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Everyone has seen high intensity strobe lights at par­ties, discos and nightclubs. The “stop motion” effect of each light flash makes dancers appear to move in a strange way. With this high-powered strobe the light can be made to flash in time with the music or at any speed between one and 20 times a second. By JOHN CLARKE For flashing lights – even synchronised to the music Build This Beat Triggered Strobe What is a strobe light anyway? It provides repetitive in­tense flashes of white light and is based on an Xenon gas dis­charge tube. These are the same sort of tube as used in camera flashes but instead of being flashed just once, as in a camera, they are flashed continuously. These days Xenon flash tubes are 54  Silicon Chip widely used in burglar alarms, shop displays, on police cars and so on and in these applications they usually flash at quite slow rates. In our Beat Triggered Strobe, the flash rate can be varied from slow to fast or it can be synchronised to the beat of the music. Our Beat Triggered Strobe uses two Xenon tubes and is housed in a wood- en box measuring 240 x 240 x 300mm. It is covered in black speaker carpet which looks good and prevents the box from being easily damaged. For the same reason, loudspeaker corner protectors are fitted. The strobe tubes and their spun aluminium reflector are mounted at one end of the box and are protected Fig.1: block diagram of the Beat-triggered Strobe. by a sheet of Perspex. The control panel is mounted at the other end of the box and is recessed to protect the controls from damage. On the control panel are two knobs, a jack socket for remote on/ off control, a pair of RCA sockets for the music sign­al, the power switch and the IEC mains socket which is the same as found on computers these days. The two knobs provide a sensitivity control for the music (beat) input and a flash rate control. There is also a small toggle switch to select either beat (music) or continuous flash operation (internal oscillator). above 200Hz; ie, only bass signals pass through. The low frequency signal is then fed to a peak detector which drives a Schmitt trigger and pulse generator for the beat triggered mode. Alternatively, when switch S2 selects the oscillator mode, the Schmitt trigger oscillates at a rate set by VR2. Again, the Schmitt trigger drives the pulse generator. The output of the pulse generator drives the optical isola­tor which fires a trigger circuit involving a Triac and Main Features •  High intensity flash •  Adjustable flash rate from 1 to 20 per second (internal oscilla­tor) •  Flash rate synchronised to music beat •  Remote on/off switching •  Rugged construction Xenon flash tubes We’ve already mentioned the Xenon flash tubes which are the heart of this project. A Xenon flash tube is a light source for producing a high intensity flash from the electrical energy stored in a capacitor. It comprises a U-shaped glass tube which is filled with a small amount of Xenon gas. It has metal electrodes at each end of the tube and a trigger electrode which wraps around the outside of the glass. A high voltage from a capacitor is applied to the outer electrodes and when a very high (4kV) voltage is applied to the trigger electrode, the tube fires by ionising the gas which then emits a burst of light. The duration of the light flash depends on the size of the capacitors and any stray inductance in the circuit and is normally just a few microseconds. Block diagram Fig.1 is the block diagram for the Beat Triggered Strobe. The left and right signals from a tape deck or CD player are mixed to produce a mono signal which is fed to VR1. From there the signal goes to an amplifier and a low pass filter which rolls off signals The two Xenon tubes are mounted at the focus of the spun alumin­ium reflector which is mounted behind a Perspex window to keep unwary fingers away from the high voltage. August 1998  55 WARNING! CIRCUITRY INSIDE DOTTED LINES OPERATES AT LETHAL VOLTAGE – SEE WARNING PANEL Fig.2: two Xenon tubes are used in this strobe lamp circuit. Note the remote control circuit which is grounded to the 0V line, while the 555 (IC2) is powered from the -9V line. Note also that the circuitry to the right of the MOC3021 (IC3) operates at lethal voltage. pulse transformer (T2). T2 produces a 4kV pulse to fire the two Xenon flash tubes. Diodes D4-D7 rectify the 240VAC mains supply to provide about 330V DC across the storage capacitors. The ±9V supply for the ICs is derived from mains transformer T1, diodes D8-D11 and two 470µF filter capacitors. Circuit description The circuit for the Beat Triggered 56  Silicon Chip Strobe is shown in Fig.2. It comprises one quad op amp (IC1), a 555 timer (IC2) and an optically coupled Triac driver (IC3). The two Xenon tubes each have two 6.5µF capacitors connected in parallel to give a high flash output over the full range of operation. The left and right audio inputs are mixed in inverting amplifier IC1a. The 47kΩ resistors and 0.22µF capacitors produce a low frequency rolloff for signals below 15Hz while the .015µF capacitor across the 47kΩ feedback resistor rolls off high fre­ quencies above 225Hz. The Beat Sensitivity control VR1 sets the level of signal fed to op amp IC1b which has a gain of 471 and a low frequency rolloff at 16Hz, as set by the 1kΩ resistor and the 10µF capaci­tor between the inverting input at pin 6 and ground. High fre­quency rolloff is again at 225Hz, as set by the .0015µF capacitor across the 470kΩ feedback resistor. IC1b is followed by a low pass filter comprising IC1c and associated resistors and capacitors. It is a 2-pole filter Warning 1 Flashing lights can initiate convulsions in people with epilepsy. They can also cause people to suffer nausea and head­aches. It is advisable to use the strobe for short periods only and it should be switched off if it is apparent that someone is suffering from the above effects. and rolls off the signal above 200Hz at 12dB/octave. This filter and the previous filtering on IC1a and IC1b ensure that signals above 200Hz are severely attenuated. The signal from IC1c charges a 1µF capacitor via diode D1. The result is that each bass beat in the music produces a posi­tive DC pulse across the 1µF capacitor following diode D1. Fig.3: these scope waveforms show how the Schmitt trigger (IC1d) responds to a burst of low frequency. The top trace is the audio waveform at the output of the low pass filter (pin 1, IC1c) while the lower trace is the output of the Schmitt trigger (pin 14, IC1d). Schmitt trigger modes IC1d is connected as a Schmitt trigger with positive feed­back applied via a 220kΩ resistor to the non-inverting input at pin 12. The 220kΩ resistor between pin 14 and 12 plus the 100kΩ resistor to +9V and the 47kΩ resistor to ground set the hystere­sis. If the input at pin 13 exceeds +4V then the Schmitt trigger output goes low and conversely, if the input voltage goes below +2.5V then the output goes high. If switch S2 is in position 1, each beat signal from diode D1 causes the output of IC1d to briefly go low. The scope waveforms of Fig.3 show the beat mode in action. The upper trace shows a burst of low frequency from the output of the low pass filter (pin 1 of IC1c), while the lower trace shows the resultant pulse output from the Schmitt trigger (pin 14 of IC1d). On the other hand, if switch S2 is in position 2, then the filtered signal from D1 is out of circuit and the oscillator components comprising VR2, the 10kΩ resistor and a 10µF capacitor are connected to the inverting input of IC1d. The 10µF capacitor is then charged and discharged via VR2 and the 10kΩ resistor from the Schmitt trigger output. It charges to the +3.6V upper threshold and discharges to the lower threshold of +2.3V. Fig.4: these scope waveforms show how the Schmitt trigger con­trols the monostable (IC2). The top trace is the output at pin 14 of IC1d while the lower trace is the monostable pulse (11ms) at pin 3 of IC2. Potentiometer VR2 sets the frequency of oscillation. It is wired as a variable resistor and when its resistance is low, the frequency is high and vice versa. Monostable pulse generator IC2 is a 555 timer wired as a mono­ stable pulse generator and while it may look fairly standard, there are some tricky aspects to it. First, while IC1 operates from the ±9V rails and its output can swing over almost the full supply range (actually about +7.5V to -7.5V), the 555 is only operated from the negative supply rail, ie; between 0V and -9V. So the 0V line is actually the positive supply rail for IC2. We’ve August 1998  57 used this supply arrangement for a particular reason which we’ll come to in a moment. Each time the output of IC1d goes low (to about -7.5V), it momentarily pulls pin 2 of IC2 low via diode D2 and the .01µF capacitor. The pin 3 output of IC2 then goes high and the 0.1µF capacitor on pins 6 & 7 charges up via the 100kΩ resistor. When the voltage reaches the trigger level of pin 6 (about -3V), the pin 3 output goes low. Thus an 11ms pulse is produced at pin 3 each time the output of IC1d goes low. The scope waveforms of Fig.4 show the monostable operation. The upper trace is the output of the Schmitt trigger while the lower trace shows the short duration (11ms) positive-going pulse from the monostable, pin 3 of IC2. On/off control Pin 4 is the reset input for IC2 and is normally tied high with the 10kΩ resistor to pin 8. When transistor Q1 is switched on it pulls pin 4 low to prevent pin 3 going high and so strobe flashing is stopped. Q1 is switched on by connecting its 2.2kΩ base resistor to the 0V line and this point is earthed to the metal chassis. Now this is the whole point of the unconventional supply arrangement for IC2. We wanted to use a grounded 6.5mm jack socket for the remote on/ off control and we wanted to use a cable which could be simply shorted at the end with a switch to stop strobe operation. Hence, when the 2.2kΩ resistor is connected to 0V via the jack socket, its plug and remote cable, Q1 turns on, pulls pin 4 low and IC2 is disabled. Q. E. F. or quod erat faciendum which is Latin for “which was to be done”. The 10kΩ resistor from base to emitter of Q1 prevents the transistor switching on when long lines are connected to this remote control input. Diode D2 prevents any voltage from IC1d’s output which is above ground from passing to pin 2 of IC2. Note that another reason for the unusual supply for IC2 is that it could not take a total supply of 18V (recommended maximum is 15V). Fig.5: this diagram shows the dimensions (in milli­metres) of the timber cabinet (made of MDF) and the general arrangement of the chassis bracket. 58  Silicon Chip High voltage optocoupler Pin 3 of IC2 drives IC3 via a 470Ω resistor. IC3 is an optically coupled Triac driver which incorporates an LED which triggers an internal Triac. This then triggers Triac1. Now why have we used a MOC3021 optocoupled Triac in a trigger circuit which only handles DC, not AC? We specified IC3 to get a device which provides a very high isolation between its input and output. The MOC3021 Triac driver is one of the few optocouplers which is safe to use for 240VAC mains operation and it has an isolation voltage rating of 7.5kV. Other common optocouplers such as the 4N28 only have an isolation voltage rating of 500V which is inadequate for this application. Diodes D4-D7 rectify the mains voltage and the 0.1µF ca­pacitor in series with the primary of trigger transformer T1 charges up to about 330VDC via the two series 270kΩ resistors. Also the 6.5µF capacitors connected across the Anode and Cathode connections of the Xenon tubes are charged via the two 470Ω 5W resistors. When IC3 is triggered by IC2, the internal Triac conducts and the Triac1 is triggered via the Neon and the series 680Ω resistor. The charged 0.1µF capacitor is effectively connected across the primary winding of pulse transformer T2 and a high voltage is induced into its secondary winding. This secondary winding is connected to the trigger winding on the Xenon tubes, and causes them to “fire” and conduct the charge from the 6.5µF capacitors. That Neon tube in series with Triac1 is an odd inclusion and one which you would not expect to find in a semiconductor circuit. Interestingly, it is there to stop the Triac from con­ ducting when it shouldn’t. Why? When the Triac is triggered on, it will dump a fairly large current from the 0.1µF capacitor into the pulse transformer. But the capacitor will not discharge completely because it is still being fed about 600µA from the series 270kΩ resistors. 600µA may not seem like a big current but it is well above the “holding current” of 250µA for the Triac in IC3. Hence, without the Neon tube, once the Triac was triggered into conduction, it would never turn off. But with the Neon in place, once the voltage across the 0.1µF capacitor has dropped below about 70-90V, the Neon goes open circuit and stops the current flow. Neat, huh? Power for the low voltage side of the Parts List 1 PC board, code 16305981, 173 x 85mm 1 panel label, 140 x 140mm 1 warning panel label, 57 x 27mm 1 spun aluminium reflector, 190mm diameter 1 clear Perspex reflector cover, 190 x 190 x 2.5mm 1 sheet of 1.6mm aluminium, 290 x 210mm 1 sheet of Medium Density Fibre board (MDF), 900 x 600 x 12mm 1 400mm length of 12 x 12mm DAR timber 1 1500 x 400mm sheet of 3mm thick speaker carpet 1 strap handle 8 speaker box corner protectors 1 IEC chassis mount socket with fuse holder 1 200mA 2AG fuse (F1) 1 3-pin mains plug to IEC female plug mains lead 1 DPST mains switch with Neon indicator (S1) 1 SPDT toggle switch (S2) 1 octal socket 1 octal plug 2 panel-mount insulated RCA sockets 1 PC board mount mono (or stereo) 6.35mm socket 2 100kΩ linear pots (16mm) (VR1, VR2) 2 16mm OD knobs 1 M2851 12.6V 150mA mains transformer 1 Xenon tube trigger transformer (T2; Altronics Cat M-0104 or equivalent) 2 Xenon tubes (see text) 1 Neon tube 4 15mm tapped spacers (use 9mm spacers if 25mm pots used for VR1&VR2) 1 50g tube of contact adhesive 12 M3 screws 6mm long 2 M3 x 9mm countersunk screws (to mount IEC socket) 7 M3 nuts 15 M3 star washers 3 No.6 x 6mm self tapping screws 9 5G 16mm round-head wood screws (to secure aluminium rear panel and reflector) 2 6G 20mm round-head wood screws (to secure timber rear panel) 32 4G 12mm countersunk wood screws (to secure corner protectors) 2 7G 16mm countersunk wood screws (to secure handle) 4 solder lugs (or crimp eyelets) 24 PC stakes 1 1500mm length of blue mains rated wire 1 1500mm length of brown mains rated wire 1 600mm length of green/yellow mains rated wire 1 100mm length of 0.8mm diameter tinned copper wire 1 100mm length of three way rainbow cable Semiconductors 1 LM324 quad op amp (IC1) 1 555 timer (IC2) 1 MOC3021 Triac optocoupler (IC3) 3 1N914, 1N4148 signal diodes (D1-D3) 8 1N4007 1000V 1A diodes (D4-D7,D8-D11) 1 BT136 500V Triac (TRIAC1) 1 BC338 NPN transistor (Q1) Capacitors 2 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 4 6.5µF 250VAC stud-mounting capacitors 1 1µF 16VW PC electrolytic 2 0.22µF MKT polyester 1 0.1µF 250VAC MKT polyester X2 class 1 0.1µF MKT polyester 1 .022µF MKT polyester 1 .015µF MKT polyester 1 .01µF MKT polyester 1 .0056 MKT polyester 1 .0015 MKT polyester Resistors 1 470kΩ 2 270kΩ 1 220kΩ 6 100kΩ 7 47kΩ 1 22kΩ 3 10kΩ 1 2.2kΩ 1 1kΩ 1 680Ω 4 470Ω 5W 1 470Ω 2 10Ω Miscellaneous Heatshrink tubing, PVA glue, nails. August 1998  59 WARNING! ALL PARTS TO THE RIGHT OF THE DOTTED LINE OPERATE AT LETHAL VOLTAGE Fig.6: this is the component layout for the PC board. The Triac’s metal tab should be fitted with a piece of heatshrink tubing to avoid accidental contact. circuit is derived from the 12.6V centre tapped transformer T1 via diodes D8-D11 and the 470µF capacitors. These provide nominal +9V and -9V supply rails. Construction The Beat Triggered Strobe is housed in a box measuring 240mm wide, 240mm high and 300mm deep. It is made of medium density fibreboard (MDF) and is covered with black Meltrim® or similar speaker car­pet. Black corner protectors and a plastic handle add to the pro­fessional appearance of the prototype. Most of the circuit components are mounted onto a PC board which measures 173 x 85mm and is coded 16305981. This board is mounted on an L-shaped bracket measuring 150 x 140 x 210mm. This wide bracket forms the rear control panel of the Strobe. The details of the box and the L-shape bracket are shown in Fig.5. The first step in assembly is to insert and solder all components into the PC board. Its component layout diagram is shown in Fig.6. Note that the PC board is effectively divided into low voltage and high voltage sections with IC3, the opto60  Silicon Chip Warning 2 The high voltage parts of this circuit are directly pow­ e red from the 240VAC mains and are potentially lethal. THE 6.5µF 250VAC CAPACITORS & THE TERMINALS OF THE OCTAL SOCKET & XENON TUBES ARE PARTICULARLY DANGEROUS! Note that lethal voltages are present at one end of the PC board. This circuitry includes IC3, the 5W resistors, the 6.5µF storage capacitors, trigger transformer T2, diodes D4-D7, the Triac (TRIAC1), the neon and all associated parts. Do not touch any part of the circuit while it is operating and always give the 6.5µF capacitors sufficient time to discharge after switching off before working on the circuit – see text. We recommend that only experienced constructors should tackle this project. coupler, being the interface between the two sections. The first step in board assembly is to insert and solder the PC stakes at the external wiring connection points. Then insert the wire links and resistors. Table 2 shows the colour codes for all the specified resistor values. Mount the 5W wirewound resistors so that they have about a 2-3mm clearance above the PC board to aid in their cooling. When inserting the diodes, take care with their orientation. Although lower voltage types could have been used for D8-D11, we have specified 1N4007 types for all eight power diodes. This is to prevent placing incorrect types in the D4-D7 positions. Install the ICs and the transistor next, taking care to orient them as shown. Note that IC1 is oriented differently to IC2. The capacitors can be installed next. Table 1 shows the codes for all the specified capacitor values. Take care with the polarity (orientation) of the electro­lytics. Triac1 can be mounted next, with the metal tab facing to­wards potent­ iometer VR2. Potentiometers VR1 and VR2 are mounted directly onto the PC board as shown in Fig.6. If 16mm pots are used, then the 6.35mm jack socket can directly mount on the PC board, as all the bush mounting holes are in-line. Fig.7: wiring details of the chassis. With the exception of the wires to the RCA phono sockets and switch S2, all the wiring should be rated for 250VAC. Use cable ties to lace the high-voltage wiring, as shown in the photographs. WARNING! ALL PARTS TO THE RIGHT OF THE DOTTED LINE OPERATE AT LETHAL VOLTAGES August 1998  61 The chassis bracket slides out of the rear of the cabinet to reveal a neat layout. Note that a section of the PC board and a lot of wiring is powered di­rectly from the 240VAC mains supply and is potentially lethal, particularly the four 6.5µF storage capacitors which are charged to about 330V DC. However, if 25mm pots are used, the jack socket will need to be raised off the PC board with PC stakes so that its mounting bush is in line with the pot bushes. T2, the Trigger transformer, is wound as an auto-transform­ er and must be mounted as shown on the PC 62  Silicon Chip diagram. The Neon tube is soldered directly into the board and can be supported with a dab of Silastic or Blu-Tak. Secure the tapped pillars to the four corners of the PC board using the 6mm long M3 screws. Note that you will need to use tapped 15mm spacers with 16mm pots and 9mm spacers with 25mm pots. Chassis bracket If you are not assembling a kit with all parts supplied, the next step is to make the chassis bracket. This is made from a sheet of 1.6mm aluminium Fig.8: this is the full-size etching pattern for the PC board. Check the board carefully before installing any of the parts. measuring 290 x 210mm. This is bent to form a right-angle bracket, with one section measuring 140 x 210mm and this becomes the rear panel. Table 1: Capacitor Codes ❑ Value ❑ 0.22µF ❑ 0.1µF ❑ .022µF ❑ .015µF ❑ .01µF ❑ .0056µF ❑ .0015µF IEC 220n 100n 22n 15n 10n 5n6 1n5 EIA 224 104 223 153 103 562 152 Strobe and the program source. Now position the PC board in place and mark out the holes for the standoff pillars on the base of the chassis. Also mark out and drill the mounting holes for the four 6.5µF capacitors, the power transformer (T1) and the earthing screw. Affix the label to the panel and cut out the holes with a sharp utility knife. Now attach the PC board in place with four M3 x 6mm screws. Secure the IEC socket with countersunk M3 screws and attach the power transformer using two M3 screws. Use shakeproof washers for each screw. The earthing solder lugs must each be held with a screw, nut and a shake­ proof washer. The RCA sockets are attached with the insulating bushes in position. Secure the pots and 6.35mm You will need to mark out the positions for the pots VR1 and VR2, the 6.35mm jack socket and for switch S2 using the panel label as a guide. Make sure the height of these components is correct by checking the PC board on its standoff pillars up against the inside of the panel. Drill and cut out holes for the power switch (S1) and the fused IEC power socket. The earth screw is positioned just below the IEC socket. Now drill out the holes for the two RCA sockets. The holes must be large enough to allow for their insulating bushes. These isolate the metal body of the RCA sockets from the chassis bracket, to prevent ground loops between the Table 2: Resistor Colour Codes ❑ No. ❑  1 ❑  2 ❑  1 ❑  6 ❑  7 ❑  1 ❑  3 ❑  1 ❑  1 ❑  1 ❑  1 ❑  2 Value 470kΩ 270kΩ 220kΩ 100kΩ 47kΩ 22kΩ 10kΩ 2.2kΩ 1kΩ 680Ω 470Ω 10Ω 4-Band Code (1%) yellow violet yellow brown red violet yellow brown red red yellow brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown red red red brown brown black red brown blue grey brown brown yellow violet brown brown brown black black brown 5-Band Code (1%) yellow violet black orange brown red violet black orange brown red red black orange brown brown black black orange brown yellow violet black red brown red red black red brown brown black black red brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown brown black black gold brown August 1998  63 types used for power factor correction in fluorescent batten fittings and as motor-run capacitors. And as with most fluorescent battens these days they use wire-capture terminations. You poke the wires into the socket holes and they are “captured”. You will need to strip the capacitor wires back by about 10mm and then “tin” them with solder before they are inserted into the capture terminals. Once the wires are captured, you cannot pull them out again. Once the wiring is finished, you should check your work very carefully to be sure that all parts and wiring are correctly positioned. Initial voltage check The rear panel has controls for audio sensitivity and flash rate. The strobe can be used in continual flash mode or beat triggered mode. Use cable ties to keep the wiring neat and tidy, as shown here. socket with the nuts provided for each. Mount switch S2 using its nuts and locking washer. The Xenon flash tubes are mounted in the spun aluminium reflector via an octal socket. The reflector is supplied with a hole in its base and this is big enough to take the octal socket. You will have to drill holes in the base for the socket’s mount­ing screws and an earth lug. We drilled the holes to accept 6G self-tappers. Wiring details Now you can do the wiring of the chassis – see Fig.7. All the wiring, with the exception of the connections from the RCA phono sockets and to switch S2, should use 240VAC mains-rated wire. We used 260mm lengths of wire from the PC board to the octal socket and a 260mm length 64  Silicon Chip of green/yellow striped wire from the earth point to the reflector lug. Place insulating sleeving over all exposed PC pins and octal pins to prevent any possibility of accidental contact. Also place a length of insulating sleeving over the Triac’s metal tab to prevent accidental contact. The wiring to the four 6.5µF 250VAC capacitors requires special mention. These are standard stud-mounting WARNING! Lethal voltages are present on all parts at one end of the PC board & on the 6.5uF 250VAC capacitors, octal socket & Xenon tube terminals. Capacitors retain lethal voltage for some time after switch off. Fig.9: this warning label should be affixed to the metal chassis, adjacent to the power transformer. Initial testing of the Strobe can be done without having the Xenon flash tubes fitted. Firstly, be aware that the circui­try at one end of the PC board, involving IC3, the 5W resistors, the storage capacitors, diodes D4-D7, trigger transformer T2, the Triac and other associated parts, is all running at 240VAC and is potentially lethal. Set your multimeter to read DC volts and connect the common lead to the blue wire connection on transformer T1. Apply power and check for about +9V on pin 4 of IC1. There should be -9V on pin 11 of IC1 and pin 1 of IC2. Now switch S2 to the oscillator position and check that the Neon flashes at the rate set by the oscillator pot, VR2. If so, then the circuit is probably all working. Now switch off the power and wait for several minutes. Carefully measure for high voltages between the Anode and Cathode leads on the octal socket. The 6.5µF capacitors specified do have internal bleeder resistors to discharge them but you need to wait several minutes for safety reasons to be sure they are discharged. Mounting the tubes The Xenon tubes can be mounted in one of two ways. If you are using the Xenon tubes from Dick Smith Electronics or Jaycar Electronics, their extra long leads will be sufficient for them to be directly soldered into the pins of the octal socket. You should have 25mm clearance between the base of the octal socket and the base of the tubes. We covered the exposed leads with a short length of tubing 25mm in outside diameter. This was obtained from a 35mm film ca­nister. Alternatively, you could use the tubing from a “METEOR Party popper”. The tubes from Altronics have shorter leads and require extra spacing to ensure that they are correctly positioned to be at the focus of the aluminium reflector. In this case use an octal plug to solder the tubes into and then insert this plug into the socket. Note that this plug is a larger diameter than the socket and will need to be inserted from the reflector side after the socket has been mounted. Before soldering the tubes in place, make sure that the red marking is placed in the Anode position. If you place the tubes incorrectly, it is probably best to change the wiring to suit rather than try to unsolder the tube wires since the glass is easily cracked. Woodwork The box is made from 12mm MDF as shown in Fig.5. We used simple woodworking tools to make this box, however we did resort to a power jigsaw to cut out the hole for the reflector. Cut out two sheets 300 x 240mm and two sheets 300 x 216mm. These form the sides and top of the box. Cut out another piece 210 x 79mm for the rear panel. Also cut out a 216mm square piece and mark out a 180mm hole central to the square with a pair of compasses. Cut out with a jigsaw or a small fret saw. The hole can be chamfered with a half-round wood rasp. The box can now be assembled using PVA glue and some nails or screws. Note that the front panel is recessed by 10mm. This is so that the speaker carpet can be folded around the front of the box. When the glue is dry (wait six hours), file and sand the box and round off the sharp corners on each edge. The rear aluminium panel is recessed in the box by 25mm. We used 12 x 12mm cleats to provide the mounting arrangement for the chassis panel. Make sure that the chassis can slide into the box with 12mm clearance beneath it to allow for the capacitor-mounting studs. Additional 12mm cleats need to be glued in position for the 210 x 79mm MDF plate which mounts at the top of the rear panel. Note that this panel is recessed by 3mm around the top and sides to allow for the speaker carpet thickness. Isolate exposed leads of Xenon tubes using a plastic 35mm film canister (or similar) – see photo on page 62. One method of mounting the Xenon tubes (from Altronics) using an octal plug and socket. This sets the tubes at the focus of the parabolic aluminium reflector. The octal socket is fastened to the parabolic reflector using two self-tapping screws. Note the earth lead which runs from the chassis to a lug which is bolted to the parabolic reflector. The chassis panel is secured to the cleats with 5G 16mm round-head screws, while the MDF plate is secured with the 6G 20mm round-head screws. The reflector and Perspex window are secured with four equally spaced 5G 16mm round-head screws around its circumference. August 1998  65 We spray painted the front and rear sections of the box with black satin enamel. It is not necessary to paint the base, top and sides of the case. Attaching the carpet The speaker carpet is attached to the box with contact adhesive. We started by coating half of the base with contact adhesive and securing one edge of the carpet on the base. This leaves the carpet join along the middle of the base. Be sure to leave sufficient carpet overhang on each end so that it can be wrapped around the front and rear of the box. Now coat the rest of the box with contact adhesive and secure the carpet in position. Note that contact adhesive works best if you coat both surfaces and wait for it to dry before sticking down. Also, if you are using a 50g tube, apply the glue sparingly or you will run out. The bottom edge is trimmed so that it meets the first carpet edge for a neat join. The ends are com­pleted by cutting the carpet to length and folding around the edges. These are secured with contact adhesive as before. Note that the corners will need Fig.10: this is the artwork for the control panel, reproduced here half-size. to be trimmed so that the carpet will fold in without puckering. Plastic corner protectors are secured with 4G 12mm counter­sunk screws while the handle is secured to the top of the box using two 7G 16mm screws. Now you are ready to attach the Xenon tubes and octal sockets into the reflector and secure the earth. Then slide the chassis assembly into place and secure with four 6G 16mm SC round-head screws. Protect Your Valuable Issues Silicon Chip Binders REAL VALUE AT ★  Heavy board covers with 2-tone green vinyl covering $12.95 PLUS P &P ★  Each binder holds up to 14 issues ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p each (Aust. only). Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 66  Silicon Chip