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Run your own disco light show with the PT.1: By MAURO GRASSI DSP Musicolour This DSP Musicolour accepts audio input signals and drives coloured lights “in tune” with the music. Its four output channels respond to different audio frequency bands and the brightness of the lights is in direct proportion to the amplitude of the frequency components. A dot-matrix LED menu display is featured on the front panel and this also functions as a spectrum analyser or VU meter. I N PRESENTING THIS brand new project, we are reviving a name which became synonymous with party light shows in the 1970s and 1980s. Long time readers will be familiar with the “Musicolour” light shows which were presented in “Electronics Australia”, the last one being the Musicolour IV in the August 1981 issue. SILICON CHIP magazine followed the same theme with the Discolight project in the July & August 1988 issues and produced a 12V version called the 30  Silicon Chip “Lightshow” in the January & February 1998 issues. All of these projects were analog designs based on op amps and logic ICs. The audio was passed through different pass-band filters for each channel and the filtered outputs were used to drive either phase-controlled Triacs in the Musicolour and Discolight circuits or power Mosfets in switchmode in the case of the 12V Lightshow. The logic ICs were used in the Discolight and 12V Lightshow to produce the chaser modes, which were alternating patterns controlling the output channels in the absence of an audio input. These chaser modes meant that you could have pleasing lighting effects without any music. Two decades on from the Discolight we now present the brand new DSP Musicolour, a full digital design using DSP (digital signal processing). It is based on a single DSP microcontroller to produce a comprehensive list of features that were simply not possible siliconchip.com.au This photo and the photo on the facing page show the completed DSP Musicolour boards fitted to the case. The displays have been lit by temporarily powering the Display Board using an external DC supply. with previous analog designs. The DSP Musicolour operates from 230-240VAC or 115-120VAC mains (detecting the 50Hz or 60Hz mains frequency automatically) and drives four channels of incandescent lights. The total power output rating is 2400W for the 230-240VAC 10A version and 1600W for the 115-120VAC 15A version. For the 240VAC version, each channel is rated at 800W maximum (as long as total power is less than 2400W). For the 120V version, each channel is rated at 400W. By making the design suitable for both 230-240VAC and 115-120VAC 50/60Hz mains supplies, we are ensuring that the DSP Musicolour can be built in any part of the world without modifications, apart from changing the power transformer’s primary connections. Incidentally, since February 23rd 2000, Australia’s mains supply has been nominally 230VAC, with a normal range from -6% to +10%, ie, 216-253V (AS60038-2000). In practice though, supply authorities try to maintain the range at between 225V and 253V. Operating features Pictured in this article is the prototype DSP Musicolour without its front or rear panels. The photos show the quite complex vertical PC board which accommodates a 7x15 (rows siliconchip.com.au x columns) dot-matrix LED display, an array of illuminated pushbuttons and three potentiometers. By contrast, the main PC board is larger but not as packed with components. It carries the microcontroller and the 240VAC Triac circuitry. The pushbuttons are used to navigate through the menus which are displayed on the dot-matrix display. This display can also be switched to function as a spectrum analyser or as a VU meter, or can be used to display “screen saver” patterns. One potentiometer on the front panel is used to vary the screen brightness, screen refresh rate, the frequency band, the gain and the quiescent current of each output channel, among other settings. Another two potentiometers are used to independently control the input sensitivity of each channel. It is then possible to mix the two channels in software before processing the audio data or to select either channel independently as the audio input. The back panel carries an IEC male mains socket (with an inbuilt mains switch and fuse) plus four 3-pin 240VAC sockets for the four output channels (these are used to connect the lights). It also carries a 4-way speaker terminal block to accept two audio input channels, an optional connector for an external microphone and an optional 10-way IDC connector that can be used to update the firmware (more on this in a later article). Block diagram The block diagram of the DSP Musicolour is shown in Fig.1. As can be seen, there are three audio input channels. Two audio channels come from the 4-way speaker terminal block on the back panel while the third channel is for an onboard electret microphone or an optional external microphone. The two audio channels pass through the front panel potentiometers and are then fed into independent AC-coupled inverting amplifiers. The output of each amplifier is then fed to a dsPIC microcontroller at inputs AN1 & AN2 and are converted to digital signals by onboard ADCs (analog-todigital converters). Similarly, the microphone signal is fed to an AC-coupled inverting amplifier and the output again connected to the microcontroller, this time at AN3. Note that the optional third audio input for connecting an external microphone is mixed with the output of the onboard electret microphone. The two audio channels are independent and as mentioned earlier, have separate sensitivity controls. They can be optionally mixed in software in different proportions (selectable balance). So there are three distinct audio channels: the two audio channels and the microphone channel. June 2008  31 MAINS OUTLETS POTENTIOMETERS OPTO COUPLED PHASE CONTROL TRIAC 15 x 7 DOT MATRIX DISPLAY OP AMP AUDIO CHANNEL INPUTS AN1 OP AMP MIC INPUT AN2 AN3 ADC INPUTS dsPIC 30F4011 DSP MICRO CONTROLLER OP AMP OC1 OPTO COUPLED PHASE CONTROL TRIAC OC2 OC3 OC4 OPTO COUPLED PHASE CONTROL TRIAC INT0 ZERO CROSSING DETECTOR FROM TRANSFORMER SECONDARY OPTO COUPLED PHASE CONTROL TRIAC 7 x USER INPUT SWITCHES AND POTENTIOMETER ACTIVE NEUTRAL Fig.1: the block diagram of the DSP Musicolour. The audio inputs are digitised and processed using a dsPIC micro­ controller to produce control signals for four frequency bands. The dsPIC then phase controls four optocoupled Triac output stages to power the mains outlets. As stated previously, the audio channels are converted to digital format by onboard ADCs in the dsPIC microcontroller. These are each sampled at around 48kHz (slightly higher than double the highest audible frequency) in order to avoid aliasing. Next, the resulting digital time domain data is transformed to the frequency domain using an FFT (Fast Fourier Transform) algorithm. The result is a list of amplitudes for the frequency components of the audio input signal. The computed frequency amplitudes are then used to drive the four output channels via phase-controlled Triacs in the output stages. These Triacs are driven via optocouplers which ensure full mains isolation for the low voltage sections of the circuit. As may be apparent from the description so far, there are no op amp filters so the frequency bands for each channel are user-selectable. Furthermore, different arithmetic operations can be applied to the digitised audio data to amplify or attenuate it. This means that each input channel is effectively post-amplified in software. Moreover, the various input channels can be selected and mixed in software with different weighting. In previous designs, the switching was 32  Silicon Chip accomplished using a toggle switch. Providing the chaser modes is also much easier with a digital design. Again, it’s all done in the software and no extra circuitry is involved. Circuit operation Let’s now look in more detail at the circuit operation of the DSP Musicolour. Figs.2-4 show the full circuit details. It’s based on microcontroller IC1, a dsPIC30F4011 from Microchip. This combines most of the common peripherals of a PIC with a high-performance RISC (reduced instruction set computer) processing core that has instructions optimised for digital signal processing (DSP). The Microchip dsPIC series of microcontrollers incorporate DSP feat­ures like MAC (multiply accumulate), variable bit shifting, bit reversed addressing modes, dual data fetching (multiple operands), high-precision arithmetic operations (including fractional hardware divide and multiply) and multiple 40-bit accumulators. All DSP instructions are single cycle, meaning the dsPIC series is surprisingly powerful, with throughputs in the tens of MIPS (millions of instructions per second). In this design, IC1 runs off an internal system clock operating at about 29MHz. Among other things, it’s responsible for digitising the audio inputs, processing the audio inputs (including computing the FFT), synchronising with the mains frequency and implementing real-time phase control, driving the multiplexed LED display and responding to key presses. The operation of the software is explained in greater detail later in this article. The output stages of the DSP Musicolour are virtually identical to those in the Discolight but have been uprated using heavy-duty BTA41-600B Triacs. These are isolated-tab Triacs rated at 600V, with continuous current ratings of up to 40A and a surge current rating of close to 400A. This high surge-current rating is necessary in order for the Triacs to withstand the large fault currents which can occur if a 150W floodlight blows its filament. This is a particular problem if the floodlight is facing upwards. In that case, the broken filament can flail around and short to the adjacent filament support. The resulting arc continues until the stem fuse inside the lamp blows. In addition, the software programm­ siliconchip.com.au ed into the dsPIC has a facility to set the filament preheat current of each output channel. This helps minimise surge currents to protect the lamps. Note that the four Triacs, which are housed in TOP3 packages, are mounted in pairs on the main board and each pair share a heatsink. These heatsinks are necessary, as the Triacs will get hot during normal operation, even if each channel is only rated at 800W. Each Triac is triggered by the dsPIC via a MOC3021 optocoupler (OPTO1OPTO4), to ensure full mains isolation from the low voltage part of the circuit. As shown, IC1’s “output compare” pins (OC1-OC4) are used to drive the internal LEDs of the optocouplers which then switch on the Triacs until the next zero crossing of the mains waveform. Following the Triacs, each channel output has a filter network consisting of a 100mH inductor (rated at 5A) and a 0.1mF 250VAC capacitor, to reduce EMI. As can be seen, the mains Active is connected to the A2 terminal of each Triac and is switched through to the mains output sockets via the 100mH inductors – see Fig.3. The outputs are then fed to screw terminal blocks CON4 & CON5 on the main board. Audio inputs As shown in Figs.2 & 4, the two audio channels are fed in via pins 2 & 4 of CON2 and are attenuated by two 270W resistors. These resistors ensure proper attenuation of the incoming signals, which are derived from the loudspeaker outputs of an amplifier. From there, the signals are fed to pins 5 & 6 of connector CON1 and then to pins 5 & 6 of CON7 on the display board (CON1 & CON7 are connected via a 26-way flat-ribbon cable). The audio signals are then further attenuated using potentiometers VR1 & VR3 on the display board and fed back to pins 4 & 3 of CON1 on the main board. These pots set the input sensitivity for the audio channels. Link LK4 is optional and allows the two audio channels to be mixed together via the 270W input resistors. It’s normally left out of circuit. The outputs of VR1 & VR2 (at pins 3 & 4 of CON1) are AC-coupled to op amps IC2a & IC2d via 68nF capacitors and 33kW series resistors. These two op amps are wired in inverting mode and operate with a gain of 0.3 as set siliconchip.com.au Main Features • • Full digital design using a DSP microcontroller (dsPIC) • • Four phase-controlled output channels (8-bit resolution) • • • Zero voltage switching of Triacs for minimum RF interference • • • • • • • • Selectable filament preheat current for each output channel • • • Software-controlled input selection with software mixing Powered from 230-240VAC or 115-120VAC mains with auto-detection of mains frequency Optocoupled Triac triggering for complete isolation of control circuitry from output circuitry Four mains outputs rated at 800W each (240VAC) or 400W (120VAC) Selectable gain plus selectable minimum and maximum frequencies for each output channel Persistent software settings Self-diagnostics Chaser modes Direct stereo inputs for audio modulation of lights Microphone input/third channel input for audio modulation of lights 7x15 pixel dot-matrix LED display for menus Dot-matrix display can function as a spectrum analyser, screen saver or VU meter Adaptive potentiometer control for software settings Optional provision for ICSP (in-circuit serial programming) by their 10kW feedback resistors and 33kW input resistors (ie, 10kW/33kW = 0.3). Because the circuit operates from a single +5V supply rail, the noninverting inputs of IC2a, IC2d and IC2c (pins 3, 12 & 10 respectively) must all be biased to half supply (Vcc/2). This is achieved using op amp IC2b. Two 10kW resistors are wired as a voltage divider to derive a 2.5V reference and this is fed to pin 5 of IC2b which is connected as a voltage follower. IC2b’s pin 7 output then provides the Vcc/2 (ie, 2.5V) rail to bias the remaining op amps. IC2c is used to amplify the signal coming from the on-board electret microphone. It’s also wired as an inverting amplifier but operates with a gain of 30. As before, the signal is fed to the op amp via a 68nF capacitor and 3.3kW resistor. Power for the electret microphone is derived from the +5V rail via 1kW and 4.7kW resistors, while a 47mF capacitor filters this supply. In addition, the signal from an external microphone can be fed to IC2c via pin 3 of CON2. In that case, the signals from the two microphones will be mixed and applied to IC2c. The outputs of op amps IC2a, IC2d & IC2c are connected to the ADC inputs of microcontroller IC1 (AN1, AN2 & AN3). Another ADC channel (AN0) is used to read the value of potentiometer VR3 which is also on the display board. This 10kW linear potentiometer is used to change settings. Resetting the micro Each time power is applied to the circuit, the MCLR-bar (reset) input of IC1 is pulled high (ie, towards the +5V rail) via diode D8 and a 1kW resistor. This releases the reset on the dsPIC30F4011 so that it can start operating. Note that pulling the MCLR-bar input low resets the microcontroller but this is not used in this circuit. Instead, an internal POR (power on reset) circuit resets the microcontroller when power is first applied. Diode D8 is included because the MCLR-bar line connects to 10-way IDC connector CON3. This connector is optional and is included to allow ICSP (in circuit serial programming). June 2008  33 CON1 +5V 11 13 100nF 14 40 100nF AVdd 15 16 CN 28 17 SER 38 19 CK2 21 G 23 CK1 26 S7 24 S6 22 S5 20 S4 18 S3 10 S1 8 S2 12 AN0 V12 3 V22 6 V13 5 V23 68nF 3 1k MCLR RF6 PGC PGD RF0 RF1 270  1W 26 25 30 29 12 IC2a 5 1 AN3 OC3 4 OC1 AN2 OC2 RE4 IC2d RE5 14 22 19 23 18 34 33 A LED8  4 R AUDIO 68nF 3 MIC 5 K 1k 1 3.3k 47 F 16V 4.7k 100k 3 470 AN1 9 68nF ELECTRET MIC LK6* 1k 13 2 L AUDIO 24 IC1 dsPIC30F4011 10k 33k 270  1W 1 LK5* OC4 68nF 1 D8 K 10k 33k LK4* 2 A 32 Vdd CN17 2 25 CON2 21 Vdd SFR 37 RE1 36 RE2 35 RE3 15 RC13 14 RC15 10 RB8 9 RB7 8 RB6 6 RB4 7 RB5 2 AN0 9 4 11 Vdd 10 IC2c INT0 8 AVss 39 + Vss 12 Vss 17 1k Vss 20 31 +5V 100nF 10k 5 10k 47 F 16V 6 * NOTE: IN THE DEFAULT CONFIGURATION, LK4 AND LK6 ARE OMITTED AND LK5 IS INSTALLED 4 IC2b 7 +2.5V D8 -D10: 1N4148 11 A K D11-D14: 1N4004 SC 2008 DSP MUSICOLOUR MAIN BOARD A LM317T K OUT ADJ OUT IN Fig.2: the Main Board circuit. Microcontroller IC1 accepts audio inputs from op amps IC2a, IC2d & IC2c, processes the signal and drives the four Triac output stages via optocouplers OPTO1-OPTO4. It also drives the Display Board via CON1 and processes the inputs from the various controls on this board. Since the MCLR-bar pin is multiplexed with the programming voltage (Vpp) for ICSP and because the typical level used is around +13V (much higher than +5V), D8 is reverse biased during 34  Silicon Chip ICSP and thus protects the circuit from over-voltage. LED8 is used as a “normal function” indicator for the microcontroller and is normally lit. It may also be used by future software versions to indicate runtime errors in the program. The 470W resistor limits the LED current to about 10mA. Note that this LED is mounted on the main board and is not siliconchip.com.au 680 6 1 OPTO2 MOC3021 +5V  2 CON3 5 A2 G 4 A1 TRIAC3 BTA41-600B 100nF 250VAC CON4 220 100 H 1 9 680 2 10 6 1 6 OPTO4 MOC3021 7 8 2 3 A2 TRIAC4 BTA41-600B  G 4 100nF 250VAC A1 220 4 100 H 680 6 1 OPTO1 MOC3021 A2 TRIAC1 BTA41-600B  2 G 4 100nF 250VAC A1 CON5 220 100 H 680 6 1 OPTO3 MOC3021 A2 TRIAC2 BTA41-600B  2 G 4 100nF 250VAC A1 220 100 H 270  1W NOTE: ALL WIRING AND PARTS IN THIS SHADED AREA OPERATE AT MAINS POTENTIAL. CONTACT COULD BE FATAL! K D10 A 5.6nF D11– D14 A D9 A K OUT K IN ADJ 100 470 F 120V 7.5V REG1 LM317T +5V T1 K 4700 F 16V A K A A K 0V 0V LK1 LK2 LK3 FOR 240V, FIT LK2 ONLY FOR 120V FIT LK1 AND LK3 ONLY 120V 7.5V 0V 0V 100 1 F 16V LED8 30VA CON6 100 N BTA41-600B 100 FUSED MALE IEC SOCKET WITH SWITCH K A A1 visible with the case lid on. LK5 & LK6 are used to set IC1 so that it operates according to the audio input mode set using LK4 (more on this in a future article). The default siliconchip.com.au A A2 G * FUSE F1 RATING: 10A FOR 240V, 15A FOR 120V configuration is to omit LK4 & LK6 and install LK5. Display board circuit Now let’s take a look at the display F1* MAINS IN E board circuit (Fig.5). It’s based on three 74HC595 shift registers (IC3-IC5) which are used to drive the 7x15 dotmatrix LED display in multiplexed fashion. This display is actually made June 2008  35 CON4 MAINS OUTPUT SOCKETS A N E N A E CON5 N A E N A E MAINS EARTH MAINS NEUTRAL Fig.3: the switched Active lines from CON4 & CON5 are connected to the mains outlet sockets as shown here. up using three separate 7x5 dot-matrix red LED modules (Kingbright TA1211EWA, distributed by Tenrod). IC3 & IC4 are cascaded to form a combined 16-bit shift register. This is controlled by the CK2 (clock), G-bar (enable) and SER (data) lines coming from microcontroller IC1. By using these three lines, any 16-bit value can be loaded into the shift register. In operation, the 16-bit shift register drives the 15 columns (ie, the anodes) of the 7x15 dot matrix display. The remaining column output, at pin 7 of IC4, is used to drive transistors Q16-Q22. These in turn drive LEDs1-7 which are inside the tactile switches (S1-S7). In greater detail, the first 15 bits of the combined shift register drive PNP transistors Q1-Q15 (all BC327s) via 470W resistors. These transistors are necessary to provide the high currents required to obtain a display that is bright enough. Because the display LEFT AMPLIFIER OUTPUT RIGHT AMPLIFIER OUTPUT + – is multiplexed, each LED is only switched on very briefly and therefore must be driven quite hard. Note that there are no series resistors to limit the current. That’s because the switching occurs very briefly and this limits the currents and protects the display modules. In addition, the cathodes are driven by a ULN2003 Darlington array (IC6) and this also limits the total peak currents. The Darlington transistors inside IC6 are driven by the remaining shift register (IC5). This is controlled in a similar way to the 16-bit register and is clocked using the CK1 (clock) line from the microcontroller. The G-bar (enable) and SER (data) lines are shared with the other two shift register ICs. Using a separate clock ensures that IC5 can be controlled independently of the two cascaded shift registers. IC5 thus functions as an independent 8-bit shift register and is used to drive the seven rows (R0-R6) of the dot matrix display via IC6. The extra bit is not used. The seven rows of the display are all cathodes and the ULN2003 (IC6) is necessary to provide the required current drive. In operation, this device can sink up to 500mA for each of its seven outputs. It’s also used to drive the “extra” row formed by the seven LEDs inside the tactile switches (LED1-LED7) – ie, IC6 drives the cathodes of LEDs1-7, while pin 7 of IC4 switches their anodes. During the display update period, the microcontroller sends the G-bar (enable) line high and this forces all the shift register outputs to also go high. This effectively blanks the display but this blanking interval is so short that it is imperceptible. The SER (data) line feeds the data from the microcontroller into the shift registers during this blanking period. Note that because the display is multiplexed, the microcontroller can control the 7x15 dot-matrix display CON2 2 + 4 – 3 OPTIONAL EXTERNAL MIC INPUT 36  Silicon Chip 5 1 Fig.4: the left & right audio input signals are derived from the speaker terminals of an amplifier and fed in via connector CON2. The external microphone input is optional. and LEDs1-7 using just four digital outputs, ie, CK1, CK2, G-bar & SER. Switches S1-S7 are all pushbutton momentary-contact types. These switches are used to change the operating modes and drive the menus (more on this next month). In practice, each switch is connected to a separate microcontroller input (via CON1 & CON7) and these seven inputs are all normally pulled high (ie, to +5V) by 1kW resistors. The other side of each switch is connected to ground and the switch contacts are debounced by the software. Diodes D1-D7 and their associated 1kW pull-up resistor form a 7-input AND gate. Its output (at the anodes) connects to the CN17 (pin 28) input of the microcontroller. This input is used to trigger an interrupt when it changes state and is used by the software to respond to switch presses. Power supply Now let’s go back to Fig.2 and describe the power supply. As shown, power for the low-voltage side of the circuit is derived using a 30VA mains transformer with dual primary and secondary windings. The two primary windings are either connected in series or in parallel, depending on the mains voltage (either 240VAC or 120VAC, respectively). For a 240V AC supply, LK2 is fitted to connect the primary windings in series. Alternatively, for a 120VAC mains supply, LK1 & LK3 are fitted to connect the windings in parallel. The selected links are soldered to the main PC board, before the transformer is installed. The incoming mains voltage is fed in by a fused male IEC socket. For 230-240VAC mains, this fuse should be rated at 10A while for 115-120V mains, it should be rated at 15A. The two 7.5VAC secondary windings are connected in parallel and feed a bridge rectifier comprised of diodes D11-D14 which are standard 1N4004 types. The rectified output of the bridge is then filtered using a 4700mF capacitor and fed to an LM317T variable-voltage regulator (REG1) to derive a regulated +5V rail. This +5V rail is used to power the low-voltage section of the circuit, including the op amps, the microcontroller and all the display circuitry. An LM317T was used instead of the common 7805 +5V fixed regulator siliconchip.com.au CON7 Q1 +5V 11,13,14 9,12 AN0 100nF VR3 10 16 Vdd Q0 Q1 Q2 Q3 MR 1F 16V 17 SER 14 19 CK2 12 Sin 47F 16V 100nF B B B C Q9 +5V E Q15 E B C E B C C 1 2 3 4 IC3 Q4 74HC595 5 10 12 11 G 13 1k 7 x 470 R6 R5 R4 R3 R2 R1 R0 +5V 9 100nF 16 Vdd Q0 Q1 Q2 Q3 MR 14 12 11 13 Sin 16 1 10 15 1 2 3 3 IC5 Q4 4 74HC595 5 4 Q5 6 Q6 7 Q7 9 So LCK SRCK OE 15 2 LED ARRAY 1 VR1 LED ARRAY 3 +5V 470 13 12 5 B B IC6: ULN2003 B  B  B LED4 K Q22 E B C A E B C A   LED3 K 470 Q21 E C A  470 Q20 E C A LED2 K 470 Q19 E C A LED1 K 470 Q18 E C A 10 7 470 Q17 E 11 6 470 Q16 6 V13 4 V12 LED ARRAY 2 14 Vss 8 C14 14 21 16 Vdd 15 Q0 1 Q1 2 Q2 3 Q3 Sin IC4 Q4 4 74HC595 5 Q5 6 LCK Q6 7 Q7 SRCK 9 So OE Vss 8 MR 8 C7 Vss C8 SRCK OE C1 11 C0 Q5 6 Q6 7 Q7 9 So LCK 13 23 CK1 Q8 E C 8 x 470 15 Q2 E LED5 K C A  LED6 K  LED7 K 8 2 V11 5 V23 3 V22 1 V21 15,16 10 8 18 20 22 24 26 1k VR2 LED1 INSIDE S1, ETC CN A A D1 K S1 S2 S3 A A K K A D4 D3 D2 K A K A K K S4 S5 S6 S7 7 x 1k 25 D1 – D7: 1N4148 A SC 2008 D7 D6 D5 DSP MUSICOLOUR S1 S2 S3 S4 S5 S6 S7 K B DISPLAY BOARD E C Q1 – Q22: BC327 Fig.5: the Display Board circuit uses three 74HC595 shift registers (IC3-IC5) to drive the dot-matrix LED displays and the LEDs inside the switches in multiplexed fashion. These shift registers are driven by the dsPIC microcontroller via CON7 using just four digital outputs. Switches S1-S7 select the various operating and display modes. siliconchip.com.au June 2008  37 Note: all the parts on the heatsink side of the Main Board operate at mains potential (ie, 240VAC). Contact could be lethal – see Fig.2 & warning panel! because it can supply greater current (up to 1.5A). The four 100W resistors set the output voltage from the regulator according to the formula: VOUT = 1.25 x (1 + R2/R1) where R1 is the resistance between the OUT & ADJ terminals and R2 is the resistance between the ADJ terminal and ground. In this case, R1 = 100W and R2 = 300W, so VOUT = 1.25 x (1 + 300/100) = 5V In practice, slight manufacturing variations mean that the 1.25 factor How The Software Works The software is responsible for most of the Musicolour’s functions and uses various interrupts to accomplish time critical tasks. For example, the multiplexed LED display is driven by the microcontroller using a periodic interrupt based on an internal timer. This allows the display refresh rate to be set by the software. Each time the timer expires, an interrupt occurs and display data is sent to the display driving hardware. The display is “multiplexed”, meaning that only one column of seven LEDs is lit at any one time. The seven LEDs inside the tactile switches effectively form the 16th column of the display. The “dead 38  Silicon Chip time” between screen refreshes can also be programmed and this effectively controls the perceived brightness of the display. Pressing any of the seven tactile switches (S1-S7) also triggers an interrupt. The software then debounces the switch and adds the key press to an internal queue implemented as a FIFO (first in, first out). This ensures that there are no missed key presses due to software latencies. Digitising the audio The microcontroller is also responsible for digitising the four analog input channels. Three of these are audio inputs can be anywhere between 1.2 and 1.3. Because of this, the PC board has been designed so that there are three resistors in series between ADJ and GND terminals. This allows REG1’s output to be fine-tuned by changing one or more resistor values, to obtain a supply rail that’s very close to +5V. Note, however, that this fine-tuning will not be necessary unless you plan to use the optional 10-way header (CON3) for in-circuit programming coming from the op amp circuits, while the fourth is used to read the setting of potentiometer VR3. This latter value is used to change various settings, according to the menu selected. Note that when changing a value with this potentiometer, the value will only begin to change when the potentio­ meter’s setting matches the current value of the setting that’s being changed. This gives the potentiometer a kind of “memory”. The analog-to-digital conversion of the four input channels is automatically handled by the ADC subsystem within the microcontroller (IC1). Each channel is sampled at around 48kHz. Most of the data produced is ignored and only one audio stream (or a mixed stream) is actually stored in an internal buffer. An interrupt is generated when- siliconchip.com.au These photos show the assembled Main Board and Display Board modules. The construction details are in next month’s issue (note: boards are shown slightly smaller than actual size). (and then only if the output voltage isn’t close to +5V). Zero-crossing detection As well as driving the bridge rectifier, one side of the transformer’s secondary winding is also fed to pin 17 of IC1 via 270W and 1kW resistors. Diodes D9 & D10 clip the positive and negative excursions of this AC signal to +5.6V and -0.6V respectively, to prevent damage to the microcontroller’s INT0 input. In addition, the signal is bypassed using a 5.6nF capacitor to filter any noise on the line. This capacitor changes the phase of the signal slightly but this effect is corrected by the software. ever the internal ADC system buffer (16 bytes) is full. The firmware then stores only the relevant channel data in an internal buffer. This happens after any mixing, amplifying or averaging is performed on the audio data. The FFT (Fast Fourier Transform) is computed in real time without loss of samples. This is accomplished by the software using double buffering. While the FFT is being computed on the data stored in an internal buffer, another internal buffer is being filled by the interrupt servicing the ADC subsystem. When the FFT is finished on the active buffer, it is then computed on the other buffer. In this manner, the two buffers are alternately filled and then processed by the FFT. Note that the microcontroller runs at close to 30MIPs using its internal clock siliconchip.com.au In operation, the microcontroller uses the clipped AC signal to detect the zero crossings of the mains waveform. This is necessary in order to correctly drive the Triacs using phase control and to minimise the interference caused by the Triacs switching the mains. Basically, the INT0 (pin 17) input of the microcontroller functions as an edge-triggered external interrupt source. This is used by the software to phase control the four output channels. That’s all we have space for this month. Next month, we’ll describe the assembly of the PC boards and the SC internal wiring. source. This is fast enough to compute the FFT on the audio channels without loss of samples. Phase control The frequency domain data output by the FFT is used to drive the Triacs using phase control. For each output channel, an 8-bit brightness level is computed from the frequency domain data, depending on the settings for its frequency band. An interrupt is used to detect the zero crossings of the mains waveform. When the zero crossing is detected, the microcontroller sets the triggering period for each of the four “output compare” (OC1-OC4) channels. When each period expires, the microcontroller emits a short pulse at each output compare pin. These OC pulses turn on their cor- Warning! All the parts in the red shaded area on the Main Board circuit diagram (Fig.2) operate at mains potential (ie, 240VAC) and contact with any of these parts could be FATAL. These parts include the PC tracks, the optocouplers (OPTO1-4), the Triacs, the 100m 100mH inductors, the 680W 680 W resistors, the 100nF 250VAC capacitors, screw terminal blocks CON4-CON6, the transformer primary and the wiring to the mains input and output sockets. DO NOT TOUCH any of these parts unless the power cord is unplugged from the mains supply. DO NOT CONNECT this device to the mains unless it is fully enclosed in the specified case. This project is not for the inexperienced. Do not build it unless you know exactly what you are doing and are completely familiar with mains wiring practices and construction techniques. responding Triacs until the next zero crossing of the mains waveform. The earlier in the half-cycle that each Triac is triggered, the greater the power delivered to its load. The effective power varies according to the area under the mains waveform from the trigger point in each half cycle. This relationship is not linear and so an internal correction factor is calculated by the microcontroller to make the channel responses linear. The microcontroller also adjusts the triggering periods of the output channels according to their quiescent current settings (ie, the user can set the “off” level for each output channel). Most lamps will respond better with a small current flowing through them at all times, so that they glow faintly in the “off” state. This will also increase lamp life. June 2008  39