3 so that the level is suitable for the following power amplifier. IC5a is biased at +5V so that when there is no signal, its pin 1 output sits at 5V. This allows a large voltage swing before the output clips. High frequency roll-off for the amplifier is set at around 16kHz to prevent high-frequency instability. Its output is coupled to power amplifier IC6 via a 4.7kΩ resistor and 10µF DC blocking capacitor. Transistor Q4 provides muting of the output signal and it is controlled by comparator IC5b and the filtered PWM signal from pin 9 of IC1. IC5b operates in the following way. When the circuit is quiescent (ie, not producing any V8 sounds), the filtered PWM signal is at 5V. This is monitored at pin 5 of IC5b and is compared with the voltage set by trimpot VR4, fed to pin 6. VR4 is set so that pin 6 is at about 4.7V and so pin 7 of IC5b will be high at around 11V. This high signal drives the base of Q4 which therefore shunts any noise signals to ground. When the doorbell is pressed, the microcontroller begins to produce the V8 sounds and the PWM signal immediately drops to 4.5V and so pin 7 of IC5b goes low and Q4 is switched off. The signal at IC5a’s output now passes to the line output socket and to IC6, the power amplifier. vides high-frequency filtering. IC4 is a CMOS 555 timer set up as a voltage controlled oscillator (VCO). Its output is fed to port RA4 (pin 3) of the microcontroller to determine the audible engine RPM. Pin 5 (threshold control) is used to set the output frequency. When pin 3 of IC4 is low, diode D2 discharges the 220nF capacitor at pins 2 and 6 relatively quickly via the series connected 2.2kΩ resistor. When pin 3 goes high, the 220nF capacitor only charges via the 33kΩ resistor since D2 is now reversed-biased. The resulting pulse waveform at pin 3 has a relatively short low period and a longer high-level period; ie, a high duty cycle. We then vary the voltage at pin 5 to control the output frequency. When pin 5 is up around 5V, the frequency is low and if pin 5 is low the frequency is higher. Power for the circuit is provided by a 12V DC plugpack. Diode D1 prevents damage if the supply is connected the wrong way around while the 470µF capacitor provides extra filtering. The 12V supply feeds IC5 and IC6 while REG1, an LM7805 5V regulator, supplies the rest of the circuit. Power amplifier IC6 is a TDA1905 power amplifier rated to produce 5W into 4Ω with a 14V supply. It includes thermal shutdown if it overheats and a very low noise output. For the intermittent use it gets in this circuit, it is ideal. Gain of the amplifier is set at 11 by the 100Ω and 1kΩ resistors connected between pin 1 and ground, with the feedback signal AC-coupled to pin 6 via a 2.2µF capacitor. The 100µF capacitor at pin 7 provides supply ripple rejection while the 47µF capacitor between pin 1 and pin 3 provides classic bootstrapping between the amplifier’s output and driver stages. A 1000µF capacitor across the 12V supply provides a reserve for transient power output while a 100nF bypass capacitor pro- Construction The V8 Doorbell is built onto two PC boards: a main board coded 05101051 (171 x 105mm) and a LED display board coded 05101052 (56 x 48mm). The two PC boards and the 100mm loudspeaker are housed inside a plastic utility box measuring 197 x 113 x 63mm. Before installing any of the parts, check the two PC boards for any shorts between the copper tracks or for any Table 2: Resistor Colour Codes o o o o o o o o o o o o o o o No. 4 5 2 1 1 11 1 2 13 1 5 8 1 1 28  Silicon Chip Value 1MΩ 100kΩ 47kΩ 33kΩ 22kΩ 10kΩ 8.2kΩ 4.7kΩ 2.2kΩ 1.2kΩ 1kΩ 560Ω 100Ω 1Ω 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown orange orange orange brown red red orange brown brown black orange brown grey red red brown yellow violet red brown red red red brown brown red red brown brown black red brown green blue brown brown brown black brown brown brown black gold gold 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown orange orange black red brown red red black red brown brown black black red brown grey red black brown brown yellow violet black brown brown red red black brown brown brown red black brown brown brown black black brown brown green blue black black brown brown black black black brown brown black black silver brown siliconchip.com.au Fig.3 install the parts on the two PC boards as shown here. Make sure that you install each part in its correct location and take care to ensure that all polarised parts go in the right way around. Note that there are two 8-way pin headers on the main board. This lets you drive two separate display boards if required. breaks in the connections. Also check the hole sizes. You will need 3mm holes for the mounting positions in the four corners of the display PC board and for the regulator screw on the main PC board. That done, begin the assembly by installing the links and resistors on the main PC board – see Fig.3. Use the resistor colour table as a guide to selecting each resistor, then check each siliconchip.com.au value using a digital multimeter. Once the resistors are in, the diodes can be installed, taking care with their orientation. Follow these with ICs 2-6, make sure that each IC goes in the correct position and is mounted the right way around. An IC socket should be used for IC1. Install it now, then solder in the three Mosfets (Q1-Q3) and transistor Q4. The trimpots and capacitors can go in next. When installing the capacitors, note that the polarised types must be installed with the correct polarity. Note also that three electrolytic capacitors have to be placed on their side, so that there is room for the loudspeaker later on (see layout diagram photos). Regulator REG1 is mounted with its metal tab flat against the PC board. This involves first bending its leads at right-angles so that they pass through January 2005  29 Fig.4: you can change the sound produced by your V8 Doorbell by making the connections shown here and then applying power – see text for further details. their matching holes in the board. That done, the regulator tab is secured to the board using an M3 screw and nut and the leads soldered. The following parts can now all be installed: the 2-way terminal block, the DC socket, the eight PC stakes (at the external wiring points shown) and the 2 x 8-way pin headers (the right-angle header is installed on the display PC board). You will need to connect the two header socket shells using 8-way rainbow cable. This is done by stripping the wire ends and crimping them to the pins supplied. These pins are then slid into the header shells. have to drill the holes yourself. This involves drilling eight holes in a “V” pattern for the eight LEDs, plus four mounting holes each for the display board and the loudspeaker. In addition, you will also have to drill holes in the lid in front of the loudspeaker cone area, to allow sound to escape. Another hole is required in the front panel for the volume control pot. And finally, two holes are required in one end of the base for the RCA output socket and the DC power plug, plus another hole in the opposite end for the doorbell switch wire entry. The four 12mm tapped Nylon spacers can now be fastened to the lid at the display board mounting points. These are secured using four M3 x 6mm screws. That done, slip the eight LEDs into their mounting holes on the PC board (make sure you get them the right way around), then secure the board to its spacers. It’s then simply a matter of pushing the LEDs through their respective holes in the front panel and soldering their leads. Finally, the loudspeaker and pot can be secured to the lid and the wiring completed as shown in Fig.3. Don’t forget to run the wire lead from the PC stake near the 8-way header on the main board to the PC stake on the display board. Test & adjustment Now for the smoke test. First, apply power to the circuit and check for 5V between pins 4 and 8 of both IC2 & IC3, between pins 1 & 4 of IC4 and between pins 5 & 14 of IC1. That done, check for about 12V between pins 4 & 8 of IC5 and pins 2 & 9 of IC6. If these voltages are correct, switch off and install IC1. However, if there are no voltages, check the polarity of the DC plug on the plugpack. The centre pin should be positive. OK, now let’s see if it actually works. To do this, connect the doorbell switch to the terminal block (using figure-8 wire) and adjust the various trimpots as follows: (1). set VR1 fully anticlockwise; (2). set VR2 & VR3 fully clockwise; (3). set VR4 so that its wiper voltage is at +4.7V with respect to ground. Now press the doorbell and slowly adjust multi-turn trimpot VR1 clockwise. The engine sound should start to increase in volume. The final setting for VR1 depends on personal preference – set it too far clockwise and the sound will become very harsh. A lower setting will produce a cleaner engine sound. Trimpot VR2 is set so that you obtain the required idle volume, compared to the “rev up” volume. It’s just a matter of slowly adjusting this pot until the idle volume is suitably lower than the “revved-up” volume. If required (ie, if you want more “ooomph”), a 100mm PVC pipe joiner (or 120mm length of pipe) can be secured to the lid in front of the loudspeaker using silicone sealant. This tuned pipe makes the sound more resonant and penetrating. If you like, you can try different lengths of pipe The unit gives a good V8 sound on its own but it’s even better with the tuned pipe installed. Display board assembly The display PC board can now be assembled. For the time being, it’s just a matter of installing the resistors, the right-angle header plug and a PC stake. Don’t install the LEDs just yet – that step comes later. If you buy a complete kit, then the case will be supplied pre-drilled, with screen printed lettering. If not, you will 30  Silicon Chip siliconchip.com.au Here’s how it all goes together inside the plastic case. You can use light-duty hook-up wire to make the connections to the volume pot and the loudspeaker. to vary the effect. Note that you may need to file some slots in the pipe so that it clears the loudspeaker mounting screws. Individual preferences There are seven setting changes that can be made to IC1’s software to produce different sounds. This involves connecting a wire and a series 1kΩ resistor between the +5V terminal (for the doorbell switch) and one of seven terminals on the 8-way header pin, as shown in Fig.4. When a connection is made to one of these pins during power up, the required software change is made siliconchip.com.au automatically. Note, however, that it’s important that the +5V supply rail is fully discharged to 0V before powering up if the change is to take effect. In fact, it’s a good idea to measure the voltage between the +5V terminal of REG1 and ground after the power is switched off, to ensure the power has been completely removed. Note also that the resistor only has to be connected at power up. It can then be disconnected when you are satisfied with the new sound. The options available are summarised below: Terminal 1: you can adjust the V8 sound to simulate different lengths of exhaust pipe between the lefthand January 2005  31 the rev range. However, it takes many applications of power to make large changes to the frequency. The frequency can be reset to its default value using Terminal 2. Terminal 7: this selects whether the doorbell includes an idling period before the two revving sequences. Other changes The PC board is secured by clipping it into the integral slots in the side of the case. Power comes from a 12V DC 1A plugpack. and righthand sides of the engine. This is the phasing adjustment. Phasing can be altered in steps of about 20°, from its original default of 180°. Terminal 2: the 180° default setting of the phase and the exhaust note frequency can be reset using this input (see Terminals 5 & 6 below). Terminal 3: this terminal alters the amount of overlap for the sound generated by each cylinder firing. It can be altered in sequence from 60° to 30° to 0° and finally to a 30° gap. Terminal 4: the 6 or 8-cylinder selection is made using this input. This alternatively selects either setting, with the LED display showing which cylinders are firing. Note that only the 0° and 30° gap settings should used in 6-cylinder mode. Do not use the 60° and 30° overlap settings, as this will simulate a 6-cylinder engine with an erratic seventh cylinder. The correct setting will be seen on the “V” display when only six LEDs light. If seven LEDs light, change the overlap setting using Terminal 3. Terminals 5 & 6: these inputs allow the exhaust frequency to be altered slightly. Terminal 5 increases the frequency, while Terminal 6 lowers it. If the frequency is increased too far from the original value, the sound will have a “raspy” quality at the top of Brand New From SILICON CHIP The idle RPM can be set by changing the 33kΩ resistor at pin 3 of IC4 – a larger resistance will lower the RPM or you can use a 50kΩ trimpot to adjust this to your liking. The ambient noise can be increased in frequency by decreasing the 1.2kΩ resistor in the “twin-T” filter of IC3a and vice versa. In addition, the 10kΩ resistor at pin 3 of IC3a sets the degree of mixing with the cylinder firing sound. A lower value will reduce the ambience and vice versa, or you can use a 22kΩ trimpot to adjust this. You can also make major changes to the doorbell sound characteristics by altering the software. To do this, you will need to be able to modify the software, reassemble the code and reprogram IC1. Some PIC programming experience will be necessary. A much fuller sound is available if you use a large loudspeaker housed in a suitable box. For more volume, you may want to use a more powerful amplifier and this can be connected using the RCA line output socket. If you do this, you can either disconnect the internal loudspeaker or you can leave it connected so that it SC operates as an extension. 160 PAGES 23 CHAPTE RS Mail order prices: Aust: $A22.50 (incl. 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