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By LEO SIMPSON BUILD You’ve seen those late-model Japanese sports cars with a row of pinpoint red lights in the spoiler. They look snazzy & they draw immediate attention to the brakes being applied. Now you can have one for your car. This Brake Light Array uses 60 high-brightness light emit­ting diodes and a few other components. The LEDs are installed on two narrow PC boards and they are driven so that they light up from the centre of the array and spread out till all LEDs are alight. This takes place in a fraction of a second and looks even more eye-catching than the brake light arrays on Japanese cars. The Brake Light Array, or BLA for short, is housed in a thin aluminium channel which is 500mm wide. It can be mounted on the parcel shelf of your car and power can be taken from one of the brake lights. The total current drain of the BLA is about 260mA which is minuscule compared to the current of several amps drawn by your existing brake lights. In fact, the BLA is so bright for such a small current that it seems likely that brake lights in the future will not use incandescent lamps – they will use high-brightness LEDs. 56  Silicon Chip Interestingly, because the circuit has a regulated supply voltage, the brightness of the Brake Light Array will always be constant, regardless of any variations in the battery voltage. Flasher circuitry Now have a look at the circuitry of the BLA – see Fig.1. This looks fairly complicated considering that it merely lights up a bunch of LEDs. However, the circuit could be used for other purposes and so can be made to flash several times in succession before the LEDs stay on permanently, until the power is removed. Power for the circuit comes from one of the brake lights. The positive supply (+12V) is fed through a 500mA in-line fuse and then to a 2.2Ω resistor and 15V zener diode which protects the circuit from any high voltage transients which could come, for example, from door solenoids or motors. The +12V supply is regulated to +8V by a 7808 3-terminal regulator which feeds all the circuitry. IC2, an LM3914 dot/bar display driver, is the heart of the circuit. It drives 30 LEDs in 10 groups of three and each group of three LEDs is in series with its particular output from the LM3914. The LM3914 is operat­ed in bar mode (pin 9 connected high) and the current through each set of three LEDs is set at 10mA by the 1.2kΩ resistor at pin 7. Normally, an LM3914 is used to A 60-LED BRAKE LIGHT ARRAY FOR YOUR CAR drive a bargraph display of LEDs in response to a signal voltage applied to its pin 5; the more signal, the more LEDs light up. And so it is in this design. The signal voltage is applied to pin 5 via transistor Q3 which is connected as an emitter follower. Its base signal comes from the emitter of Q1, a unijunction transistor. Q1 is connected as a relaxation oscillator to produce a sawtooth waveform at its emitter. What happens is that the 22µF capacitor at the emitter is charged up to about +5V via the 10kΩ resistor and 100kΩ trim­pot, VR1. Each time the capacitor reaches the threshold voltage of around +5V, the unijunction (Q1) discharges the capacitor and the cycle begins again. So Q1 is the source of signal voltage applied to pin 5 of IC2 via Q3. If we neglected the effect of Q2 and IC1, the action of the circuit presented so far would be to repeatedly light up the full row of LEDs. Clearly, this would be no good for brake light use as it would send the drivers of following cars mad (as well as being illegal). This is where IC1 comes into the picture. Each time Q1 discharges the 22µF capacitor at its emitter, it produces a brief positive pulse at its base 1 (B1) terminal. This pulse is amplified, inverted by transistor Q2 and fed to the clock input of IC1, a 4017 decade counter. With the aid of a link on the PC board from its pin 13 (enable) input, IC1 can be made to count any number of pulses up to six whereupon it will stop counting and its selected output will go high. This selected output is fed via diode D1 to the emitter of Q3 and to pin 5 of IC2. This stops Q3 from responding to the sawtooth signal from the emitter of Q1. Thus, IC1 will turn all LEDs on until power is removed from the circuit. Master & slave circuit The description so far tells how LEDs 1-30 are driven. LEDs 31-60 are driven by IC3, another LM3914 which is “slaved” to the signal from the emitter of Q3. Thus, IC3 is forced to mimic Below: this close-up view shows the master board of the LED Brake Light Array. It carries 30 high-brightness LEDs, while the slave board carries another 30 LEDs. August 1993  57 58  Silicon Chip B 4.7k Q3 BC548 E 10k 100k VR1 22 16VW C 100W B1 3 1k MODE 6 RHI 9 Q1 2N2646 E B2 8.2k +V1 1 B K K A K A A    K K A K A A    K K A K A A    K K A K A A 1.2k E C REF OUT 7 CLK 16 2 Q1 4 2 Q2 IC1 4017 Q3 7 15 10 15 RST Q4 1 Q5 5 Q6 EN 8 13 14 14 5.6k RLO 4 LED1LED30    K K A K A A BRAKE-LIGHT ARRAY 100k 0.1 REF ADJ 8 IC2 LM3914 18 17 16 15 14 13 12 11 10 10k    Q2 BC548 SIG 5 K K A K A A 6 5 4 3 2 1    K K A K A A SELECT SWEEPS K K A K A D1 1N914    A    K K A K A A    8.2k    CHASSIS +12V FROM BRAKE LIGHTS K K A K A A 6 9 F1 500mA +V2 ZD1 15V E 2. 2  RHI MODE 3 B 1       K K A K A A    K K A K A A    K K A K A A C 22 16VW 1.2k REF OUT 7 B2 IN 5.6k E B1 REG1 7808 GND REF ADJ 8 IC3 LM3914 22 16VW OUT RLO 4 I GO +8V 2 18 17 16 15 14 13 12 11 10 K K A K A A VIEWED FROM BELOW SIG 5 K K A K A A    22 16VW K K A K A +V1 LED31LED60    A K K A K A A IN K K A K A REG2 7808 GND    A 22 16VW OUT    K K A K A A +8V    +V2 K K A K A A    CHASSIS 500mA IN-LINE FUSE +12V FROM BRAKE LIGHTS ZD1 VR1 REG1 10k 22uF Q1 22uF 100 1k 10k Q2 Q3 8.2k 4.7k 1.2k 5.6k D1 1 IC2 LM3914 100k COMMON 0.1 22uF 2. 2  IC1 4017 1 2 3 4 5 6 SWEEP LED1-LED30 K A 22uF REG2 8.2k 1.2k 5.6k 1 IC3 LM3914 22uF LED31-LED60 K A everything done by IC2 in driving its LEDs. IC3 and its 30 LEDs are fed by their own regulator (REG2). Note that we do not recommend that this circuit be set up to provide more than one sweep of the LEDs before they turn on fully. Multiple sweeps of the LEDs will be quite distracting to following drivers and is illegal in Australia, as far as we know. Construction As noted above, the LED Brake Light Array is built on two narrow PC boards which each measure 230 x 27mm. These boards are supplied with a full component overlay so assembly is quite straightforward. One board has LEDs 1-30 on it and it becomes the “master” while the second board accommodates IC3 and LEDs 31-60 and is the “slave”. We suggest you build the master board first and get it going before doing the slave board. Assemble the small components such as links, diodes and resistors and capacitors first, then the transistors and in­ tegrated circuits. The highbrightness LEDs come last. ▲ LED polarity trap Fig.1 (left): the circuit uses two LM3914 dot/bar display drivers which respond to the ramp voltage generated at the emitter of unijunction transistor Q1. Decade counter IC1 controls the number of times that the LEDs are swept before they come on fully. You will need to use care in assembling the LEDs so that they are all lined up – if even one is not lined up with the others it will stick out like a sore thumb. The way to line them up is to install each LED so that its leads are just long enough so that they can lie flat on the top surface of the board, with the LED body butted up to the edge of Fig.2: install the parts on the two PC boards as shown in this wiring diagram. Note that the leads used to connect the boards together must be long enough to allow the boards to be mounted end-to-end. the board – the accompany­ing photos show the general idea. Before we leave the LEDs, there is a big trap to watch. We normally show a pinout diagram on the circuit which shows the LED polarity. The normal convention is that the longer lead is the anode and the lead adjacent to a flat on the side of the lens is the cathode. However, it is not always the case and it could be most frustrating to assemble 30 LEDs onto the board and find that they are all the wrong way around. In particular, the LEDs supplied with this project kit will be the reverse of normal convention – the shorter lead will be the anode. To be sure that you assemble them correctly, check at least one LED with a 9V battery and RESISTOR COLOUR CODE ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 2 2 1 2 1 1 1 Value 100kΩ 10kΩ 8.2kΩ 5.6kΩ 4.7kΩ 1.2kΩ 1kΩ 100Ω 2.2Ω 4-Band Code (1%) brown black yellow brown brown black orange grey red red brown green blue red brown yellow violet red brown brown red red brown brown black red brown brown black brown brown red red gold gold (5%) 5-Band Code (1%) brown black black orange brown brown brown black black red brown grey red black brown brown green blue black brown brown yellow violet black brown brown brown red black brown brown brown black black brown brown brown black black black brown not applicable August 1993  59 These two views show how the master & slave boards are mounted end-to-end so that the LEDs form a single bargraph. This version used a channel made from two angle aluminium sections. a 4.7kΩ limiting resistor. You have been warn­ed. Finally, fit the short link adjacent to the 4017 which selects the number of sweeps at six; ie, install a link connect­ ing “common” to “6”. After having checked your work carefully, connect a DC power supply set to 12V. The row of LEDs should sweep towards the regulator end of the board six times before all flick on and stay on until power is removed. You can change the rate at which the LEDs sweep by adjusting trimpot VR1. We suggest you set it for a sweep rate of several times a second. This then completes your work on the master PC board. Now you can assemble the slave PC board. As can be seen from the photos and the wiring diagram of Fig.2, the slave board has quite a few components omitted. To be specific, those omitted are the 2.2Ω resistor and zener The slave board is mounted upside down on the aluminium channel, so that the LEDs light from the centre outwards. Ignore the resistor shown tacked on the back of this prototype board – the final version has the resistor mounted on the component side (see Fig.2). 60  Silicon Chip diode at the input to the regulator, transistors Q1, Q2 & Q3, IC1, diode D1 and all of the associated resistors except for the 4.7kΩ and 1.2kΩ values associated with the LM3914. Begin the slave board assembly by installing the two resistors, the two wire links, the two 22µF capacitors and the 3-terminal regulator, then install the LM3914 and the 30 LEDs. To complete the slave board, you will need to run three insulated wires from it to the master board. These include the common ground wire (0V) and a wire from the input terminal of the 3-terminal regulator on the master board to the input of the regu­lator on the slave board. Finally, a lead must be run from the emitter of transistor Q3 on the master board to the same position on the slave board; ie, the emitter pad of Q3. This is the control signal wire for the slave board. Now check all your work carefully again and apply 12V DC once more. The LEDs on both boards should now sweep towards the regulator six times in identical fashion before flicking on perma­nently. Now we strongly suggest that the BLA be set for only on sweep of the LEDs before they come on permanently. To accomplish this, remove the link between pins 13 and 5 of IC1 that was installed previously and connect a short link underneath the master PC board between pins 2 and 13 of IC1. This done, apply power again and check that the LEDs make one sweep and then flick on fully. Finally, set the rate at which the LEDs sweep on by ad­justing trimpot VR1. Mounting the boards To make up the Brake Light Array, the two assembled PC boards must be positioned end-to-end with the regulators on the outermost ends. Mounted in this way, the resulting display will start in the centre of the two boards and spread out to the ends until all LEDs are alight. We had two prototypes of the BLA. One had the PC boards mounted in an aluminium channel measuring 40 x 25 x 500mm long. The boards were glued together and then secured in the channel with small blocks of foam plastic. The channel was mounted on a short upright made from metal towel rail fittings. The whole assembly was then sprayed with flat black enamel. The second prototype BLA used a channel made from two angle aluminium sections measuring 25 x 50 x 500mm and secured together with self-tapping screws. The boards were mount­ed end-on on the bottom section using suitable screws, spacers, Protect your valuable issues Silicon Chip Binders PARTS LIST 1 aluminium channel, 500mm wide (see text) 2 PC boards, 230 x 27mm 1 in-line 3AG fuseholder 1 500mA 3AG fuse 1 100kΩ trimpot (VR1) Semiconductors 60 5mm high brightness red LEDs (LED1-60) 2 7808 8V 3-terminal regulators (REG1,REG2) 1 4017 CMOS decade counter (IC1) 2 LM3914 dot/bar LED drivers (IC2, IC3) 1 2N2646 unijunction transistor (Q1) 2 BC548 NPN transistors (Q2,Q3) 1 15V 1W zener diode (ZD1) 1 1N914, 1N4148 silicon diode (D1) Resistors (0.25W, 1%) 1 100kΩ 2 1.2kΩ 2 10kΩ 1 1kΩ 2 8.2kΩ 1 100Ω 2 5.6kΩ 1 2.2Ω 0.5W 1 4.7kΩ ★ High quality ★ Hold up to 14 issues Miscellaneous Aluminium channel mounting hard­ ware, hook-up wire, screws, nuts, spacers, washers. ★ 80mm internal width Where to buy the kit A kit for this project with all parts except the metalwork is available from Oatley Electronics, PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985 or fax (02) 570 7910. The kit price is $65 plus $3 for postage & packing. Note: copyright of the PC artwork for this project is retained by Oatley Electronics. Price: $A11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form ➦ Capacitors 5 22µF 16VW electrolytic 1 0.1µF monolithic These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. Enclosed is my cheque/money order for nuts and washers. A similar support assembly was made from towel rail fittings and again the whole assembly was sprayed with flat black enamel. The assembled BLA can be mounted on the parcel shelf of your car, as close to the rear glass as possible. You will then need to make a connection to the chassis for the 0V supply line and to one of the brake light wires to pick up the +12V supply. This can most conveniently be done using a “Contact” connector. This connector is simply wrapped around and new wire and the wire to the brake light and then the connector is squeezed to make a safe and insulated connection. These connectors are available in a pack of four for $1.50 from Jaycar Electronics (Cat. HP-1206). Points to note Two important notes about the connection to the brake light: (1). Make sure you make the connection to the stop light filament line, not the tail light; and (2). Don’t forget to fit a 500mA inline fuse to the +12V line, as specified SC on the circuit diagram. $________ or please debit my ❏ Bankcard   ❏ Visa   ❏ Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ August 1993  61