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Ideal for use with Dolby 5.1-channel digital decoders, this 6-channel volume control will allow you to adjust the volume of all channels simultaneously. It also includes infrared remote control so that you can adjust the volume from your armchair. Pt.1: By JOHN CLARKE O K, SO YOU’VE JUST bought yourself a fancy new DVD player with inbuilt Dolby 5.1-channel decoding. A matching multi-channel amplifier would also be nice but what if your budget won’t stret­ch that far? For many people, the answer is quite simple – use existing hifi stereo amplifiers that they either already own or can scrounge for next to nothing. Most of us have progressively upgraded our hifi systems over the years and typically have old – but still working – stereo 56  Silicon Chip amplifiers stashed in a cupboard or in the garage. For example, you can use your main hifi stereo amplifier for the two front (left and right) channels, plus a second stereo amplifier for the two rear channels. A third stereo amplifier is then used to amplify the centre and subwoofer outputs from the DVD player – see Fig.1. By the way, you don’t need integrated stereo amplifiers with preamplifier and tone control stages in this application. If you have one or more stereo power amplifiers, they will do the job just fine. Adjusting the volume Using existing stereo amplifiers might be economical but there is one big disadvantage – you have to adjust three separate volume controls (more if an amplifier has separate volume con­trols for each channel). And that’s fiddly. Wouldn’t it be nice if you could adjust the volume of all six channels simultaneously using just one control? That’s just what this project does and as a bonus, it throws in infrared (IR) remote control as well. A low-cost commercial IR remote provides the necessary control signals. Basically, the SILICON CHIP 6-Channel Remote Volume Control ac­cepts all six channels from the DVD player and provides outputs that can be uniformally varied in level with excellent tracking accuracy. These outputs are then fed into the stereo amplifiers, which have their volume controls set to fixed positions. www.siliconchip.com.au As shown in the photos, the unit is housed in a 1U-high rack-size metal case. It’s easy to drive, with just a few controls on the front panel: an on/ off switch plus three pushbuttons for volume up, volume down and mute. A LED bargraph display lets you check the current volume setting. Another feature of the unit is its excellent performance, so that it doesn’t degrade the high-quality sound from your DVD player. It has extremely low noise and distortion and also provides accurate tracking between each channel. This means that the sound balance between the various chan­ n els is maintained regardless of the volume setting. In addition, there is virtually no coupling between channels so that the signal in one channel is not heard in another. Specifications The accompanying specifications panel lists the performance of the unit. In particular, the volume can be adjusted over a 76dB range, with 1dB steps from 0dB (maximum volume) through to -48dB. After that, the volume is adjusted in 2dB steps down to the -76dB attenuation level. The volume can also be Muted at any volume setting to provide an attenuation of at least -96dB. As stated earlier, the volume can be adjusted using either the front-panel pushbutton switches or by the matching buttons on an IR remote control. These controls allow adjustment from full volume to minimum volume in about seven seconds. Alternatively, you can use the Channel Up/Down buttons on the remote control for even Fig.1: this diagram shows how the 6-Channel Remote Volume Control fits into a surround sound system. It accepts the Dolby decoded outputs from a DVD player and in turn drives three stereo amplifiers. faster volume changes. The LED bargraph display consists of 20 LEDs, each representing a 4dB volume step. “In-between” settings are indicated by lighting two adjacent LEDs, which means that the display is actually capable of 2dB resolution – ie, it can in­dicate 38 different volume settings from 0dB to -76dB. Note that the LED bargraph does not indicate the 1dB volume steps that are available down to the -48dB attenuation level. These smaller 1dB changes are provided to give a fine volume adjustment, so that the exact level can be selected for this important range of the volume control. Below the -48dB level, the volume is quite “soft” and so the 2dB level steps are more than adequate. Muting is indicated by flashing the current volume level LED (or LEDs) on the display. It is released by pressing the Mute switch again or by pressing the Up Volume switch. However, the muting remains on if the Down Volume switch is pressed. This allows the volume to be reduced to a more comfortable level while the muting is still in effect. SPECIFICATIONS Total Harmonic Distortion (THD) ������������ 0.002% at 1kHz and 1V RMS (see graph) Frequency Response ����������������������������� -0.1dB at 20Hz and 20kHz Signal-to-Noise Ratio �������������������������������110dB with respect to 1V RMS with 20Hz to 20kHz filter (112dB A weighted) Separation Between Channels ��������������� 80dB at 20kHz, 94dB at 1kHz and 96dB at 100Hz worst case with 1kΩ loading resistor on measured channel and 1V RMS input on other channel Attenuation Steps ����������������������������������� 1db from 1-48dB attenuation, 2dB from 48-76dB attenuation Volume Tracking ������������������������������������� Typically better than ±0.7dB to -60dB, ±1.5dB >60dB attenuation Mute Attenuation ������������������������������������ 96dB minimum Signal Handling �������������������������������������� 4V RMS Display Resolution ���������������������������������� 2dB steps www.siliconchip.com.au March 2002  57 Parts List For 6-Channel Remote Volume Control 1 1U rack case (Altronics H-5035 or equiv.) 1 Universal remote control with Mute, Channel and Volume Up/Down buttons (Altronics A-1007, Jaycar AR-1703, DSE G-1223, etc) 1 2 x 12V 15-30VA mains transformer (Altronics M-4912 or equiv.) (T1) 1 SPST mains power switch (S4) 1 M205 mains panel-mount safety fuseholder (F1) 1 0.5A slow blow M205 fuse 1 7.5A mains lead with moulded plug including earth pin 1 mains cord grip grommet 2 M3 crimp lug eyelets 2 insulated spade connectors (female) 1 .001µF 250VAC X2 MKT polyester capacitor 1 10A 3-way mains terminal block (chassis mount) 1 piece of Elephantide insulation, 35 x 35mm 1 30mm length of 10mm-dia. heatshrink tubing 4 stick-on rubber feet 10 100mm long cable ties 4 8-way 0.1-inch pin header plugs 1 270mm length of 8-way rainbow cable 4 M3 tapped x 12mm spacers 4 M3 tapped x 10mm spacers 2 6mm untapped spacers 17 M3 x 6mm screws 3 M3 x 12mm screws 1 M3 x 10mm screw 5 M3 nuts 2 M3 star washers 6 6g x 10mm self tapping screws for RCA sockets 1 PC stake 6 PC-mount RCA sockets (Altronics P 0210) 1 600mm length of 0.7mm tinned copper wire AUDIO ATTENUATOR BOARD Semiconductors 1 PIC16F84 microcontroller programmed with REMVOL.HEX code (IC1) 1 PC board, code 01103021, 121 x 85mm 2 8-way 0.1-inch pin headers Another feature of the unit is that it remembers the cur­rent volume level when the power is switched off. It then au­tomatically reverts to this volume level when power is next applied. The muting is always initially off when the unit is switched on, even if the unit 58  Silicon Chip Semiconductors 2 LM1973N 3-channel audio attenuator (IC4,IC7) 4 TL072 dual JFET op amps (SGS Thomson or Motorola brands) (IC2,IC3,IC5,IC6) Capacitors 2 1000µF 16VW PC electrolytic 6 10µF bipolar (non-polarised) electrolytic 4 1µF 25VW PC electrolytic 6 1µF bipolar electrolytic or MKT polyester 4 0.1µF MKT polyester Resistors (0.25W, 1%) 6 10kΩ 6 150Ω 2 2.2kΩ 1 1.2kΩ DISPLAY BOARD 1 PC board, code 01103022, 231 x 56mm 1 8-way 0.1-inch pin header 1 TO-220 heatsink, 19 x 19 x 10mm 3 SPDT momentary subminiature pushbutton switches (S1-S3) (Al­tronics S 1498) 3 7.5mm diameter caps for switches 1 M3 x 6mm screw 1 M3 nut 1 18-pin DIL IC socket 1 4MHz crystal (X1) 1 250mm length of 0.7mm tinned copper wire was muted at switch-off. Commercial transmitter As mentioned earlier, this unit can be operated using a commercial IR remote control, which can also be used to control other equipment (eg, a TV, 1 infrared remote control demodulator IC (IRD1) 1 7805 5V 3-terminal regulator (REG1) 6 BC328 PNP transistors (Q1-Q6) 6 1N4148, 1N914 diodes (D6-D11) 4 5-segment green LED bargraph displays (Altronics Z 0972) (LEDs1-20) 1 3mm green LED (LED22) Capacitors 1 1000µF 16VW PC electrolytic 3 10µF 25VW PC electrolytic 1 0.1µF MKT polyester 2 18pF ceramic Resistors (0.25W, 1%) 1 100kΩ 1 470Ω 2 10kΩ 1 220Ω 4 1kΩ 5 120Ω 4 680Ω 1 100Ω 5W POWER SUPPLY BOARD 1 PC board, code 01103023, 71 x 45mm 1 8-way 0.1-inch pin header 4 PC stakes 1 25mm length of 0.7mm tinned copper wire Semiconductors 1 7812 regulator (REG1) 1 7912 regulator (REG2) 1 7806 regulator (REG3) 1 7906 regulator (REG4) 5 1N4004 diodes (D1-D5) Capacitors 2 1000µF 25VW PC electrolytic 2 470µF 16VW PC electrolytic 4 10µF 25VW PC electrolytic Resistors 2 39Ω 0.25W 1% 1 33Ω 5W VCR or satellite receiver). In fact, quite a few IR remote transmitters will work with the 6-Channel Remote Volume Control and some of these are in the parts list. At one end of the scale, you can use a TV-only remote con­trol unit with just www.siliconchip.com.au Fig.2: the block diagram for the 6-Channel IR Remote Volume Control. A microcontroller (IC1) decodes the signals from the switches and the IR receiver and provides a control signal for the audio attenuators (IC4 & IC7). It also drives the LED bargraph display. Volume Up/Down, Channel Up/Down, Mute and Operate controls. Alternatively, you can use a more elaborate unit which can also used to operate some of your other equipment. The main proviso with the IR remote control is that it is able to output Philips RC5 codes. There are three possible RC5 code sets that you can use, so if you already have a Philips TV set and/or VCR, you can select an unused code to avoid control­ling both devices at once. In that case, you will need to purchase a preprogrammed IR remote control that can be set to control satellite receivers (as well as TVs and VCRs). OK, enough of the preamble. Let’s find out how it works. fed to an audio attenuator block. This then atten­uates all six channels, according to the control signals applied via a 3-wire interface from the Infrared Decoder/Controller block (IC1). IC1 is actually a microcontroller. Its job is to decode signal inputs from the Down, Up & Mute switches and from Block diagram Take a look now at Fig.2 which is the block diagram of the 6-Channel Remote Volume Control. It’s really quite simple in concept, with just a few main circuit blocks (you didn’t really want anything too complicated and expensive, did you?). Here’s how it works. First, the six audio signals from the DVD player are www.siliconchip.com.au Fig.3: total harmonic distortion (THD) vs. frequency. As can be seen, the performance is excellent, with THD less than .02% up to around 10kHz. March 2002  59 Fig.4: the audio attenuator section is based on two LM1973 3-channel attenuator ICs. The attenuation level is adjusted according to a 3-wire digital control signal on pins 9-11 and op amps IC2b, IC3a & IC3b buffer the attenuated outputs. the infrared receiver (IRD1) which picks up signals from the remote control. It then generates the 3-wire control signals (Data, Clock & Load) which are applied to the Audio Attenuator. In addition, the microcontroller also drives the 20-LED bargraph display. Finally, the signals from the Audio Attenuator are buffered by op amps IC2, IC3, IC5 & IC6 and fed to the output sockets. These outputs in turn are fed to the audio amplifiers. Circuit details Figs.4-6 show the complete circuit details of the 6-Channel Remote Volume Control. We’ll look at each of 60  Silicon Chip these sections in turn. Fig.4 is the audio attenuator section. For the sake of clarity, only channels 4-6 are shown – channels 1-3 are identical except for the IC numbering which is shown in brackets. IC4 (and IC7) are “3-Channel 76dB Audio Attenuators with Mute” (from National Semiconductor). Each channel includes a resistive array with various tappings to select the requisite amount of signal attenuation. The input impedance is a nominal 40kΩ, while the output impedance varies from 25kΩ to 35kΩ, de­pending on the attenuation level. As indicated previously, the atten- uation level is adjusted using a 3-wire serial digital control signal. The audio signals from the DVD player are fed to IC4 via 1µF bipolar capacitors. These serve two purposes: they roll off the low-frequency response below 4Hz (in company with the input impedance); and they prevents any DC current flow in the resis­tors that make up the attenuators. The outputs from the attenuators appear at pins 6, 20 & 16 (OUT1, OUT2 & OUT3) and are fed to JFET op amp stages IC2b, IC3a & IC3b. These op amps are wired as unity-gain buffer stages and have several functions: (1) they provide low-impedance output signals which are suitable for driving power amplifier stages; (2) their high input impedance prevents loading of www.siliconchip.com.au Fig.5: the control display section is based on microcontroller IC1. This processes the inputs from the switches and IR receiver and provides the 3-wire control signal for the attenuator ICs. It also drives the LED displays. the attenua­tors, thus maintaining the accuracy of the volume setting; and (3) the negligible bias currents flowing in the op amp inputs prevent unwanted clicks in the audio signals as the output im­pedance of the attenuators changes with volume Note that the output of each op amp buffer is isolated from its output terminal using a 150Ω resistor, to prevent instabili­ ty. In addition, the outputs are AC-coupled via 10µF bipolar capacitors to prevent any DC offsets from being fed through to the power amplifiers. The 10kΩ resistors tying the op amp outputs to ground are included to ensure that the outputs discharge to ground when power is removed from the circuit. This prevents switch on clicks when power is reapplied. Note that IC2 and IC5 are actually TL072 dual op amps even though we are using only one op amp in each package. They are specified because the dual-package versions produce a lower switch-on click than TL071 single op amps. For the same reason, it is necessary to use either Motorola or SGS Thomson (ST) TL072s in preference to those from other manufacturers, as these produce the lowest output transients at switch on. Each op amp package is powered from ±12V supply rails. These rails are decoupled at each op amp’s supply pins using a 1µF capacitor, while two 1000µF 16VW capacitors provide overall decoupling of the supply. These relatively large capacitors ensure that the op amp supply rails decay away at a slow rate when power is removed from the circuit, to prevent switch-off thump. The two audio attenuator ICs (IC4 & IC7) are powered from ±6V rails which are decoupled using 0.1µF capacitors. Their Clock, Load and Data inputs are at pins 9, 10 & 11 and these are controlled by microcontroller IC1 (see Fig.5) to set the attenua­tion values. Note that each of the three channels in each IC is individually addressable and could theoretically be loaded with any attenuation value. www.siliconchip.com.au March 2002  61 REG3 7806 +17V FUSE F1 0.5A SLOW POWER S4 T1 M 4912 30VA A 12V IN D1-4: 1N4004 D1 D4 D5 1N4004 IN 12V D2 D3 1000F 25VW +6V 470F 16VW REG1 7812 .001F 250VAC OR 275VAC N OUT GND 39 OUT GND +12V 10F 25VW 10F 25VW 0V 1000F 25VW E 10F 25VW 330 5W (CASE) IN GND OUT 39 REG2 7912 REG1, REG3 GND IN OUT REG2, REG4 GND IN 10F 25VW OUT -12V 470F 16VW IN GND OUT -6V REG4 7906 SC 2002 6-CHANNEL REMOTE VOLUME CONTROL Fig.6: the power supply circuit uses four 3-terminal regulators to deliver ±6V and ±12V rails for the audio circuitry. The +17V rail feeds a 5V 3-terminal regulator in the display section. In this case, however, both IC4 and IC7 have these three control lines connected in parallel. As a result, the attenuation value set for channel 6 is also set in channel 3. Similarly, channels 5 and 2 have the same attenuation value, as do channels 4 and 1. In addition, the software programmed into the microcon­troller sets all channels to the same value. Control & display circuit This circuit section is based mainly on a PIC16F84 micro­controller (IC1) – see Fig.5. It primary function is to decode the signals from switches S1-S3 and from the infrared receiver (IRD1) and provide the 3-wire control signals to IC4 (and IC7). It also drives the 20-LED bargraph display in multiplex fashion. This enables the LEDs to be driven via five common lines at the RB0-RB4 outputs of IC1. The RB5-RB7 outputs and transistors Q1-Q5 select which bank of five LEDs will be driven at any one time. For example, when RB7 is low, Q5 turns on and the anodes of LEDs 16-20 are all high. As a result, any low lines at RB0-RB4 drive the correspond­ing LEDs in this group via series 120Ω 62  Silicon Chip resistors (eg, if RB0 goes low, LED16 will light). When RB7 subsequently goes high, RB6 goes low and selects LEDs 11-15 via Q4. RB6 then goes high and RB5 goes low to select LEDs 6-10 via Q3. When ever one of the RB5-RB7 lines is low, transistor Q1’s base is pulled low via a 10kΩ resistor and an OR gate made up of diodes D6-D8. This turns Q1 on which means that Q2 is off (as are LEDs1-5). However, when RB5-RB7 all go high, Q1 turns off and Q2 turns on (due to its 680Ω base resistor) and pulls the anodes of LEDs1-5 high. These LEDs can now be driven by the RB0-RB4 outputs of IC1, as before. So this simple gating technique allows us to drive four banks of LEDs using only three outputs (RB5-RB7) from IC1. When ever one of the RB5-RB7 lines is low, the RA0 input is monitored to check whether a switch has been pressed. A closed switch causes the normally high RA0 input to be pulled low via either D9, D10 or D11 when an RB5-RB7 line goes low. For example, if S1 (Volume Down) is pressed, RA0 is pulled low via D11 when RB5 goes low. Similarly, if S2 POWER SUPPLY (Volume Up) is pressed, RA0 is pulled low via D10 and RB6. And if S3 (Mute) is pressed, RA0 is pulled low via D9 when RB7 goes low. By this means, the software in IC1 detects which switch has been pressed and generates the appropriate control signals at the RA2-RA4 outputs which are then fed to IC4 & IC7 Note that the 3-wire control outputs are fed to IC4 & IC7 via 1kΩ resistors. These slow down the control signal rise times so that they don’t cause noise in the audio signal. Note also that RA4 of IC1 is an open drain output and requires a pullup resis­tor. This output is high at power up and this ensures that the attenuators are initially set to minimum volume, after which the software quickly takes over and sets the volume to the required level. IR control The infrared receiver circuitry is simplicity itself and is based on infrared receiver module IRD1. This 3-lead device ampli­fies, filters and demodulates the coded signals picked up from the IR transmitter and feeds the demodulated signal to the RA1 input of IC1. It also drives Q6 which flashes the Acknowledge LED (LED21) each time pin 1 of IRD1 goes low. As a result, the Acknowledge LED www.siliconchip.com.au flashes (to indicate that infrared signals are being picked up) each time you press a button on the transmitter. IC1 decodes the signals applied to its RA1 input and, pro­vided the coding is correct, changes its outputs accordingly. The default code is for the TV1 address but this can easily be changed so that you can use either the SATellite 1 (SAT1) or SATellite 2 (SAT2) code (eg, if you are already using TV1 to control your TV set). A 4MHz crystal connected between pins 15 & 16 provides the timing for IC1. The two 18pF capacitors ensure that the crystal is correctly loaded, so that it starts reliably. Power for the microcontroller is supplied via 5V regula­tor REG5. This is isolated from the incoming +17V supply using a 100Ω 5W resistor and decoupled at its input and output using 10µF electrolytic capacitors. In case you’re wondering, the 100Ω resistor causes the 5V supply to rise relatively slowly at power up, to ensure a “soft” start. It also dissipates power which would otherwise have to be dissipated by REG5 (which, in turn, would require a bigger heat­sink). In addition, a 1000µF capacitor is used to decouple the +5V supply line at IRD1. This prevents any switching noise on the +5V supply from being amplified within the infrared receiver. The 220Ω resistor connected across the 5V supply ensures that the voltage falls to zero at switch off. This is neces- Fig.7: install the parts on the audio attenuator (signal) board as shown here, taking care to ensure that the ICs and electrolytic capacitors are correctly oriented. The bipolar capacitors can go in either way around. sary to ensure that IC1 resets correctly when power is reapplied. Power supply circuit Fig.6 shows the power supply circuit. It uses a 24V centre- tapped mains transformer, the output of which is rectified using diodes D1-D4 and filtered by two 1000µF capacitors to produce nominal ±17V rails. These rails are then fed to regulators REG1-REG4 to derive ±12V and ±6V supply rails. Note that REG1 and REG3 are isolated from the +17V supply using diode D5. This ensures that the regulated positive supply rails fall at the same rate as the negative rails when power is switched off. In addition, the outputs of REG3 & REG4 are decoupled using 470µF capacitors rather than 10µF capacitors, as used at the outputs of REG1 & REG2. This ensures that the ±6V supply rails fall slower than the ±12V supply rails at switch off. And that in turn ensures that the inputs to the op amps are tied to ground via the attenuator output resistances while ever power is applied to the op amps. The +17V rail pro- Left: this is the completed audio attenuator board. Note the orientation of the two pin header sockets. www.siliconchip.com.au March 2002  63 Left: another view of the completed audio attenuator board. Make sure that the RCA sockets are seated correctly before soldering their leads. Fig.8 (below): be sure to place the 3-terminal regulators (REG1REG4) correctly when building the Power Supply Board. vides power to the display circuit and this load is balanced by including a 33Ω load resistor across the -17V supply. This is done to produce similar decay times for the ±17V rails at switch off. REG1 39 10F D2 1000F D4 330 5W Note that the +12V and -12V outputs from REG1 & REG2 are decoupled with 39Ω resistors before being applied to the signal circuitry. These resistors slow down the risetime for the op amp supply rails at power up. 470F 10F 10F 470F REG4 0V 32030110 0V 12V 1000F D1 REG2 D3 12V REG3 39 D5 +12V q12V +6V q6V +17V GND GND GND 10F This is the completed Power Supply Board. Mount the 5W resistor slightly proud of the board to aid cooling. FROM POWER TRANSFORMER 6-CHANNEL VOLUME POWER Basically, the power supply has been designed to deliver its various supply rails without causing excessive voltage excur­sions in the op amp outputs during power up and power down. As a result, switch-on thumps Table 1: Resistor Colour Codes  No.   1   8   2   1   4   4   1   1   6   5   2 64  Silicon Chip Value 100kΩ 10kΩ 2.2kΩ 1.2kΩ 1kΩ 680Ω 470Ω 220Ω 150Ω 120Ω 39Ω 4-Band Code (1%) brown black yellow brown brown black orange brown red red red brown brown red red brown brown black red brown blue grey brown brown yellow violet brown brown red red brown brown brown green brown brown brown red brown brown orange white black brown 5-Band Code (1%) brown black black orange brown brown black black red brown red red black brown brown brown red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown red red black black brown brown green black black brown brown red black black brown orange white black gold brown www.siliconchip.com.au and switch-off instability problems are avoided. Power on/off switching is provided by mains switch S4, with a 0.5A fuse protecting the transformer primary. The .001µF ca­ p acitor across S4’s contacts prevents arcing at switch off, again to avoid sudden transient voltage excursions at the op amp out­puts. Construction The unit is easy to build, with separate PC boards for each of the circuits shown in Figs.2-4. These boards are as follows: (1) an audio attenuator board coded 01103021; (2) a display board coded 01103022; and (3) a power supply board coded 01103023. Figs.7-9 show the assembly details for the PC boards. As shown in the photo, the PC boards are interconnected using 8-way flat cables that plug into pin headers. Signal board assembly Begin the assembly by building the audio attenuator board – see Fig.7. Install the wire links first, then the resistors, ICs and capacitors. Make sure that the ICs and the electrolytic capaci­tors are all oriented correctly. The bipolar (BP) capacitors are non-polarised and can be installed either way around. Table 1 shows the resistor colour codes but we suggest that you also check each value using a digital multimeter as some of the colours can be difficult to decipher. The audio attenuator board can now be completed by installing the two 8-way pin headers and the RCA sockets. Note that it will be necessary to cut off the plastic locating clips at the base of each RCA socket pair before installing it. You can do this using a pair of sidecutters. Push the RCA sockets all the way down onto the board and make sure they are properly seated before soldering their leads. Display board assembly Fig.9 shows the parts layout on the Control & Display board. Again, Table 2: Capacitor Codes      Value IEC Code EIA Code 1µF   105   1u 0.1µF   104   100n 0.001µF 102   1n 18pF   18  18p www.siliconchip.com.au begin by installing the wire links, then install the resistors, diodes and capacitors. Note that the 100Ω 5W resistor should be mounted about 3mm proud of the PC board, to allow the air to circulate underneath it for cooling. The six transistors (all BC338 or BC337) can go in next, followed by the 4MHz crystal (X1). That done, you can install the three pushbutton switches (S1-S3). Next, install an 18-pin DIL socket for IC1 but don’t in­stall IC1 (the PIC microcontroller) at this stage. That step comes later. The 7805 regulator (REG5) can go in now. This device is mounted horizontally which means that you have to bend its leads down by 90° before mounting it on the PC board. This is best done by slipping an M3 screw through the device tab, positioning it on the board and then gripping one of the leads with a pair of needle-nose pliers just before it reach­ es its mounting hole. The device can then be lifted clear of the board and the lead bent down at right angles, after which the procedure can be repeated for the two remaining leads. REG5 can now be fitted to a small mini-U heatsink and the assembly bolted to the PC board using a 6mm M3 screw, nut and star washer. Don’t forget to solder its leads after bolting it down. The 5-segment LED bargraph displays have a plastic moulding that has a raised section at one end and a recessed section at the other. This allows them to be locked together to form one continuous bargraph. Before mounting the displays, first orient each segment so that the anode leads (the longer of the two for each LED) are towards the right. This done, bend the leads down at right angles about 5mm from the plastic body, so that the front faces of the LEDs will sit about 4mm from the edge of the PC board. Now mount the LED segments so that they sit about 2.5mm proud of the board surface. A 2.5mm-thick “standoff” (made from a strips of cardboard, for example) will make this job easy – just sand­wich the standoff between the LED segment and the PC board, solder the two outer leads, then remove the standoff and solder the remaining leads. LED21 is mounted similarly by bending its leads at right angles and Fig.9: the parts layout for the control and display section. Refer to the text when installing the LED displays. March 2002  65 This view shows the assembled Control and Display Board, ready for installation in the case. The four 5-way LED bargraph segments lock together to form a continuous display. installing it so that it lines up with the bargraph LEDs. Similarly, the infrared receiver (IRD1) is mounted by first bending its leads down at right angles close to its body, and then down at right angles again before soldering it to the PC board. The front of this device should be about 1mm out from the front edge of the PC board when it is installed. The Display Board can now be completed by installing IC1 in its socket. Make sure that it is correctly oriented. Power supply board The LED bargraph segments must each be mounted about 2.5mm proud of the PC board and this can easily be done using some folded cardboard to act as a spacer. This board is assembled as shown in Fig.8. The main thing to watch out for here is that you use the correct 3-terminal regulator at each location and that the regulators are correctly oriented – their metal tabs all go towards the 1000µF electrolytic capaci­tors. Note too that diode D5 faces in the opposite direction to diodes D1-D4. The 33Ω 5W resistor should be mounted about 3mm proud of the PC board to aid cooling. That’s all for now. We’ll complete the construction and SC give the test procedure in next month’s issue. The completed modules are installed in a 1U rack chassis and interconnected using two cables fitted with pin headers (details next month). 66  Silicon Chip www.siliconchip.com.au