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By THOMAS SCARBOROUGH Want a flashy piece of jewellery for the love of your life? Then build the “JAZZY HEART”, an eye-catching LED flasher in the shape of a heart. It could be worn as a brooch or as a pendant. H ERE’S A PIECE OF JEWELLERY that you can be sure is unique. Go to a party and you will know that nobody else will be wearing something like the Jazzy Heart. It’s a heart-shaped LED flasher using two ICs and eight different-coloured high brightness LEDs. It is powered by a 12V miniature battery and turned on and off by a mercury switch. The Jazzy Heart randomly flashes eight LEDs using just two common CMOS ICs. These LEDs are arranged around the perimeter of a red plastic “heart” template (or for Christmas, a green plastic “Christmas tree” template) to pulse eight water-clear LEDs. A special feature of the design is that all colour LEDs (red, green, blue, etc) may be used in all eight positions provided on the PC board while using just a single current-limiting resistor. The Jazzy Heart really is jazzy. It needs to be seen to be appreciated – preferably accompanied with a fast disco beat to accompany it! All eight LEDs essentially flash at random – but for fractions of a second, discernible patterns emerge. The LEDs may briefly whirl clockwise or anticlockwise, or bounce to and fro, sparkling in their water-clear encapsulations. Circuit description In concept, the circuit is very simple. At its heart lies a CMOS 4051 8-channel analog multiplexer. This can be thought of a single-pole, 8-position switch, with the important difference that the 4051 allows random access to each of the eight switch positions. This means that it does not need to sequence through each of the eight positions as a normal 70  Silicon Chip siliconchip.com.au Fig.1: the beauty of the circuit is its simplicity, which means it can be made nice and small for an eye-catching display! switch would do but has the ability to jump randomly from one position to the next. Pin 3 is the centre or common pole of the switch, which is connected to any given switch position (numbered 0 to 7). This is done by means of a three-bit binary number (or “word”) which is presented to three “select” terminals (pins 9 to 11). The “select” terminals accept binary numbers ranging from 000 to 111 and decode them to the eight separate switch positions. Since only one 3-digit word can be entered at a time, only one of the output terminals can go “high” at any time. Each of the “select” terminals is fed separately by an oscillator running at about 5Hz. This means that each binary digit alternates between a binary 0 and 1 – independently of the other two binary digits. Thus a practically random 3-digit binary word is generated, with the LEDs dancing more or less at random across switch positions 0 to 7, with fleeting patterns emerging. Since pin 3, the centre or common pole in this circuit, is connected to 0V, each of the switch positions goes “low” when connected. Therefore the anodes of all the LEDs are connected to +12V, through a common 1kΩ current-limiting resistor. Note that when a switch position is not connected to the common pole, the corresponding LED is disconnect- The two versions of the Jazzy Heart – electronically they’re the same but the one on the right is meant for the Festive Season. All it takes is a change of cover (see inset below). With just a bit more judicious trimming, it could also be made into a Shamrock for St Paddy’s Day. siliconchip.com.au August 2003  71 Parts List – Jazzy Heart 1 heart-shaped PC board, 63mm x 60mm, code 08108031 1 Miniature mercury switch 4 10µF 16V electrolytic capacitors (or tantalums) 2 8mm crimp terminals for battery holder “end brackets” 1 Round head (No.2) paper fastener for battery holder negative terminal 1 red plastic sheet for Jazzy Heart fascia, 65mm x 65mm 1 green plastic sheet for Jazzy Christmas Tree fascia, 65mm x 65mm 1 5mm drill bit to drill plastic fascias (if required) 1 MN21, 23A or equivalent 12V (alkaline) battery 1 Suitable length of flexible wire or fishing line for “necklace” Semiconductors 1 40106B hex Schmitt trigger (IC1) 1 4051B single 8-channel multiplexer (IC2) 1 1N4148 signal diode 2 5mm ultra-bright red water-clear LEDs 2 5mm ultra-bright yellow water-clear LEDs 2 5mm ultra-bright green water-clear LEDs 2 5mm ultra-bright blue water-clear LEDs Resistors (0.25W 1%)  4-Band Code (1%)   5-Band Code (1%) red red orange brown red red black red brown 1 22kΩ brown green orange brown brown green black red brown 1 15kΩ   brown black orange brown brown black black red brown 1 10kΩ brown black red brown brown black black brown brown 1 1kΩ ed from the power supply. This is in contrast with the 4028 CMOS IC, a BCD-to-decimal decoder, which serves a very similar function in digital circuits, but whose output terminals will only go “high” or “low”. Had a 4028 IC been used, the reverse voltage across the LEDs would then have been 12V, which exceeds the rating (of typical LEDs). While the supply voltage could have been reduced to overcome this, the design could not then have accommodated all colour LEDs. Each of the three oscillators, based on a Schmitt NAND gate, is very simple, requiring only one resistor and one capacitor. For the purpose of preventing “frequency lock” (the tendency of oscillators to “lock on” to one another in close proximity), the values of the capacitors are relatively large (10µF). A 10µF supply decoupling capacitor is included for “good practice”, although this is not strictly necessary. For a less jazzy (that is, more sedate) display, increase the values of the capacitors, and vice versa. Due to the relatively high supply voltage (12V), and since only one LED is flashed at a time, a single current-limiting resistor can be used for all eight LEDs combined, thus simplifying and compressing the circuit. While it would be possible to use four or eight resistors, thus perfectly matching them to each colour LED, this would considerably increase the component count, and is not necessary in practice. The result is an exceedingly compact circuit, using just over one component for each randomly flashing LED. Do note, however, that when selecting LEDs, the luminous intensity should be roughly the same – or test first with 12V and a 1kΩ ballast resistor. A miniature 12V battery is used (an MN21 or 23A or similar) and a 1N4148 diode is employed for reverse polarity protection. The reason why the diode is inserted in the 0V rail here is simply because this suits the circuit layout best. A mercury switch was chosen to switch off the circuit, partly because a standard switch (even a miniature one) would have taken up considerably more space on the PC board. With the mercury switch as shown, the circuit is switched off as soon as the PC board is laid flat or turned upside-down. The inhibit pin (pin 6) of the 4051 CMOS IC is tied “low”. When this pin is taken “high”, all switch positions are disabled. Finally, a question that is commonly asked about the 4051 is what purpose pin 7 (VEE, or A/D) serves. When this terminal is tied “low”, the IC will handle digital signals, as it does in the present circuit. On the other hand, when analog signals need to be routed through the Fig.2: here’s the PC board layout and a near-same-size photograph. Between the two of them, you should have no assembly problems! 72  Silicon Chip siliconchip.com.au Fig.3: same-size templates for either the Jazzy Heart or the Christmas Tree. IC, this pin would normally be connected to the lowest voltage level in the circuit. So, for instance, pins 16 and 7 could be connected to +6V and -6V respectively, while pin 8 could be connected to 0V. Thus analog voltages of up to 12V could be handled with 6V digital control signals. Assembly The Jazzy Heart is on a single PC board measuring 63mm x 60mm, and this is further cut and filed to shape as shown. To begin, the battery holder is constructed of two round 8mm crimp terminals which are inserted into the holes provided on the PC board. In the prototype, a brass round head (No.2) paper fastener was inserted into one of the crimp terminals as shown and soldered into place, to accommodate the negative terminal of the battery. Next, the resistors and capacitors are soldered to the PC board, as well as the 1N4148 diode and mercury switch. Be careful with the polarity of the capacitors and the diode. Note that the mercury switch may need its legs to be raised a little, so that it is “off” when the circuit is laid on its back. Then solder the two ICs, observing anti-static precautions (most importantly, touch your body to earth immediately before handling). Be quick with the soldering iron, so as not to damage siliconchip.com.au the ICs. Alternatively, use dual-in-line (DIL) sockets. The CD40106BCN IC is recommended for the oscillator section – other makes of the same IC may affect the “speed” of the Jazzy Heart. If other types are used, the capacitor values may need to be changed (probably reduced). Solder the eight LEDs into place, noting their correct orientation (the “flat” on the encapsulation is the cathode). These LEDs are given fairly long legs, such that a “heart” or a “Christmas tree” template can be slipped over them after soldering. Thus they will just stand proud of the other components on the PC board. Some ultra-bright LEDs are static sensitive, and anti-static precautions may need to be observed. Finally, you may wish to attach a “necklace”, which may be made from a flexible length of wire or fishing line inserted through the holes provided on the PC board and held with a knot at each hole. Then fit the battery into its holder, taking care to insert it the correct way round. The Jazzy Heart should “fire up” as soon as it is vertical. When laid flat, it will “go to sleep”. Battery life Since the circuit draws a current of about 10mA, and the capacity of the specified battery is typically 33mAh, Fig.4: this is the full-size PC board artwork. All of the “stripey” bits are trimmed off. the Jazzy Heart should flash for three hours or more continuously before the battery is exhausted. These 12V batteries can be expensive but we have found a very cheap source to be at bargain stores and markets, where you can often pick up a pack of two or three “no name” Asian imports for a couple of dollars or so. If a “Christmas tree” template is used, the circuit will of course be positioned “upside-down”. In this case, the position of the mercury switch will need to be adjusted accordingly, so that the Christmas tree is “on” when it is SC stood up vertically. August 2003  73