I’ve wanted a third brake light for my car for some time, but have never liked the added-on appearance of many of the solutions I’ve seen, even many factory ones. So, after thinking about it recently, I came up with this:


https://www.youtube.com/watch?v=Z_9JoKi9dzk


The sequence shown is, brakes on, brakes on, left turn, right turn, left turn with brakes on, right turn with brakes on, emergency flasher on. The light isn’t a part adapted from another vehicle. It’s just a 1”x1/2” rectangular bar of black plastic cut to the right length to fit closely under the roll bar with a ¼” wide slot down the middle of the long ½” edge that contains a one meter length of flexible LED strip of 5mm red LEDs. The bar is held in place by four 6mm flat head bolts, two on each side, that thread into the tapped holes that are already there on the underside of the roll bar, so no cutting of any metal on the roll bar is required, and the whole thing can be cleanly removed to return the car to a more original condition if that should be desired at some point in the future.

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When all assembled, the device is pretty inconspicuous. People don’t tend to notice it, even in broad daylight, unless they know it’s there and are looking for it and are pretty close to the car. It’s also invisible to the driver as he looks in the rearview mirror or out the rear window because the padding inside on the underside of the roll bar extends down more than the ½” thickness of the plastic bar.

The most important part of constructing it is making the two groups of holes for the bolts the right distance apart, so that, when the bolts are tightened down, the bar is forced up into a curve that closely duplicates that of the roll bar. The radius of that curve works out to be about 185”, and the two inner holes need to be precisely 42” apart. Shown below is a drawing of the bar as I made mine.

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I used Delrin for the material for its free-machining properties because I machined the slot that holds the LEDs on a milling machine. To reduce costs, ABS could probably be used instead. The ends of the bar are cut off at 30° to match the sides of the roll bar, and this can be done with an ordinary table saw using a carbide-tipped blade. The dual width slot, .250” wide to a depth of .375” and then .125” wide for another .285” of depth, is an attempt on my part to hold the LEDs aligned so they all point in more or less the same direction. The narrow, deeper part contains the flexible circuit and whatever wires are attached to it and holds it near the center of the bar, while the wider part fits the LEDs closely so that they are aimed straight back. Some way of doing this is advisable, since the LEDs have integral lenses and radiate nearly all their light into a 15° cone, which ideally should be aimed directly at the drivers of following vehicles. I wanted the light to be as inconspicuous as possible when it’s off, so I made a filter from dark red transparent 1/8” acrylic sheet to match the shape of the bar when it’s mounted, and attached it over the slot for the LEDs with a flat head screw at each end and a clip in the middle. I laser cut the plastic, which made it easy to make the precise shape, but also forced me to make it in two pieces due to size limitations of the laser cutter I had access to. Since the bar will flex back to straight when its mounting bolts are removed, the filter has to be removed first to avoid being broken, and this mounting arrangement makes it easy to do that, as well as accommodating its being two pieces.

The LED strip I used came from superbrightleds.com, and came on a reel like this:

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Unfortunately, I had to buy five times as long a strip as I really needed, but that gave me extra material both to experiment with and as a backup in case I ruined one or more strips. You definitely want LEDs with integral lenses for this application to keep the apparent brightness of the light to the intended viewers as high as possible. If I’d only been interested in making a third brake light, I could’ve just snipped off a one meter length of this strip, mounted it in the bar, run the two wires out and down into the engine compartment, hooked the wires to ground and to one of the brake light connections on the tail light assemblies, and been done. I wanted sequential turn indication too, however, so I needed to do some surgery on the LED strip to electrically isolate the groups of emitters that flash together and bring out additional wires to allow them to be turned on independently. The LED strip has its emitters hooked up electrically as parallel-connected groups of three with a current-limiting resistor in each group, like this:

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If you want all the LEDs to illuminate, you must break the power and ground connections only at the points between the groups of three emitters. So, any segment you create has to contain a multiple of three LEDs. For sequential indication, you have to have at least three groups on each side, so six total groups. A one-meter length of the LED strip, which is what fills the space between the mounting bolts nicely, contains 96 emitters. So, the six groups would each need to have 16 LEDs, but 16 is not divisible by three. So four segments, eight total, each with 12 emitters or four groups of three, is a better choice. This meant I had to bring out ten wires, eight from the positive connection to each group of 12, and a common ground for each side. Ten wires like those that come attached to the LED strip form a large bundle that would be difficult to run down to the wiring in the car in a concealed fashion. To get around this, I used 30 gauge wire wrap wire which is a single solid strand with a tough, close-fitting Teflon insulation. This wire is only .010” in diameter, counting the insulation, and yet able to safely carry more than 750ma of current. The wires called on to carry the most current are the two common grounds, each of which carries the current from all four of the groups on that side. Each group of 12 LEDs running on battery voltage draws about 60ma, so with all emitters on the grounds carry ~240ma each, which is three times less than their maximum.

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I used an Xacto knife to cut out a small piece of the flexible copper conductor and pry it off of the plastic backing at each location where I needed electrical isolation, then soldered a wire on to the part of the trace I’d just disconnected (solder pads not coated with protective varnish are thoughtfully provided by the manufacturer) and coated the exposed conductors with RTV to insulate and protect them against moisture. When all the connections were made, I had a group of wires that could be formed into a flat ribbon .100” wide and .010” thick, or into a round bundle less than .050” in diameter. As shown in the drawing, I cut a shallow slot in the top of the bar, the side that goes up against the roll bar, and connected it to the slot for the LEDs with a couple of intersecting ¼” holes to provide a channel to get the wires out. After I installed the strip of LEDs and ran the wires out this channel, I filled the narrow rear part of the slot, both holes, and the channel with RTV as a way of holding everything in place and making the assembly more weatherproof. I made no use of the adhesive strip that comes attached to the opposite side of the flex circuit, and left its release layer in place when I inserted the wired strip into the slot in the bar. The adhesive would otherwise stick to anything it touched and make the insertion very difficult. From the edge of the bar, the wires run straight to the roll bar trim on the driver’s side, down behind the trim to the point of closest approach to the small grille on the left side of the engine opening, and then down into the engine compartment. You can see the bundle where it runs from the bar to the trim and where it emerges at the bottom to go down into the engine compartment in the following photos. To protect the wires in the short run at the top where they’re exposed, I bent up a small metal jacket out of .010” stainless shim stock, painted it body color, and crimped it around the bundle of wires.

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The sequential signaling requires an active controller, and, although such controllers are offered by several aftermarket sources, I didn’t find one with the functionality I wanted, mainly the ability to use the same emitters for both turn signals and brake lights with preference given to the turn signaling function so that you still have turn indication when you’re slowing down to make a turn or sitting at a stop sign or traffic light with your foot on the brake. So, I built my own using parts of circuits I found on the web and modifying them to fit the 914 application. Here’s the circuit I used:

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The sequential drive signals are generated by the 555 timer IC and the 4017 counter chip. The 555 is wired as an oscillator that produces pulses at the rate determined by R3 and C2, and this is the rate at which the groups of LEDs are cycled off and on. The values shown result in about a 3Hz rate. Increase R3 to slow this down or decrease the value to speed it up. The pulses drive the 4017, which is a resettable decade counter that’s wired to reset after counting to four. Its output isn’t coded, which means it has a separate output pin for each number it counts up to. The four active outputs are now the sequential signals that drive the LEDs. Because the chip won’t drive enough current to power the LEDs directly, the outputs are fed to the bases of transistors Q3 – Q6 for the left side of the strip, and Q7 – Q10 for the right, which switch battery voltage to the appropriate groups of LEDs. We want the LEDs to light up with this sequential pattern only when the corresponding turn signal is on, however, so we need signals that indicate that the left and right turn signals are flashing. That’s the function of the 14538 chip, which is a dual retriggerable one-shot. Each section, when it detects a pulse on its input, generates a single pulse of length determined by C1 and R1 or C6 and R9. Both these pairs are chosen to cause an output pulse length of about one second. The flasher in my 914 outputs pulses at about a 3Hz rate, so, when one of the turn signals is active and the first pulse from the flasher has arrived, before the end of the one-shot’s output pulse another input pulse arrives, retriggering the one-shot and starting the one second-long pulse over again. This continues until the turn signal cancels and the pulses from the flasher cease. Then the one-shot pulse is allowed to run to completion, and a maximum of a second after the last flasher pulse occurs, the output drops back to ground. The result is a signal that is continuously high instead of intermittently so while the turn signal is running. These “turn signal true” signals are used as enable signals for the drive to the LEDs of the sequential signals, and that is accomplished by Relay 1 and Relay 2. The signals from the 14538 are buffered by Q1 and Q2 and then drive the two relays, which switch battery voltage to the banks of driver transistors that drive the LEDs with the sequential signals. The relays also ground the collectors of the driver transistors when the enable signals are not present to prevent any of the sequential signals from leaking though and producing visible output.

The circuitry for the brake indication is much simpler. When battery voltage appears on the connection to the brake lights, that voltage is applied directly to all the groups of LEDs through diodes D5-D12. The diodes are there to isolate the groups from each other when the sequential drive signals are present. As I mentioned before, I wanted the turn indication to take precedence over the brake signal, and this is also accomplished by the relays. While one pole of each double-pole relay switches power to the banks of driver transistors for the sequential signals, the other pole simultaneously disconnects the brake signal to that bank of LEDs. So, when indicating a turn with the brakes on, we get the turn signal running on the appropriate side and the full-on brake indication on the other.

Although this sounds like a lot of circuitry, I found it fits easily on a small (3.8” x 2.0”) PC board which in turn fits into a ~5” x 2.5” plastic enclosure that I mounted on the upright in the engine compartment that normally mounts one of the rain downspouts on a 4cyl car. Should anyone want to actually build one of these, I can share the PC board layout.

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I ran the bundle of small wires from the LEDs to a 12-pin Molex panel mount connector, which I supported with an aluminum bracket attached to the same upright to create a strain relief. Aside from connections to these ten wires, the controller needed five others: battery power and ground, left and right turn signals, and brake light connections. I ran wires along the existing wiring harness to the left and right tail light assemblies to pick up the brake and turn signal connections via piggyback tab connectors, and ran power and ground wires the opposite direction to pick up those connections from the relay panel. These five wires interface to the controller box through another Molex connector, so the box can be removed easily without disturbing the wiring.

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I’ve had this light on my car for a few weeks now, and, based on comments from those that have seen it in operation from the driver’s seat of another vehicle, I’m satisfied that it’s attention-getting enough. I’m also aware, however, that there are many on this board that would probably like such a light to be more so. There are several options for making the light more noticeable, in order of increasing heavy-handedness:

1. Animate the brake indication too. My personal impression is that animation is more effective than brute force brightness in attracting attention. Everyone I’ve talked to who’s seen it in operation comments first on the sequential turn signals. Consequently, the most effective measure may also be the easiest: simply connect one of these in series with the existing brake light input to the controller:

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This is a simple brake light strobe module sold by the same place that sold me the LEDs, superbrightleds.com. When the brakes are applied, this module will cause all the LEDs to flash rapidly (8Hz) for about 1/3 second, then flash at a lower rate (2Hz) for the remainder of three seconds, and then stay on continuously until the brakes are released. For $4.95 it’d be really hard to beat it with circuits you build yourself.

2. Drive the LEDs harder. Although everyone I asked about the light’s visibility said they thought it was more than bright enough, there’s no question that it would be more noticeable were it even brighter. The resistors on the strip are all 360 ohms, which, when driven by the ~12.8V car battery voltage, results in ~15ma through each LED. This is a very conservative drive level for these LEDs. They will probably run very reliably at 30-35ma, but, with the 360 ohm resistors, you would have to power the strip with ~18V to force this much current through each emitter. This could be done with a compact DC-DC boost converter that would probably fit on the same small circuit board. At this level, the total current to all the emitters would be about an amp, so each of the common ground wires would have to carry about 500ma and the resistors would be dissipating around 400miliwatts – still safe, but nearing the limit. I wouldn’t care to push those tiny 30AWG wires much further on a quasi-continuous basis like this. It would also be necessary to make some changes to the controller circuitry, namely, add a third relay operated by the brake light signal instead of driving the emitters directly with it, and have that relay switch the higher voltage to the LEDs. Doing all this would boost the brightness by about a factor of two.

3. Do both. You could obviously combine the previous two since the strobe module will work with up to 24V. The module would have to be connected to the controller differently, however, since you want it to work with the higher voltage which presumably is now generated within the controller itself.

4. Pulse the LEDs hard, but at a low duty cycle. This would take some experimentation to see what the LEDs, the resistors, and the small wires could reliably stand. What I mean is hit the LEDs with narrow pulses, maybe just a few milliseconds wide, up to something like 45V, resulting in a peak current of ~100ma through each emitter and an instantaneous power input to each resistor of 3-4watts, or perhaps even harder, but space the pulses out so that the average current and power levels are within the continuous limits for all three components. Also, use a slow enough pulse rate that the eye will see the pulses as separate and not simply average their brightness together, probably 10Hz or less. The LEDs would then look like rapidly-firing very bright strobe lights. This would make you look a bit like an emergency vehicle, but it would certainly be eye-catching. So much so, it might even be illegal.

nice

I'd be curious to see the sequence shot at night and the camera about 75' behind the car.
Building this is above my pay grade, but looks interesting.

Holy cow...that's a lot of work. Great write-up!

I'm not a big fan of the sequence for the turn signal, but love everything else!

For those of us that don't have the machining capabilities, any chance you'd be willing to supply some of these to interested members of 914World? This does look like an excellent safety addition for our beloved little cars!

Nice job.

These cars need a 3rd brake light. I'm looking for one that can mount inconspicuously like yours.

BTW, where did you get the PCB fabricated?

That is very nicely done!
Tom

OOOOOhhh! I love a 555 chip! Still within my grasp for understanding yet able to do so many things!

Thanks for the kind reaction everyone.

Spoke, I used expresspcb.com. I didn't have anyone to do surface mount assembly for me, so I used all thru-hole components. You could shrink the whole thing a lot by going to surface mount.

George, manufacturing, even in small quantities, isn't my cup of tea. I'd be happy to contribute all my design data if someone else wanted to do that, however. I've tried to think of ways others could build something very similar with more generally-available tools, though, so that a DIY project wouldn't be unreasonable. For example, I think you could probably cut the groove for the LED strip with an ordinary table saw instead of milling it, and you could definitely jig or band saw the filter instead of laser cutting it. TAP Plastics will laser cut pieces like this for you, but they have a $125 minimum charge. If you just wanted a 3rd brake light, you wouldn't need the controller or any of the delicate wiring. And if you wanted brake and turn indication, but didn't care about the sequential thing, you could simplify both quite a lot.