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By ROSS TESTER T This little lighting gimmick was used at a recent school eisteddfod. It uses a 12V 20W halogen lamp/reflec­tor fitting mounted in a plastic drink bottle filled with orange/red cellophane. It gives a convincing imitation of fire, hence the name “Flickering Flame”. Why not set your next performance on fire? 60  Silicon Chip HEATRICAL PRODUCTIONS often call for flaming torches and similar lighting effects. The problem is that for fairly obvious safety reasons most theatres and halls have very strict rules regarding the use of naked flames on stage. In fact, in most halls, naked flames are taboo. We had just this situation recently when our local high school needed some props for its Rock Eisteddfod act. One scene was a castle wall lit by flaming torches – except they couldn’t have flames! Also they (and the Rock Eisteddfod itself) had a couple of other curly requirements which made life just that much harder! For example, mains power was not an option because the props on which the flaming torches were placed were to be moved to various positions on the stage. And the lights themselves needed to be completely portable because the props were stored offstage and assembled in a rush! One other request was that they had to cost as close to nothing as possible and be reasonably sized – not too big to get in the way but conversely not too small either. And they had to look realistic! For those not familiar with the Rock Eisteddfod, perhaps a word or two of explanation is in order. The challenge is open to all secondary schools and entails an act of dance and drama set to contemporary recorded music. The act itself can be no more than eight minutes and most importantly, any set or props used must be capable of being brought onto stage and set in less than four minutes (and con­versely, removed in the same time). There are strict limits on the amount of money a school can spend and there are also limits on the number of LEFT: it mightn’t look too spectacular up close and as a still photo but from an audience viewpoint it looks just like the real thing when it is working. backstage crew allowed to assist the “perform­ers”. Those who have never seen a Rock Eisteddfod performance before marvel at the spectacle, the professionalism, the choreog­raphy, the costumes and the props. Ah, the props. This is where we came in, with a request to help out with those flaming torches (no pun intended!). The props people at the school had come up with the basic design for the torch (and as you will see, it’s amazing what you can get away with from a distance!). What they wanted was something to make them flicker. “Easy”, we thought. First of all we looked at a real flick­ering flame (a candle, to be precise). Effectively, the “light” was on all the time but every now and then it dimmed a bit as the flame was caught by a breeze. All we needed to do was emulate that. A microprocessor could easily be programmed to do the job nicely. “Whooaa! Too expensive”, they said. OK then, how about a 555 timer configured as an astable multivibrator driving a cheap power Mosfet? You couldn’t get much cheaper than that! The problem with that idea was that while it certainly flickered the lights, it was far too regular: looked more like a flamin’ lighthouse than a flaming torch! What about two 555 timers running at different, unrelated frequencies? Would this give the random effect we wanted? We tried this idea and . . . sure would! Calculating values gave us roughly the right oscillation rates, trial and error gave us the effect we wanted. Fig.1 shows the final circuit. The duty cycle (on time to off time) is set by the ratios of R1 to R2 and the oscillation rate is set by R1 and R2 in conjunction with C1. The duty cycles were set very high – around 10:1. Any longer than this and the lights actually went right off – not very realistic at all! The oscillation rate was set at about 1Hz or longer. The values of R1 & R2 are significantly higher than one might “normally” expect to be used in 555 circuits. The reason for this is that high values of resistance allow low values of capacitance. A high value resistor costs the same as a low value resistor while a high value capacitor costs significantly more than a low value unit. We used the same values for R1 The 20W halogen lamp can be directly soldered to the PC board as shown here, or connected via flying leads. The component at top right of the PC board is a pair of header pins with a shorting link – this formed our on/off switch. Fig.1: the circuit employs a 556 dual timer to drive a Mosfet. The two oscillators run at different rates to give a random flickering effect from the halogen lamp. & R2 in the two oscillator circuits but picked different capacitor values to ensure that the operating frequencies were not too close together. While on the subject of costs, we looked at the lolly shop catalogs and found that a 556 (two 555s in one package) was a few cents cheaper than a pair of 555s, so we went this route. The outputs of both astable oscillators are fed to a diode “OR” gate and these feed the gate of the Mosfet. In effect, the difference between the two oscillators is fed to the Mosfet gate. What happens is that when either of the oscillator outputs goes low, one of the diodes is forward biased, taking the Mosfet’s gate low. When the gate is taken high, which will be most of the time, the Mosfet is turned hard on and is a very low resistance. Therefore, the lamp lights at full brilliance. When the Mosfet gate is pulled low by either of the oscil­lator outputs, the Mosfet turns off, turning off the lamp. But because of the very short “off” time and the thermal inertia of the lamp filament, it doesn’t actually turn off but flickers. So we have random flickering of the lamp, which is just what we want. Viewed up close, it doesn’t look all that impressive. From more than a few metres away though, the effect is quite convinc­ing – there’s fire in that thing! In the end, we made quite of few of these torches, varying the timing capacitors in each to ensure they never flickered “in sync”. Making the torch You would be surprised at how much you can get away with in designing props! Looking at the photograph, you’ll see that our torch appears exactly what it is: crumpled cellophane in a plas­tic drink bottle, fitted to a length of cardboard tube. But to the audience, it looks just like October 1997  61 Fig.2: the halogen lamp was soldered directly to the PC board but it would be easier if you used a standard halogen lamp socket base. a flaming torch! We cut the bottom off a PET 1.25l Coke bottle with a sharp serrated knife. Did we forget to mention that we drank the cont­ents first? Next, we cut some thin strips, about 15-20mm wide, of red cellophane and laid these down the inside of the bottle. About five or six strips seemed to work best but you can experi­ment for the desired effect. Then we crumpled a sheet of orange cellophane and placed this inside the red strips. Presto, a torch! By the way, PET stands for Polyethylene Terephthalate, which is the long-winded moniker for polyester. Now you know. The other end of the bottle was removed to suit the lamps used. We mentioned safety before as this was a major concern. The lamp we used was especially chosen for the job: a 12V 50mm dia­ meter halogen type with dichroic reflector, normally used in low voltage downlights or shop display lights. However, we used a specific type. Most 12V downlights have a 50 watt rating; we used a 20 watt type to keep heat to a minimum. And to ensure that the hot halogen lamp would not be able to ignite the cellophane (we’ve seen that happen before!) we chose a lamp with an integral clear glass cover. Should these lamps not be available at your normal shop Jaycar stores have them (Cat No SL-2732). They are coded “BAB” on the reflector, indicating that they have a beam width of 38 degrees which is pretty well optimum for this job. Their price is right, too at $4.95 each (we were quoted $15 each at a 62  Silicon Chip Fig.4: this is the full-size etching pattern for the PC board. lighting store!) Incidentally, Jaycar have a similar 12V/20W lamp just 35mm in diameter if space is a problem. They also have bases to suit these lamps but at $2.95 each we decided to forego these to keep cost to a minimum and solder directly to the lamp pins. In retrospect, that may not have been such a brilliant idea. The reasons will follow shortly . . . The lamps were fitted to the neck end of the bottle, shin­ing upwards through the cellophane. To do this, we cut the neck off, leaving an opening about 25mm in diameter. What you want to achieve is an opening not too large for the lamp to slip through. We used contact adhesive to hold the lamp in place. Yes, the lamp does get rather hot; in fact, enough to distort the PET bottle but we were able to operate our torches for half an hour or more without any problems. Allow the adhesive to dry completely. While this is happen­ing, you can fashion the mounting hardware. We used 100mm card­board tube long enough to take the battery pack (see below) and the lamp itself. Eight slots were cut in the tube, about 40mm down from the top, which allowed the lamp to be simply pushed into place. It is quite important that the tube be made deep enough to ensure that no white light can be seen by the audience – this ruins the effect completely. We said before that our idea was to solder the pins of the lamp directly to the PC board. This is not quite as simple as it seems the pins simply did not want to solder! They’re probably nickel plated or similar which is certainly not designed for ease of soldering. Eventually, after much scraping of the pins and with a very hot iron we were able to make a soldered joint but this was definitely the weakest link in the chain. If you can afford to invest another couple of dollars in the bases, they would make life much easier. The circuit is designed to operate from 12V DC. The elec­tronics drain is negligible but the lamp itself draws the best part of 2A (ie, 20W/12V). We used a battery pack made of six 2.5A SLA rechargeable cells (mainly ‘cos the school had these on hand), giving an operating time of more than an hour. As the whole Rock Eisteddfod performance was over in eight minutes, this was more than enough. If you need longer times, you will need a suitably larger battery. Assembling the PC board This view shows the 20W halogen lamp and its integral reflector. We designed a PC board to suit the project. It is coded 11410971 and measures 58 x 38mm. This is quite straightforward to assemble as the only polarised parts are the IC, Parts List 1 PC board, code 11410971, 58 x 38mm 2 PC Stakes 1 SPST switch (optional) 1 12V 20W sealed halogen reflector lamp (see text) 12V battery capable of supplying 2A Semiconductors 1 556 dual timer 1 MTP3055E power Mosfet 2 1N914 diodes Fig.3: this diagram shows the overall scheme. The battery and PC board/lamp assembly is mounted in a cardboard tube with the lamp illuminating a PET soft drink bottle filled with crumpled cel­lophane. Mosfet and diodes. Make sure you get these right, along with the battery connections Therefore, construction is very simple – apart from solder­ing the lamp, as mentioned above. We made provision for an on/off switch on the board or you could simply connect and disconnect the battery as required. Before you solder in the Mosfet and the lamp, you can check the operation of your circuit with an analog multi-meter. Apply 12V and check the voltage at pins 5 & 9 of the IC using a multimeter (set to measure 12V DC). At both pins, you should see the meter dipping regularly – but not down to zero volts, unless you have chosen a much higher value resistor for R2. When you measure at the junction of the two diode anodes, you should see the meter dipping more or less randomly, indicat­ ing that the two outputs are being added, or rather Capacitors 1 0.1µF MKT polyester or ceramic 1 .039µF MKT polyester or ceramic 2 .01µF MKT polyester or ceramic Resistors (0.25W, 1%) 2 10MΩ 1 100kΩ 2 1MΩ subtracted, through the diodes. With the 20W lamp suggested, a heatsink for the FET is not really necessary. It does get reasonably warm to touch but should be quite happy at this level. Do not use a 50W lamp. There is no doubt that it will melt the PET bottle and it could well set fire to the SC cellophane. October 1997  63