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FLASHER CIRCUIT FOR AUXILIARY BRAKE LIGHTS Reduce your chances of a rear-end shunt by building this simple brake lamp flasher circuit. It drives an auxiliary brake lamp mounted on the rear parcel shelf. By BOB FLYNN & GREG SWAIN Auxiliary brake lamps mounted on the rear parcel shelf are all the rage these days, especially in the major cities. Because they are mounted more or less at eye level, they are much more visible than conventional brake lights and so can significantly reduce the chances of a rear end collision. Although some of the latest cars feature an auxiliary brake lamp as standard (eg, some Honda models), most devices fitted to cars are aftermarket ''add-ons''. These devices are available for about $10 (or less) from stores such as K-Mart and Target (or from auto-accessory shops) and are easily installed. All the motorist has to do is mount the brake lamp on the rear parcel shelf and connect it to the existing brake lamp wiring in the boot. Other units, like the one featured here, come with a self-adhesive mounting surface so that the unit can be affixed directly to the window glass. The angle of the red lens surface can then be set for best visibility by undoing a screw and rotating a small knob on the side of the unit. Making it flash By adding the simple flasher cir220 100 + 16VWr AUXILIARY BRAKE LIGHT 4.7M 4.7M IC1 555 0.15:r. .,. 0.33! 01+ . 0.47! .,.. ~ BCE SAFETY BRAKE LIGHT FLASHER Fig.1: when power is first applied, 555 timer IC2 drives Darlington transistor Ql to flash the auxiliary brake lamp. After about 2 seconds, pin 3 of monostable stage ICl goes low and stops IC2 from oscillating. The lamp then stays on continuously. 30 SILICON CHIP cuit described here, you can make the auxiliary brake lamp even more eye-catching. A flashing lamp is much more easily seen than a lamp which is on continuously whether in daylight or at night time. Our flasher circuit can either be added to an existing auxiliary brake lamp or used in a new installation. The original car brake lamps will be unaffected by the flasher circuit. The circuit doesn't flash the brake lamp continuously though. That would be too annoying for the driver behind in bumper to bumper traffic. Instead, when you first apply the brakes, the Brake Lamp Flasher quickly flashes the auxiliary brake lamp five or six times. After that, the auxiliary lamp stays on in the conventional fashion for as long as the brake pedal is depressed. The brake pedal must then be released for about 1.7 seconds (or more) for the flasher circuit to resume operation. If, on the other hand, braking is reapplied inside the 1.7 second period, the auxiliary brake lamp will simply turn on and remain on until the brakes are released. This delay time makes the light much less annoying for the following driver. About the circuit Two 555 timer ICs and a BD645 Darlington transistor form the basis of the circuit. Fig.1 shows the details. In practice, the two 555 timers (ICl and IC2) function quite differently. ICl is wired as a monos t able timer, while IC2 operates in astable mode. Let's ex- PARTS LIST 0 1 PCB, code SC051 0489 1 , 73 x 55mm 2 2-way PC-mounting terminal blocks 1 plastic utility case, 130 x 68 x 41mm 1 T0-220 mica washer and insulating bush 4 1 0mm PC standoffs 1 5mm rubber grommet +12V FROM BRAKE LIGHTS--411a AUXILIARY ---:-111-1+,,,.Jl:'J BRAKE LIGHT-..:.tli-lli. EARTH TO CHASSIS Fig.2: here's how to install the parts on the PCB. Be sure to install the two ICs the right way around. The Darlington transistor (Ql) mounts underneath the board (see Fig.3). plain those two terms before we go any further. A monostable timer is one that produces a single pulse output of predetermined time. For example, a one-second monostable is a circuit that produces a single pulse one second long. An astable timer is a freerunning oscillator with an output which alternately switches high and low. Now let's see how the circuit works. When power is first applied (ie, the brake pedal is depressed), pin 3 of ICl goes high and turns on Q1 The PCB is mounted on the aluminium lid of the case to provide heatsinking for the Darlington transistor. You can buy the auxiliary brake lamp from auto accessory shops or from stores such as K-Mart and Target. and the auxiliary brake lamp. IC2 then begins oscillating with its pin 3 output switching alternately high and low at about 5Hz. In turn, this switches Q1 on and off at about five times a second to flash the auxiliary lamp. · ICl controls the time for which IC2 can oscillate and therefore flash the lamp. By functioning as a monostable its output at pin 3 is initially high and stays high for a time determined by the 4.7MO resistor and 0.33µF capacitor at pin 6. This time period is 1. 7 seconds, after which pin 3 goes low. This then pulls pin 6 of IC2 low via Dl. Therefore after 1.7 seconds, ICl stops IC2 from oscillating and the brake lamp stays on continuously Semiconductors 2 1 1 1 555 timer ICs (IC1 , IC2) 8D645 Darlington transistor 1N4148 diode (D1 ) 15V 1 W zener diode (ZD1) Capacitors 1 1 1 1 2 1 OOµF 16VW electrolytic O.4 7 µF metallised polyester 0.33µF metallised polyester 0.15µF metallised polyester .O1µF metallised polyester Resistors (0 .25W, 5%) 2 4.?MO 1 1 MO 1 1 OOkO 1 1k0 1 220 Miscellaneous Cable ties, automotive cable, automotive connectors, machine. screws and nuts, split washer,' star washers. APRIL 1989 31 Fig.3: this diagram shows the mounting details for the BD645 Darlington transistor. The metal tab of the transistor must be isolated from the lid using an insulating bush and mica washer. 0 Ho + t_.i -$- I 0 + + 29 ALL HOLES 3.9mm DIA. 0 31 DIMENSIONS IN MILLIMETRES Fig.4: here is the drilling pattern for the metal lid. Drill all holes to 3.9mm (ie, 5/32-inch). When pow&r is removed from the circuit (ie, after you take your foot off the brake pedal), the 0.47 µ.F capacitor needs time to discharge via the 4. 7MO resistor. If it is not almost completely discharged at the next brake application, pin 2 will inhibit the monostable function. This means that pin 3 will remain low and pin 6 of IC2 will also be held low via D1. Thus, IC2 will be prevented from oscillating and so the auxiliary brake lamp will light up without flashing. In practice, for brake applications greater than about two seconds apart, the auxiliary brake lamp will be allowed to flash. Conversely, for rapid brake applications less than two seconds apart, the auxiliary brake lamp will not flash but will come on at the same time as the main brake lamps. Power for the circuit is derived from the + 12V rail which runs to the main brake lamps (ie, after the brake lamp switch). A 220 resistor and 100µ.F capacitor decouple the supply while zener diode ZDl clips any supply line spikes to + 15V to protect the 555 timer ICs. Note that since power for the circuit comes from the brake lights, no supply voltage will be present in the circuit unless the brakes are applied. Building it Use your multimeter to check that the metal tab of Qt is correctly insulated from the metal lid after the assembly has been bolted together. The PCB is mounted on four 10mm-long standoffs. while ever the brake pedal is depressed. Resetting ICt As noted above, we don't want the lamp to flash if the brakes are released and quickly reapplied, to avoid annoying the driver behind. This is taken care of by the trigger 32 SILICON CHIP function at pin 2. At the first power application, pin 2 is held low by the 0.47 µ.F capacitor. This allows the monostable function just described to take place. But while the 0.33µ.F capacitor is charging, the 0.47 µ.F capacitor at pin 2 is also charging via its associated 4. 7MO resistor. Construction is straightforward. All the parts are mounted on a small printed circuit board (PCB) coded SC 05104891 and measuring 73 x 55mm. This is housed in a small plastic utility case with a metal lid which acts as a heatsink for the BD645 Darlington transistor (Ql). Install the parts as shown on the layout diagram of Fig.2 but don't mount Ql at this stage. It's best to install the resistors, diodes and the two ICs first and then move on to the capacitors. Be sure to mount the two ICs the right way around. Pin 1 of an IC is always adjacent to a dot or notch in one end of the body moulding (see Fig.2). The two diodes and the 100µ.F electrolytic capacitor are also polarity conscious and must be correctly oriented. Check the resistor values on your digital multimeter if To identify the brake lamp lead on double-filament lamps, connect a multimeter between one of the lamp terminals and chassis. The meter should read about + 12V when the brake pedal is pressed and OV when the pedal is released. you have difficulty reading the colour bands. A PCB-mounting screw terminal block is used to terminate all external wiring connections to the PCB. Four-way terminal blocks are not reaily available so we suggest that you simply use two 2-way blocks side-by-side. Since more than 1.5 amps flows through the BD645, it must be attached to the metal lid of the case for heatsinking. Fig.3 shows the mounting details. To install this transistor, first bend the leads at 90° at the point where the leads suddenly narrow. The transistor should then be soldered in position so that the ends of the leads just sit flush with the top surface of the board. Fig.4 shows the drilling details for the metal lid. Mark out and drill all holes to 3. 9mm diameter (5/32-inch). Once this has been done, carefully deburr all holes us- +0 + SC05104891 0 Fig.5: full-size artwork for the printed circuit board. ing _an oversize drill. In particular, make sure that the mounting hole for the BD645 is free of metal swarf. A TO-220 mica washer and insulating bush must be used to isolate the metal tab of the BD645 from the heatsink. Smear all mating surfaces with heatsink compound before bolting the assembly together. It may be necessary to trim the length of the insulating bush so APRIL 1989 33 This auxiliary brake lamp is affixed directly to the window glass but some units are designed for mounting on the rear parcel shelf. + 12V FROM BRAKE LIGHT SWITCH CRIMP CONNECTOR '-sTOP LIGHT Fig.6: once you've identified the brake lamp lead, it can be connected to the + 12V flasher lead using an automotive crimp connector. Don't forget to connect the earth lead to chassis. that the screw can be tightened correctly. Don't forget the split washer under the nut. If you leave it out, the nut will eventually shake loose. The PCB can now be secured to the lid using four 10mm standoffs, machine screws and nuts, and shakeproof washers. After that, it's simply a matter of drilling a hole in one end of the case to allow for cable entry. A rubber grommet should be fitted to this hole to prevent the cable from chafing. Installation To ensure proper operation, the 34 SILICON CHIP brake lamp flasher should be connected as shown in Fig.6. Begin installation by mounting the auxiliary brake lamp inside the vehicle according to the manufacturer's instructions. Once this has been done, you can mount the flasher circuit under the rear parcel shelf. In most cases, it can be secured using two self-tapping screws through the rear of the case. Now run the power cable from the auxiliary brake lamp down behind the back edge of the parcel shelf and route it inside the boot to the flasher circuit. Pass the cable through the rubber grommet and trim the two leads before connecting them to the terminal block. Nylon cable ties or electrical tape can be used to secure the cable in position. If your car is a station wagon or a hatchback, the auxiliary brake lamp can be mounted at the top of the rear window and the power ea ble run along next to the window moulding. In some cases, it may even be possible to pull back the moulding and tuck the cable beneath it. The earth lead can now be connected between the terminal block and chassis. To do this, connect the earth lead to an automotive eyelet terminal, then secure it to the car body using a self-tapping screw. The + 12V power lead must be run from the flasher circuit to one of the tail light assemblies. If you can, route this lead alongside the existing wiring loom. Ideally, this lead should be the same thickness as the car wiring, as this makes for a more reliable connection when using a cable squeeze connector. Double filament lamps On most cars, double filament globes are used for the tail and brake lamps. You can easily identify the brake lamp lead by using a multimeter switched to a voltage range. Just connect the meter between one of the lamp terminals and earth while someone else presses the brake pedal. An automotive squeeze connector is the best way of connecting the flasher lead to the brake lamp lead. A suitable connector will usually be supplied with the auxiliary brake lamp, along with cable ties and an eyelet terminal and self-tapping screw for the earth connection. Finally, test the installation thoroughly. The auxiliary brake lamp should only come on when the brake pedal is pressed. Check that the lamp flashes five or six times and then remains on until the brake pedal is released. If the auxiliary lamp comes on when the parking lights are switched on or is activated when the turn indicator is used, the flasher circuit is conn1;Jcted to the wrong lead at the tail light assembly. It