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Daytime Running Lights For Cars This circuit automatically switches on your car’s headlights during the day, so that your vehicle is more visible to other road users. It drives the low-beam circuit at 80% duty-cycle to prevent unnecessary glare but switches to full bright­ness in low light conditions. By JOHN CLARKE One of the first things the visitor to Canada notices is that all cars have their headlights on during the day. The Cana­dians call them “daytime running lights” and claim that they have significantly reduced the accident rate. The headlights turn on automatically when the engine is started but are not quite as bright as a conventional low-beam circuit. Instead, they 26  Silicon Chip run at only about 80% brightness so that the glare doesn’t annoy other drivers. It’s certainly a very effective system and you really do notice other vehicles on the road much sooner than would other­ wise be the case. And that can only be a good thing when it comes to improving road safety. In Canada, daytime running lights make a lot of sense. Canada has long winters with very short hours of daylight and light levels are generally lower than in Australia. But daytime running lights make sense in other countries as well. Several state government authorities in Austra­lia now encourage motorists to drive with their headlights on during the day, particularly on long trips. It certainly works – cars coming towards you with their headlights on are much more noticeable than other vehicles. It stands to reason that the sooner you are noticed, the better. It gives other drivers more time to make decisions and that greatly reduces the chances of an accident. And in some situations, having the lights on can make the difference between being seen or not being seen at all. On a related theme, just think how many drivers neglect (or forget) to turn on their lights at dusk or when it’s raining heavily. An automatic “lightson” circuit solves this prob­lem. Main features In Canadian cars, the daytime running lights are provided by a separate filament in the main headlight housing. When the engine is started, both the daytime running lights and the tail lights come on. In addition, there is a sensor that automatically switches the headlights to full low-beam in lowlight conditions but they can also be turned on at any time by the driver. The circuit described here provides all these features and is completely automatic in operation. However, because Australian cars don’t have separate headlight filaments for daytime running lights, our circuit drives the low-beam filaments. It doesn’t drive the low-beam lights at full brightness though. Instead, it pulses the lights with an 80% square-wave duty cycle and this reduces their brightness to a comfortable level for other drivers. The accompanying panel shows the main features of the Day­time Lights circuit. Note that it also activates the tail-lights, although these are driven at normal brilliance. Why do we acti­vate the tail-lights as well? The reason is that we don’t want to be driving around at night with the headlights on while remaining blissfully unaware that the tail-lights are off. Our circuit also incorporates a light sensor and this automatically switches the low-beam lights to full brightness when it gets dark. This is an important safety feature – it means that you cannot drive around at night with the headlights only operating at 80% of normal brightness. Another important feature of the unit is that the daytime lights only come on if the battery voltage is above 12.7V. This ensures that the lights remain off while you are starting the engine, since the battery voltage will be below this figure. It also prevents the lights from coming on if the car is being serviced and the ignition switch is simply turned “on” (but the engine not started). Once the engine has started, the voltage from the alternator will exceed the 12.7V threshold and so the daytime lights will come on. The headlights switch operates normally. It effectively overrides the Daytime Lights circuit, so that the headlights can be manually switched Main Features • • • Headlights automatically switch on at 80% bright­ness when car starts. • • • • • Powers headlights rated up to 200W total. Automatic switch off with ignition. Dark sensor switches lights to full brightness at night-time and in lowlight conditions. Headlight switch works normally and overrides circuit opera­tion. Daytime lights activated only after engine starts. Efficient circuit has minimal losses. EMI suppressed. on by the driver. The Daytime Lights circuit immediately takes over again if the light switch is turned off. Finally, the circuit is designed so that when the engine is stopped, the daytime lights automatically switch off. This means that you cannot accidentally leave the lights on and flatten the battery, unless you leave your conventional lights switch on (and even here, many modern cars have you covered). Basic operation Fig.1 shows the basic operating principle of the Daytime Lights circuit. It’s based on Mosfet Q1 which is connected across the existing headlights switch. When the Mosfet is turned on (ie, conducting), the headlights are lit via the +12V supply. Convers­ely, when the Mosfet is switched off, the lights are off. By pulsing the Mosfet on and off at a fast rate, the average voltage applied to the lamps is reduced. This voltage will depend on the duty cycle of the waveform applied to Q1’s gate. The gate driver circuit connects between Q1’s gate (G) and source (S) terminals. When the gate voltage is about 12-15V above the source, the Mosfet switches on and current flows from the positive supply rail to drive the low-beam headlights. Converse­ly, when the voltage between gate and drain is 0V, the Mosfet is open circuit and the lights are off. Note that the gate driver must be capable of floating above ground and must follow the source voltage. When Q1 is on, the source is at +12V and when Q1 is off, the source is at 0V as it is pulled low by the lamp filaments. We used a Mosfet rather than a transistor here because a Mosfet switches on with a considerably lower resistance than a transistor. This both reduces power dissipation in the device and ensures that almost the full rail voltage is applied to the lamps. A Mosfet also requires much less drive current. Block diagram Fig.1: the Daytime Lights circuit uses a Mosfet (Q1) to pulse the lowbeam headlights on and off, with a duty-cycle of 80%. Now take a look at Fig.2. This shows the block diagram for the Daytime Lights for Cars. Its basic operation is quite simple but as they say, the devil is in the detail. IC1 is a 555 oscillator which produces a pulse waveform with a duty-cycle of 80%. Its output drives opto­coupler IC2 via a gating block (D1, D2 & Q2), which then feeds the oscillator signal to Mosfet driver stage IC3. The signal from this stage then drives Mosfet Q1 to activate the low-beam circuit at about 80% of normal brightness. In addition, the Mosfet output stage AUGUST 1999  27 Fig.2: this diagram shows the main circuit blocks in the Daytime Lights circuit. It uses an oscillator (IC1) to drive an optocoupler via a gating circuit. The optocoupler then pulses the Mosfet (Q1) via a driver stage. turns on relay RLY1. This activates the parking lights circuit, so that the taillights switch on. The gating circuit determines whether or not the oscillator output is fed through to the optocoupler. This is controlled by the 12.7V voltage detector block (IC4a, ZD2), which prevent the lights from coming on when the engine is being started. The dark detector block automatically switches the lights to full brilliance in low-light conditions. Circuit details Refer now to Fig.3 for the full circuit details. IC1, a 555 timer, is the oscillator and is wired in conventional fashion. Its frequency of operation is set to 1.14kHz by the RC timing components on pins 2, 6 & 7 and this is high enough to prevent any flicker in the headlight filaments. In operation, pin 3 of IC1 is high while the capacitor charges via the 8.2kΩ and 2.2kΩ resistors and low while it dis­charges into pin 7 via the 2.2kΩ resistor. This gives a duty cycle of just over 80% (82.5%, to be exact). The 1.14kHz square wave signal drives pin 2 of optocoupler IC2 via diode D1 and a 470Ω resistor. The LED inside the optocou­pler is switch­ed on 28  Silicon Chip when pin 3 is low, assuming that the +12V switched rail is present on pin 1. Each time the LED switches on, the internal phototransistor also switches on and pulls pin 3 of inverter stage IC3a low. Conversely, when the LED turns off (ie, the oscillator output is high), the transistor turns off and pin 3 of IC3a is pulled high (to the +12V supply) via a 10kΩ resistor. Note that the base terminal of the internal transistor is tied to the emitter via a 100kΩ resistor. This improves the response time of the photo­trans­istor at the expense of sensitivi­ty. IC3a buffers and inverts the signal from the optocoupler. Its output appears at pin 2 and is fed to parallel inverter stages IC3b-IC3f. These inverters drive the gate of Q1 via the 47Ω resistor. Each time the buffer outputs switch high, Q1 turns on and current flows through the low-beam lamps via inductor L1. L1 is included to suppress any electromagnetic interference which would otherwise be heard in the car radio. Diode D8 is included to suppress any switching spikes from the inductor, which could damage Q1. The scope shot of Fig.4 shows the signal applied between the gate and source of Q1. Its duty cycle is shown as 84% and with a 13V peak-to-peak amplitude. Note that the gate drive voltage follows the voltage on pin 5 of IC2. This means that a non-inverting buffer (a 4050) could be used in place of the 4049 inverter without any changes to circuit operation. Fig.5 shows the drive to the lamp filaments on the Ch1 (top) trace and the gate drive to the Mosfet on the Ch2 trace. Note that the gate drive is shown here as 27.6V, since we are now referring the signal to ground rather than to the source voltage of Q1. This means that the gate voltage is 13.4V (27.6-14.2) above the source when Q1 is on. Battery voltage detector IC4a is the battery voltage detector. This stage functions as a voltage comparator, with positive feedback via a 1MΩ resis­tor to give the circuit a small amount of hysteresis. As shown, IC4a’s non-inverting input (pin 3) monitors a 4.7V reference (ZD2) via a 68kΩ resistor, while the inverting input (pin 2) monitors a voltage divider connected across the +12V supply line from the ignition switch (ie, from the battery). When the battery voltage is less than 12.7V, pin 3 is higher than pin 2 and so the com- Fig.3: the final circuit includes an LDR which, in company with IC4b, switches the headlights to full brilliance when it gets dark. ZD2 and IC4a prevent the lights from coming on when the engine is being started. parator output at pin 1 will be high. As a result, the voltage on pin 3 will be about 5.1V (ie, slightly higher than the 4.7V reference) due to the positive feedback. When the ignition is first switched on and the vehicle is being started, you can expect the battery to be below 12.7V. Thus, pin 1 of IC4a will be high and this turns on transistor Q2 which now shunts the signal from IC1 to ground via D2. At the same time, pin 5 of comparator stage IC4b is pulled low via D3 and so its pin 7 output will also be low. This low output from IC4b turns on PNP transistor Q3 and so the +12V from the battery is applied to pin 1 of the optocoupler (IC2). The internal LED will thus be permanently on, since there is a path to ground via the 470Ω resistor, D2 and Q2. As a re­sult, pin 5 of the optocoupler will be low and Q1 is held off. When the engine is started, the battery voltage quickly rises. When it exceeds 12.7V, the output of IC4a switches low and Q2 turns off. The output of IC1 now pulses the opto­ coupler LED on and off via D1 and so Q1 drives the lamps with an 80% duty cycle, as described previously. When pin 1 of IC4a switches low, its pin 3 input is pulled down to about 4.36V due to the 1MΩ feedback resistor. This means that the battery AUGUST 1999  29 Fig.4: this scope shot shows the waveform applied between the gate and the source of Q1. It has an amplitude of 13V peak-to-peak and a duty cycle of 84%. voltage rail must drop below 10.9V before IC4a’s output switches high again and the lights go off. Normally, this could only happen if the vehicle is just idling and there is a heavy load on a battery which is “on the way out”. Dark detector IC4b and light dependent resistor LDR1 form the dark detec­tor circuit. The op amp is wired as a comparator with positive feedback, just like IC4a, and its inverting input (pin 6) is biased to 4.7V by ZD2. The non-inverting input (pin 5) monitors a voltage divider consisting of a 47kΩ resistor, trimpot VR1 and the LDR. During daylight hours, LDR1 will have a low resistance and so the voltage on pin 5 of IC4b will be lower than that on pin 6. As a result, pin 7 will be low, Q3 will be on and the +12V supply Fig.5: the top trace of this scope shot shows the drive to the lamp filaments, while the bottom trace shows the gate drive to Q1 with respect to ground. will be switched through to IC2, so that the circuit can operate. When it gets dark, the resistance of the LDR rapidly increases (up to several megohms in total dark­ness). As the resistance of the LDR rises, so does the voltage on pin 5. When this voltage rises above 4.7V, pin 7 of IC4b goes high and Q3 switches off the +12V supply to IC2. VR1 sets the light level at which the circuit operates, while the 1MΩ feedback resistor provides a small amount of hysteresis so that the circuit doesn’t oscillate if light levels fluctuate rapidly close to the trigger threshold. is provided by IC1, T1, diodes D4-D7 and ZD3. In operation, pin 3 of IC1 drives transformer T1 via a 1µF capacitor. T1 is a standard isolation transformer with 3kΩ wind­ings and its primary winding is centre-tapped. By driving only half the winding, we can use the transformer to step up the output voltage. D4-D7 rectify the AC voltage on the secondary winding to produce a DC rail and this is filtered by a 1µF capacitor. ZD3 regulates the output voltage to 15V and this rail supplies the optocoupler transistor and IC3. Power for the entire circuit is derived from the +12V igni­tion rail. This rail is decoupled using a 4.7Ω resistor and a 100µF capacitor, while ZD1 protects the circuit from voltage transients above 16V. A 10µF capacitor provides Power supply Because Q1’s source must be floating, we need a separate isolated power supply to provide the gate-source turn-on voltage. This isolated supply Table 1: Resistor Colour Codes                No. 2 1 4 1 1 3 1 1 2 1 1 1 1 1 30  Silicon Chip Value 1MΩ 150kΩ 100kΩ 68kΩ 47kΩ 10kΩ 8.2kΩ 4.7kΩ 2.2kΩ 1kΩ 470Ω 330Ω 47Ω 4.7Ω 4-Band Code (1%) brown black green brown brown green yellow brown brown black yellow brown blue grey orange brown yellow violet orange brown brown black orange brown grey red red brown yellow violet red brown red red red brown brown black red brown yellow violet brown brown orange orange brown brown yellow violet black brown yellow violet gold brown 5-Band Code (1%) brown black black yellow brown brown green black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown brown black black red brown grey red black brown brown yellow violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown orange orange black black brown yellow violet black gold brown yellow violet black silver brown LOOK AT THIS JUNE SALE!!! Did you miss it? Well you were not the only one!!! SUGAR CUBE SIZED CAMERA The ads we placed were so small that most people missed the ads BIGGER So we are going to run it again as the Much September Sale. To see just what’s on sale just check out the September Sale link on our new web page or if you have a polling fax you can see our text list of sale items on 02 95843562 or 02 95707910. But don’t forget our web page BARGAIN CORNER where we sell all of our regular specials like runout end of stock & special one or few of items like used security cameras with an incredible zoom lens Canon "C" mount, motor driven zoom lens. zoom, aperture and focus. F2.8 and the zoom range is 15-150mm!! or a large Pan / Tilt unit. 280 x 280 x170mm: 8Kg DRAW ACTUAL SIZE 16 X16 X14mm The smallest monochrome camera we have offered yet. They don’t have the greatest resolution but are very small and only draws 10mA <at> 5V (a 9V bat. + regulator would run one of these for days) Camera in its own plastic housing plus free VHF modulator and suitable power adaptor for special intro price $80 NEW SUPER LOW PRICE + LASER AUTOMATIC LASER LIGHT SHOW KIT: MKIII. Automatically changes every 5 - 60 secs. Countless great displays from single to multiple flowers, collapsing circles, rotating single and multiple ellipses, stars, etc. Easy mirror alignment with “Allen Key”. Kit inc. PCB, all on board components, three small DC motors, mirrors, precision adjustable mirror mounts: (K115) + very bright 650nM laser (LM2) module. Kit with laser module $55 Kit + laser module + plug-pack + instument style case all at a special price of $70 ***NEW*** *HIGH QUALITY 4 FREQU. CRYSTAL LOCKED 2.4GHz AUDIO / VIDEO LINK KIT COMING SOON. Will suit VCRs or Video cameras. Range of up to 50 M 2.4 GHz. 12V operation VCRs.. ***NEW KITS *** PCB plus all on-board components, connectors, switch, metal case, telescopic antenna, twin RCA A/V lead, all that is needed to complete the full kit. 12Vdc <at>10mA operation. Ideal for transmitting audio and video around you home.. Complete Kit for just $25 NEW ULTRA-SONIC RADAR KIT Just like the top European cars you can fit a reversing radar that will sound a buzzer or flash a light on your dash to let you know when your car is near another car or object. Features include adjustable range upto1M output to drive relay or buzzer. kit includes PCB plus all on-board components including Ultra-sonic transducers and buzzer for $16 $55 NEW MOSFET STEPPER DRIVER This kit is designed to work below 5V & greater than 35V (higher voltage MOSFETS avail.)Very efficient (very little heat) & work with software like DANCAD etc.(for step/dir-ection signals) & is ideal for CNC projects. It works well with the stepper motors in our famous German printer $45 or$35 with new or previous printer purchase $199 PAIR ***NEW*** 35-140 LED IR ILLUMINATOR KIT Switches on when it gets dark or can be controlled by alarm system. Kit includes mount ing tray & universal swivel mount. 35 LEDs $25. Extra 35 LED pack (3extra packs max) $14 per pack. 140 LED kit:$67 Ideal for use with our monochrome cameras to see in the dark. NEW...PC MOTHERBOARD UMC-486 CACHE ISA SX 40Mhz. Original package, 486-40Mhz CPU, booklet & QA report. inc..., 5 X 16 bit & 1 X 8 bit slots, space for 4 X 30 pin & 1 X 72 pin Mem. 220 X 170mm $18 GREAT TEST GEAR BARGAINS $25 KEY-CHAIN LASER POINTER in a presentation box. Quality metal housing + 3X LR44 /AG13 bats. FREE. Extra bats. 50c Ea. $10 Line lens+$0.80...X-hair lens +$0.80...Module (no case) only $8 suitable plugcack $5 UHF AUDIO / VIDEO TRANSMITTER KIT Kit includes all components needed...... X 465 100Mhz used TEKTRONIC CROs $440......HP 54501A 100 Mhz used digitizing CROs $970... HP3300A used Function Generators with 3302A plug-in $280 SEE WEB PAGE FOR MORE BUILD YOUR OWN COMPUTER CONTROLLED 2/3 AXIS MACHINE using our now famous $46 surplus GERMAN PRINTER & CNC shareware (DANCAD) Using the parts of our printer that is chock full of steppers, toothed belts, pulleys, bearings etc (see EA June 99). we have plans/notes for $9 (on floppy) & links to find lots of info on the net . LASER LEVEL Kit includes laser module with columnating lens plus battery holder plus suitable case plus construction notes $14 NICAD BATTERY PACK Removed from equipment for routine maintenance. We can’t fault them. Some 4 some 6 cell. $0.20 / cell. Guaranteed! CHARGER PCB (to suit above 6 cell packs) 7.2V trickle charger add $5 16 X 2 LINE LCD CHARACTER DISPLAY LAS ER LE VE L + 1M IDC ext. cable, TWO MOTOR LASER LIGHTSHOW KIT LED, buzzer Kit includes motors, mirrors, reversing & switch on $12 or 3 for $30 switch and all electronic components. Can a PCB. be controlled with a variable DC input.Lots TOLL FREE PHONE NUMBER of patterns, flowers, stars etc. $16 Sorry but we don’t have one but if Laser module to suit $8 you call 02-95843564 24hrs & (NEW) 12V / 2.3Ah AUDIOVOX LEAD leave a message & your number ACID BATTERY (Model BTR-1900). Priced at a fraction of their real value (as we will call you back ASAP at our used in video cameras & older mobile cost. (orders only please) phones - same as Panasonic batteries we sold before). 180 (L) x 60 (H) x 22 (W) mm, 0.67Kg, made in Japan. The contacts PO Box 89 Oatley NSW 2223 (which are easily solderable) are at one Ph ( 02 ) 9584 3563 Fax 9584 3561 end of the battery. 2 batteries + suitable orders by e-mail: oatley<at>world.net 500mA float www.oatleyelectronics.com charger. major cards with ph. & fax orders, Post & Pack typically $6 Prices subject to change without notice CAUTION LASER!!! OATLEY ELECTRONICS OATLEY ELECTRONICS $20 $25 + $16 4093 + + + INFRA-RED SHOP DOOR MINDER IR transmitter & receiver kits (2 separate PCB’s), basic range is 20M can be increased by adding a lens. Output to drive piezo buzzers or relays etc. 2 PCB’s + all onboard parts: $17. 2 X suitable boxes + 2 swivel mounts: $6, Buzzer: $3, 12A relay: $3 (fits on PCB) Lens: $0.80 12V Automotive Relays with 30A SPDT Contacts (73 ohm relay coil). RRP $7. our price $3 ea. $10 for 4 ***NEW***WHITE LED 5mm 3500mcd. Very bright Ideal for mini torch etc.... $4 POWERFUL IR ILLUMINATORS With strong universal swivel mount & 50X50X50mm housing:10 LED $10... 30 LED $20...80 LED $36 AMAZING MOSFET BARGAINS IRFZ-44...$2.50 60V/50A/0.028 ohm IRF-540...$2.50 100V/28A/0.077 ohm IRFP460...$2.50 500V/20A/0.27ohm IRF-820...$5 500V/2.5A/3.0 ohm NEW***NEW***NEW***NEW PELTIER CONTROLLER: This kit is a swmode design & correctly controls temp. of peltiers to 10A (very efficient design) PCB + onboard parts + new surplus case. $15 NEW AUSTRALIAN PLUG PACKS AT BELOW WHOLESALE PRICES GENERAL ELECTRIC 20VA 14VDC <at> 700mA..... AUDIOVOX 9V <at> 500mA AUDIOVOX 12V <at> 400mA.... $5 Ea. or 5 for $20 ***KIT SPECIAL*** FM FM FM TRANSMITTER TRANSMITTER TRANSMITTER MKII MKII KIT / RADIO MIC. This kit has good range and stability & can be configured as a hand held mic or lapel mic or musical instrument transmitter. Kit includes PCB, all onboard 88-108MHz 88-108MHz com-ponent,suitable small case, lapel OATLEY OATLEY microphone with clip. ELECTRONICS ELECTRONICS (02)-95843563 (02)-95843563 $17 OATLEY ELECTRONICS OATLEY ELECTRONICS 4 CHANNEL VIDEO SWITCHER KIT This kit can switch manually or sequentially up to 4 audio/video sources. Features inc. VCR relay output for STOP / REC, can be switched with PIR or alarm inputs Add a security channel to your TV with a VHF modulator, watch TV & flick channels & see who’s at the door can be auto switched using PIR units Kit + PCB + all on-bourd parts $50. Optional VHF modulator / mixer $18 PELTIER EFFECT DEVICES Make a solid state food cooler / warmer for the car etc. with 2 heatsinks, a fan and one of the following. Could be used for cooling overclocked PC CPUs. All 40 X 40mm. 4A T 65deg. Qmax 42W $25 6A T 65deg. Qmax 60W $27.50 8A T 65deg. Qmax 75W $30 Device comes with instructions to build cooler / heater plus data. Some used surplus heatsinks avail. ***NEW*****NEW*****NEW*****NEW*** QUALITY AUSTRALIAN MADE FEATURE PACKED MINI ALARM SYSTEM. Features inc. boot release, central locking output, imobiliser output, indicator flash relay. Has with 2 key-fob transmitter keys. Drawn in proportion ***NEW******NEW*****NEW******NEW*** SAW RESONATOR LOCKED. NO TUNING 433 MHz UHF DATA TX & RX MODULES +ENCODER PCBs TO SUIT. Many security codes, 4 zones, multi channel. 100 See our WEB SITE for more TX module $11 TX + encoder $18 RX module $18 RX + encoder $25 AT LAST! A COLOUR CMOS CAMERA WITH GOOD RESOLUTION + BUILT IN AUDIO + FREE PLUG PACK + F R E E V H F M O D U L AT O R . Available with swivel mount or dome mount housing. $160 $160 BNC connector (video), DC connector (power), RCA connector (audio). 330000 pixel. 330 TV line res. 7-12Vdc 55mA max. INTRO PRICE $160 NEW 12VDC-240VAC/300VAINVERTER This new design is very efficient, is rated at 300VA constant not peak (when our transformer is used). It has auto switch on and uses High power MOS-FETS that require very minimal heat-sinking. The kit inc. PCBs, all onboard components, 4 high power MOSFETs and all for $35 To save money you can use your own transformer or we can supply the Kit + a high quality compact toroidal transformer plus wiring kit plus a used large electrolytic capacitor for $89 ** CCD CAMERA SPECIAL ** WITH A FREE UHF MODULATOR The best "value for money" CCD camera on the market! 0.1 lux, High IR response & hi-res. Better than most cheaper models. 32 X 32mm $99... With 1of these lenses pinhole (60deg.), 92 deg.; 120 deg.A orUGUST for 1999  31 (150 deg) add $10 SC-AUG-99 Fig.6: here are the mounting details for the Mosfet (Q1). Its metal tab must be insulated from the case using an insulating pad and bush. further supply decoupling for IC1. Power for the headlights is obtained from the +12V rail via the lights fuse. The ground for the circuit is connected to the vehicle chassis. simplified. In practice, the high and low beam circuits usually operate via relays but the circuit shows the basic scheme. Parking lights relay Fortunately, the circuit is a lot easier to build than to understand. All the parts, except for the LDR and the relay (RLY1), are installed on a PC board coded 05408991 and measuring 87 x 57mm. This is housed in a metal diecast case which provides the necessary heatsinking for Mosfet Q1. Fig.7 shows the assembly details for the PC board. Before installing any of the parts, check the board carefully for de­fects by comparing it with the published pattern. You should also check that the board fits into the case – you may need to round the corners off using a small file, so that it fits correctly. You may also have to file three slots into each long side of the board, to clear the vertical ribs along the case walls. Begin the assembly by installing PC stakes at the six exter­nal wiring points on the PC board. Once these are in, install the three wire links (one runs under IC4), then install the resistors. Table 1 shows the resistor colour codes but you can also use a digital multimeter to check the values. Next, install the diodes and zener diodes, taking care to ensure that they are all correctly oriented. The 16V zener (ZD1) will probably be marked 1N4745, the 15V zener (ZD3) 1N4744, and the 4.7V zener (ZD2) 1N4732. The ICs and transistors can all be installed now. Again, take care with their orientation and be sure to install the correct type in each location. Mosfet Q1 is mounted with its metal tab towards the edge of the PC board. The hole in the metal tab should be about 16mm above the board surface, although this is not critical. The capacitors can go in next but Relay RLY1 turns on the parking lights, although it’s the tail-lights that we really want. Its normally open (NO) contacts are wired in parallel with the parking lights switch. When Q1 is being pulsed, RLY1 turns on, the NO contacts close and the parking lights come on. Note that RLY1 does not pulse on and off as Q1 does. Its response time is too slow and the pulse frequency too high for it to do that. Instead, when Q1 is pulsed, RLY1 turns on and stays on. Finally, note that the circuitry inside the dotted line, showing the connections to the headlights and parking lights, has been considerably This close-up view shows the mounting details for the Mosfet (Q1) and for inductor L1. Secure the toroid to the board using silicon sealant and keep the winding away from the metal case so that it cannot short out. 32  Silicon Chip Construction make sure that the posi­tive leads of the electrolytic types go towards the positive (+) terminals marked on the overlay. The transformer T1 is a standard part and can only go in one way. On the other hand, you will have to wind L1 for yourself. It’s made by winding 12 turns of 1.25mm enamelled copper wire onto the specified toroid (see parts list). This winding should be installed so that it only covers about one half of the core. Be sure to install the toroid so that the windings are clear of the side of the case. If the wires touch the case, the enamel insulation will eventually wear through and the inductor will short the supply to the headlights to ground (taking out the fuse). Terminate the leads from L1 to the positions shown and scrape away the enamel insulation before soldering. The toroid can be secured using a cable tie. This loops through the centre of the toroid and passes through two holes in the PC board, on either side of the toroid. Now that all the parts are in position, temporarily place the assembly inside the case and mark out the position for the Mosfet mounting hole. This done, remove the board and drill the hole, plus an extra hole for the earth lug screw. You will also have to drill and shape a hole at one end of the case for the cordgrip grommet. Carefully deburr the Mosfet mounting hole using an oversize drill. The area around the mounting hole must be perfectly smooth to prevent punchthrough of the insulating washer. Before installing the board in the case, attach the flying leads to the external wiring points. The leads to the LDR The LDR connections are covered with heatshrink tubing, to make a neat assembly. Mount the LDR inside the vehicle and facing the floor, so that it doesn't pick up street lights. can be run using light-duty figure-8 cable, while all other leads should be run using heavy-duty automotive hookup wire. With the excep­tion of the chassis lead, these external leads should all be about one metre long or more. You can now fasten the PC board to the four mounting posts on the bottom of the case using the supplied screws. This done, attach the earth solder lug to the side of the case and fit the cordgrip grommet. Fig.6 shows the mounting details for the Mosfet. Note that its metal tab must be electrically isolated from the case using an insulating pad and bush. If you are using a mica washer for the insulating pad, smear all mating surfaces with heatsink compound before assembly. This isn’t necessary if you have a silicone impregnated glass fibre washer. After mounting the unit, use your multimeter (switched to a high ohms range) to confirm that the metal tab of the Mosfet is isolated from the case. The meter should indicate an open circuit between the two. Fig.7: install the parts on the PC board as shown in this wiring diagram. Inductor L1 is made by winding 12 turns of 1.25mm enamelled copper wire onto the specified toroid. Testing The circuit can be tested using a 12V adjustable power supply and a small 12V lamp. Tie the two +12V inputs together and connect these to the positive terminal of the power supply. The 0V rail of the power supply connects to the case of the unit. Connect the 12V lamp between the headlight/relay output and the case. Set the supply voltage to 12V and apply power. Now use a multimeter to check for +12V on pins 4 & 8 of IC1, pin 8 of IC4 and pin 1 of IC2. Pin 1 of IC4a should be high at about 10V (or more), while pin 7 of IC4b 7 should be low, at about 0.6V. You can also check that ZD2 has 4.7V across it and that ZD3 has 12-15V across it. This same voltage should appear between pins 1 & 8 of IC3. Note that you cannot measure these latter voltages with one multimeter probe connected to the case, as this is a fully floating supply. Instead, you must measure between the points indicated. Now slowly wind the 12V supply up to above 13V and check that pin 1 of IC4a goes low (0.6V) and that Q1 lights the lamp. The voltage across the lamp should measure about 10.4V. This represents the average voltage applied to the lamp (due to the 80% duty cycle). Finally, cover up the LDR so that it Fig.8: the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts. is in darkness. Check that pin 7 of IC4b goes high and that the lamp brilliance in­creases. The voltage across the lamp should now be close to 13V. If you don’t have a variable power supply, you can test the unit by connecting it to the car’s battery instead. Starting the engine should be sufficient to raise the battery voltage above 12.7V, so that the test lamp comes on (but be sure to do this in a well-ventilated area). Installation The completed unit can be installed either under the dash­board or in the engine compartment, which ever is the easiest for your car. Either way, the case should be secured to the vehicle chassis using self-tapping screws. The ground connection to chassis can be run via an automotive eyelet connector, secured with a self-tapping screw. Do not rely solely on the case connec­tion to chassis to make a good earth. The external relay for the parking lights can be mounted in any convenient location, while the LDR can be mounted facing the floor in one corner AUGUST 1999  33 Parts List 1 PC board, code 05408991, 87 x 57mm 1 diecast metal box, 115 x 65 x 55mm 1 iron-powdered toroidal core, 28mm OD x 14mm ID x 11mm (Jaycar LO-1244) or Neosid 17-742-22 (L1) 1 coupling transformer, 3kΩ-3kΩ, centre-tapped (T1) 1 cordgrip grommet 8 PC stakes 1 cable tie 2 crimp eyelets 2 M3 x 10mm screws, star washers and nuts 1 TO-220 mounting kit (insulating pad and bush) 2 extra self-tapping screws to mount PC board 1 1m length of 1.25mm diameter enamelled copper wire 1 100mm length of 0.8mm tinned copper wire 4 1m lengths of automotive hookup wire, various colours 1 1m length light-duty figure-8 cable 1 light dependent resistor (LDR1) 1 200kΩ vertical trimpot (VR1) 1 12V 20A automotive relay (RLY1) – Jaycar Cat. SY-4068; DSE Cat. P8035; Altronics Cat. S4335 Semiconductors 1 555 timer (IC1) 1 4N28 optocoupler (IC2) 1 4049 hex inverter (IC3) 1 LM358 dual op amp (IC4) 1 BUK456-60A N-channel Mosfet (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 1 16V 1W zener diode (ZD1) 1 4.7V 1W zener diode (ZD2) 1 15V 1W zener diode (ZD3) The completed unit can be installed close to the fusebox, either under the dashboard or under the hood (keep it away from the engine). If you do mount it under the hood, waterproof the case by running silicone sealant around the edge of the lid and over the cord entry grommet. of the dashboard (so that it doesn't pick up street lights). You will need to locate the following four wiring points: (1) the +12V ignition supply after the fuse; (2) the headlight supply after the fuse; 34  Silicon Chip (3) the lead between the lights switch and the dipswitch; and (4) the parking lights supply lead after the fuse. Use automotive cable for all wiring connections and termi­nate all leads in automotive-style crimp connectors. When the installation is complete, 7 1N914, 1N4148 switching diodes (D1-D7) 1 1N4936, FR104 1A fast recovery diode (D8) Capacitors 2 100µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 2 1µF 50VW RBLL electrolytics 1 0.1µF 250VAC X2 class polyester 1 0.1µF 63VW MKT polyester Resistors (1%, 0.25W) 2 1MΩ 1 4.7kΩ 1 150kΩ 2 2.2kΩ 4 100kΩ 1 1kΩ 1 68kΩ 1 470Ω 1 47kΩ 1 330Ω 3 10kΩ 1 47Ω 1 8.2kΩ 1 4.7Ω Miscellaneous Automotive connectors, etc. check that the low-beam headlights and tail-lights come on automatically when the engine is started. If they do, check that the lights switch overrides the circuit. The headlights should increase in brightness as soon as the lights switch is turned on and dim slightly when it is turned off again. Now check that the low-beam headlights come up to full brilliance when you cover up the LDR. Finally, check that all the lights go out when the engine is stopped (assuming, of course, that you’ve turned off the lights switch). When you are sure the circuit is operating correctly, it is a good idea to secure inductor L1 and its windings in place using some non-corrosive neutral cure silicone sealant (eg, Selleys “Roof and Gutter Sealant”). This will prevent the solder joints cracking due to vibration. Finally, you will have to adjust VR1 so that the headlights come up to full brightness at the desired light level. This is a trial and error adjustment and will have to be carried out at dusk. Please note: a modification to allow thus circuit to be used with cars having headlight switching in the negative line was published in Circuit NoteSC book, November 1999.