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B 'Illf. PARTY LI Would you like some coloured flashing lights at your next party but con 't afford a full light show? Here's how you con modify a 12V-powered coloured strobe light to get a similar effect. By STEPHEN DAVID Modern rock groups certainly do make effective use of flashing light to create a good atmosphere for their performances. You can go part of the way to achieving the same effect by having flashing lights at your next party. There's no need to spend a lot of money on strobe lights though. You PARTS LIST 1 coloured strobe light (Arista WL3 or equivalent) 1 1 2V DC plugpack (battery eliminator) 1 1 MO pot, 16mm diameter 1 180k0 0.25W resistor 1 0.33µF 1 OOVW metallised polyester capacitor can modify a standard strobe very easily. The specific model we are talking about is an Arista product (Arista Cat. No. WL3) and sells for about $35. It is widely available from Arista outlets and has a fixed flash rate of about 120 flashes per minute. It runs from 12VDC, battery or mains plugpack, and draws about 150 milliamps. As it stands it can be used as a warning beacon on cars or boats, for shop displays, or as an exterior indicator for a burglar alarm. It has a screw-on lens cap which is available in red, orange or blue. And since it is intended for outdoor use, it is weatherproof. When we had a look at one of these units recently we thought it could be an attractive unit to ac- Modifying this commercial strobe unit to give a variable flash rate is simply a matter of adding a pot and two other components. company the music at a party. But the fixed flash rate would be a drawback; it would have to be variable. As it happens, that is easy. We'll show you how to install a variable flash control on the strobe. Then you can set it up next to your stereo system at a party, set the flash rate to match the music beat and dance away. How it works Before we tell you how to do the mods, let's have a look at the unit in its original condition. Pulling it apart is easy. Just unscrew the coloured lens cap and you'll see two small screws which retain the flat reflector underneath the Xenon discharge tube. Undo these two +o------------. 220 16VW 12V + 47 on w 01 2SC10 1N4002 D1 0.33 10 16VW o--11-ff-t-------+--l_ll+_....__ _ _ _ _ _ _ _ _...__~T._RI-GG_.ER_ _ ___._ TRANSFORMER PARTY FLASH __,:Jt MODIFICATION FOR ADJUSTABLE FLASH RATE Fig.1: the circuit of the Party Flash uses a ringing choke inverter to drive a Xenon flash tube. The parts to be added are on the righthand side of the diagram. 72 SILICON CHIP screws, lift of the reflector and a layer of insulation and you'll find the printed circuit board. The pattern side is uppermost, the components underneath. The full circuit is drawn out in Fig.1. It is a masterpiece of minimalist circuit design with only six active components, including the Xenon tube. Diode Dt is there to prevent the supply being connected up with the wrong polarity. So if you connect the supply the wrong way around nothing happens and nothing is damaged. The 220µF capacitor is there as a filter and reservoir to supply the DC to DC inverter. This uses transistor Qt and transformer Tl. Qt and TI comprise a "ringing choke" inverter which works in the following way. When power is first applied, base current flows via winding W2 and the 4700 resistor. This causes Qt to conduct heavily via Wt, so that the base end of W2 is swung up to + 24V. Then, when the current flow through Wt reaches its maximum value, the transformer action stops and the voltage across Wt suddenly collapses, turning off the base current to Ql. This turns off Qt and so the voltage across the windings is suddenly reversed. The process then repeats itself as conduction occurs via W2 and the 4700 resistor, Qt turns on, and so on. This all happens extremely rapidly, at several thousand cycles per second, which you can hear as a high pitched whistle. The voltage swing across winding Wt is stepped up in winding W3 and then rectified by diodes D2 and D3 to give about 400 volts DC across the 2µF 630V capacitor. This is the high voltage supply which powers the Xenon discharge tube. Then we have another oscillator which is based on the neon tube. This is a "relaxation" oscillator which works because the neon is an open circuit for low voltages but when the voltage across it rises to a threshold value of about 90 volts or so, it suddenly breaks down to become a low resistance. The oscillator works as follows. The .033µF 200V capacitor is charged from the 400V supply via The 1MO pot is mounted in the base of the strobe unit near one of the mounting pillars (see text). Be careful of the 2J.LF capacitor - it could give you a very nasty shock if you touch its leads. TRACK SIDE DF PCB STROBE CASE \ 0 FLASH TUBE Fig.2: this diagram shows how to modify the circuit board. You need to remove two 8.2MO resistors and a .033J.LF capacitor. Substitute a 180k0 resistor and a 0.33J.LF capacitor as shown. the two series 8.2MO resistors. When the voltage across the neon reaches the neon's threshold value, the neon breaks down and discharges the .033µF capacitor via the primary of the trigger transformer. The secondary of the trigger transformer then applies a high voltage pulse to the electrode wrapped around the Xenon tube, causing it to fire and discharge the 2µF capacitor. At the same time, the tOµF capacitor delivers a positive pulse to the emitter of Qt so that the DCDC inverter is stopped. If it wasn't stopped, it would continue to deliver current via W3, Dt and D2 and the Xenon tube would never stop conduction. When the Xenon tube stops conduction, the Tt inverter recharges the 2µF capacitor and the two 8.2MO resistors then begin the recharge the .033µF capacitor until the neon breaks down again. This gives the overall flash rate of about two per second. Modifications To make the flash rate variable, we alter the resistor charging path to the capacitor across the neon. continued on page 93 MAY1988 73 Ltd, Suites 3 & 4, 67 Scott Street, Liverpool, NSW 2170. Phone (02) 82 1 1622. Engraving tool from Tandy Need to label your hand tools to identify them? Perhaps you want to label your VCR and audio equipment in line with the recommendations of your local Neighbourhood Watch scheme. This little ba ttery powered engraving tool is just the ticket for these jobs. It takes two AA cells (preferably alkaline for longer cell life) and has a hardened serrated steel tip which is rotated at high speed to do the work. Pressing a button on the side of the housing turns on the motor. You just hold it like a pen. It works well although for best results you need a light touch; pressing too heavily will just stall the tool. With a little practice you can print or write. It costs just $5 .29 (not including the cells) and is available from all Tandy stores. TDK "limited edition" audio cassette New logic analyser uses 100-LED arrays Logic analysers are usually dedicated oscilloscopes which may have six, eight or more traces for displaying logic pulse waveforms. As such they are fine for design work in the laboratory but are often too cumbersome for production or service work in the field or workshop. To meet the need for a compact logic analyser with visible displays, the LogicBridge was developed. This is a dedicated instrument with four 100-LED arrays which give the ability to display two logic waveforms. The effective real time pulse bandwidth of the LogicBridge is 10MHz although its inbuilt glitch Cheapie motors for toys and models Jaycar have two economy motors available which are good value if you're involved in robotics, model cars or model trains. One type runs from 6V at 2400 RPM (35mm long with 38mm diameter) while the other runs from 3V (nominal) at 5100 RPM. Jaycar must have bucketloads of these because they're flogging them off at a dollar each. At this price it's worth snaffling half a dozen just in case. You never know when they might come in handy. Flashing party light TDK has released a new version of its popular SA formulation tape which is housed in a more rigid cassette. It also comes in a new stylised case with rounded corners to make it easy to carry in your pocket. The new cassette is only available in a 90 minute version. Called the TDK SA-LTD, the new tape has a very low bias noise of - 63dB. Recommended retail price is $8.60. catcher captures pulses down to -50 nanoseconds. The unit also has sufficient memory to allow it store up to 100 traces of 100 bits each, for later display and analysis. For further information on the LogicBridge, contact Emona Instruments, 86 Parramatta Road, Camperdown, NSW 2050. Phone (02) 519 3933. continued from page 73 and we alter the value of that capacitor. We used a 0.33µF capacitor and a 180k0 resistor in series with a lMO pot. The pot allows the flash rate to be varied from about two per second up to about 13 per second. We u::ied a 16mm diameter pot which fits easily into the lower part of the strobe casing. The two 8.2MO resistors and the 0.033µF capacitor need to be removed from the board. Install the 180k0 resistor and the 0 .33µF capacitor in their place, as shown in the wiring diagram of Fig.2. The lMO pot is wired to the PCB with two short lengths of hookup wire (you could use a twostrand length of rainbow cable instead). We also suggest you wire a 12V DC plugpack permanently to the strobe light which will make the whole unit self contained. The plugpack should have rating of 300mA or more, to cope with the higher cur rent dr ain when the flash rate is increased. ~ MAY 1988 93