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Build a rope light for party fun & frolic You’ve seen those rope lights at discos and in shop displays. Now you can build your own with some plastic tubing, a bunch of lights and a simple driving circuit. Design by ROBERT RIEDE Rope Lights are quite intriguing to look at but essentially they are just another form of light chaser. This one is based on 12V lamps which are driven by SCRs (silicon controlled rectifi­ ers). The circuit has two refinements though. As well as have a variable speed it has a built-in electret microphone to provide triggering from the beat of the music – each beat of the 22  Silicon Chip drums is seen to move the rope lights on by one step. As is usual with most light chasers, the circuit of this Rope Light is fairly simple, although it does have a few inter­esting twists (no pun intended). For example, it uses a programmable unijunction transistor, a device rarely seen these days, and as already men­ tioned, it uses SCRs instead of tran- sistors to drive the low voltage lights. Have a look at the circuit of Fig.1. The core of the cir­cuit is the 4017 decade counter. It is clocked by transistor Q3 and four of its outputs are used to control lamps. Its fifth output, DO4, is used to drive its reset line. Each of the four outputs of IC1 drives the gate of an SCR so that while ever an output is high, its respective SCR will be turned on to drive its lamps. The lamps are not supplied from pure DC because if they were, the SCRs would be unable to turn off. Instead, the lamps are fed raw DC from the bridge rectifier (diodes D1-D4) and the 12VAC plugpack transformer. The beauty of this arrangement is that the SCRs are relatively cheap and it avoids the need for expensive electrolytic filter capacitors. The SCRs are also ideally suited for turning incandescent lamps on and off. The specified C106s have a rating of 4A RMS and a whopping peak repetitive surge current rating of 75A. This makes the C106 far more rugged than any equivalent 4A transistor and it easily handles the repetitive surges of the incandescent lamps. It also means that the circuit has no need of such niceties as filament preheating. Instead of using a 555 or other pulse generator IC to pro­vide the clock source for IC1, this circuit uses a programmable unijunction transistor or PUT. Essentially, this can be regarded as an “anode gate SCR”; it turns on whenever the anode voltage is higher than the gate. The PUT is wired as a relaxation oscillator which produces very brief positive pulses at its gate at a rate determined by potentiometer VR2, resistor R11 and capacitor C8. The beauty of the PUT oscillator compared with, say, a 4093 Schmitt trigger oscillator, is that its frequency is highly predictable. This is not really an issue in this application but it means that the PUT is still a valid approach. Each time the PUT produces a pulse at its cathode it turns on transistor Q3 and this drives the clock input of IC1. So why have the two transistors and other circuitry which appears to control the PUT? The answer is that this part of the circuit provides beat synchronisation of the lights, via the electret microphone. The electret microphone is biased from the DC supply via the 3.3kΩ resistor R1 and its signal is coupled to the base of Q1. Q1 and Q2 operate as simple common-emitter amplifiers with no feedback. Q2 has a gain of about 20 (ie, 10kΩ/470Ω) while Q1’s gain is adjustable up to a maximum figure of 20. This only ap­ plies to low frequencies (bass) since the high frequency gain is severely curtailed by the .068µF capacitors, C4 & C5. The result­ant bass signal at the collector of Q2 swings high and low, pulling the gate of PUT1 with it. When the audio signal swings high, there is no effect on PUT1 but when the gate of PUT1 is pulled low, its anode is liable to be higher than the gate and so it turns on to clock IC1 on by another step. This process means that the clocking of IC1 is effectively synchronised to the bass beat of the music. Construction There are two aspects of the construction for this project: the assembly of the controller and wiring up the “rope” in its plastic tube. We’ll deal with the controller first. It uses a PC board measuring 53 x 82mm and is housed in a plastic utility box measuring 129 x 68 x 42mm. Before inserting any components, check the Fig.1: this circuit is essentially a chaser. Four out­puts from IC1 are cycled continuously and drive four SCRs. Each SCR drives a bank of incandescent lamps from rectified but unfil­tered DC. The PUT provides the clock oscillator for IC1 and is synchronised to the bass beat of the music by the electret micro­phone and amplifier stages Q1 & Q2. PUT clock generator June 1996  23 Fig.2: follow this diagram when building the PC board and take care with component polarity. No heatsinks are required for the four SCRs. board for any defects such as undrilled holes or breaks and shorts between tracks. If any are found they should be fixed before proceeding further. Then start by inserting and soldering the small compon­ ents such as resistors and diodes. Then insert the capacitors and transistors, making sure that the semiconductors and electrolytic capacitors are installed the right way around. Finally, install the IC and the four SCRs. No heatsinks are required for the latter components. You will need to drill three holes in the lid of the case, one for the electret microphone insert and one each for the sensitivity and rate controls, VR1 & VR2. You will also need to drill one hole in each end of the case, to take the power input and output cables. There is no need to run shielded cables to the electret microphone or to the poten­tiometers VR1 & VR2 – ordinary hook-up wire will suffice. Our prototype had the electret fixed to the lid of the case with a blob of epoxy adhesive – a fairly crude but perma­nent approach. Checking the board The Rope Light consists of a length of plastic tubing with a lamp wired into the loom at intervals of about every 30cm or so. Once the wiring is complete, you will want to check the circuit operation with just four lamps connected. To do this, wire up one side of a miniature 12V lamp to each of the SCR outputs. The other side of each lamp then connects to the common line from the board; this actually connects to the +V unfiltered DC line. Now connect a 12V plugpack and switch on. Check with your multimeter for the presence of +5.6V across ZD1 and at pin 16 of IC1. The lamps should be switching on and off at a rate which is variable by VR2. Try tapping the lid of the case with a pencil or your finger nail. Each time you do so, a lamp should switch off and another should switch on. If all these checks are OK then the board is functioning correctly. Rope light assembly Fig.3: actual size artwork for the front panel. 24  Silicon Chip There are several ways of approaching the assembly of the rope light but regardless of how you do it, there will be a number of common aspects. You need a length of 12mm OD clear plastic tubing, say 6-7 metres. You will need at least 5-8 times that length of hookup wire and you will need 60 or more miniature incandescent lamps, in at least four colours. Kit Availability Kits for the Rope Light described in this article are available from Oatley Electronics who own the design copyright. The pricing details are as follows: PC board with all on board components...........................................$24.00 Two pots with knobs............................................................................$5.00 Case to suit board...............................................................................$4.00 16VAC 1.5A plugpack.......................................................................$25.00 7 metre assembled Rope Light.........................................................$40.00 60 miniature coloured lamps.............................................................$12.00 Postage & packing..............................................................................$6.00 For further information on pricing and availability, contact Oatley Electronics, PO Box 89, Oatley NSW 2223. Phone (02) 579 4985 or fax (02) 570 7910. PARTS LIST 1 PC board, 53 x 82mm (from Oatley electronics) 1 plastic utility case, 129 x 68 x 42mm 1 12VAC 1.5A plugpack transformer 6 metres 12mm OD clear plastic tubing Miniature 12V coloured incandescent lamps (see text) 1 6-way Molex plug 1 6-way Molex socket 2 knobs 1 10kΩ linear potentiometer (VR1) 1 2.2MΩ linear potentiometer (VR2) Semiconductors 1 4017 decade counter (IC1) 2 BC548 NPN transistors (Q1,Q3) 1 BC558 PNP transistor (Q2) 1 2N6028 programmable unijunction transistor (PUT1) 4 C106D1 silicon controlled rectifiers (SCR1-4) 5 GIG silicon rectifier diodes (D1-D5) 1 5.6V 400mW zener diode (ZD1) 1 electret microphone Capacitors 6 100µF 25VW electrolytic 1 0.47µF monolithic 7 .068µF ceramic Resistors (0.25W, 5%) 1 220kΩ 7 3.3kΩ 1 150kΩ 5 470Ω 1 56kΩ 1 100Ω 4 10kΩ Miscellaneous Hook-up wire, cable ties, solder, plastic sleeving. Inside the box, showing details of the PC board and its wiring. The controller has only two knobs, one for the rate at which the lamps switch on and the other a sensitivity control for the inbuilt electret microphone. Since there are five outputs from the controller PC board, you might think that five wires inside rope light cable would be adequate but that depends on the cross-section of the hook-up wire and the current rating of the lamps. If you use very light duty hook-up wire, (ie, 10 or 13 strands of 0.12mm) it should be cap­able of carrying about 500mA on a continuous basis. That means you could use one hook-up wire for each output, up to a maximum lamp load of say 1A, on the basis that the duty cycle is 25%; ie, each lamp is on for 25% of the time. However, since the common cable carries current for 100% of the time, you would need to run two or three cables together, so you would have a maximum of six or seven wires in the rope. All these can be wired up to a 6-way Molex socket. This then mates to a 6-way cable and plug from the controller. If you want to double the length of the rope light, the far end of the cable can terminate in a Molex plug which can then mate up to a further length of rope light. However, if you do this, you will need to use heavier duty hook-up wire or double up on the light duty hook-up wires. On the other hand, if you don’t fancy making your own rope light cables, you can buy them ready-made from SC Oatley Electron­ics. June 1996  25