HOME-MADE SLOTCAR LIGHTS

I'm sold on lighted slot cars, but if you've got a lot of cars you want to light, it can get awfully expensive to buy store-bought kits. The kits are kind of a paradox anyway: Unless you've got a relatively drop-in installation, fitting the lights and circuit board in the limited interior space of a slot car is probably harder and more time-consuming than scratch-building the electronic circuit. Scratch building the circuit also lets you position individual components where there's room; you aren't forced to find a single large space for a circuit board. In that sense, it makes the installation easier.

At the end of the previous lighting article, I'd breadboarded my version (first pic) of the full circuit shown at Brent Carlson's website. I'd made a few component substitutions, and modified the taillights so that they'd function as both running lights and brake lights. Using a 9-volt battery, everthing appeared to work as it was supposed to. However, when I tested the circuit on the higher voltage range of my track, the brake light portion of the circuit showed a bug: The brake/running lights would remain constant for the lower voltage range, but suddenly vary in brightness above that range. My guess is that the generic PNP transistor I'd used couldn't handle the higher voltage and would turn into a full-open spigot. Rather than trying to find the right transistor and the right values for the biasing components, I decided to simplify the circuit and remove the brake portion. This had a couple of benefits: It would be easier, quicker, and cheaper to make, and I wouldn't have to figure out where to mount the transistor and electrolytic capacitor in the car. Interior space is scarce and I wanted to mount a circuit in/out switch. While the brake function would have been nice, it wasn't essential.

Besides the in/out switch, there were a few other things I wanted to try. I've been curious about the proper color of the headlights since I've heard that Le Mans headlights were amber. The amber headlights in Scalextric's Ferrari P4 330 (right) look weird in my opinion (though better in the picture than in real life), so I rewatched "Le Mans" and Googled "Le Mans 1970" for pictures of headlights (Be sure to visit IMCA SLOT-RACE WEB if you like this vintage Le Mans stuff!). While some photos did show a reddish tint, or halo, it was very hard to guess what the lights might have really looked like since film, cameras, and monitors affect color rendition. Although amber LEDs would have been ideal since they come in a clear package, the light they cast somehow looks wrong-- maybe it's too pure a shade of orange? My yellow/greenish tinted LEDs aren't even close to amber, but IMO, the light looks more "real". At any rate, I wanted to add a touch of red this time. Unfortunately, adding red makes the LED stand out in the lamp reflector, which looks funky if the lights aren't on.

There are a couple of photos of the 1970 Le Mans winner (the red & white Salzburg #23 Porche 917K) that show the outer headlight lens with an odd golden glaze. I don't know what caused this effect: Dirty lens? Moisture condensation? Reflection from the sky? Nevertheless, it gave me the idea of lightly dusting the interior side of the lens with gold dust. The coating is translucent enough so that, depending on external lighting, viewed dead on or from the top, the lens is totally transparent. If viewed from an angle, the gold reflects light, giving an odd golden glaze. This is what convinced me to try a more boldly tinted LED-- from most viewing angles, the red tint isn't obvious. Although I'd intended to apply these ideas on Fly's 1970 Le Mans winner, I wanted to test the ideas first-- especially since I wasn't sure if my funky home-made lighting circuit was going to melt through to China. What better car to subject to this treatment than Fly's recent 1970 Le Mans Coda Lunga SF Test car? (Glad I did because I learned that you shouldn't use a Bic lighter on heat shrink tubing that's already mounted in the plastic chassis!)

 

The circuit's resistor values were trial and errored-- (I'm just a common-sense tinkerer, not an engineer!) The taillights were too bright and needed to be toned down a bunch; I don't know about the design wisdom of running LEDs in parallel through a single resistor, but it doesn't seem to develop any noticible heat. The 10-ohm resistor in line with the headlights may seem risky, but it does produce some bright headlights, and the voltage is less than 3.3 (not by much though). When power's removed, the Gold Cap drops almost instantly to about 2.8 volts, where the voltage begins to drop more slowly.

You don't get a true impression of interior space from looking at the two halves, but once they're screwed together, you can't see anything! Although it looks like there's a lot of room, the cockpit fits very tightly against the chassis, and the magnet pocket eats up a prime piece of real estate. While there's plenty of space at the rear, it's not a good idea to add extra weight there unless you want to increase fishtailing and do wheelies. Black-painted aluminum tape keeps the rear area opaque so that the taillights shine only where they're supposed to.

The area in front of the driver's feet seemed almost tailor-made for the LM7805 voltage regulator, with an angled cut that the regulator can almost fit into. I cut off most of the heat sink, reasoning that the current draw would never be very high. However, the part that's left is positioned facing outward (not against the plastic), just in case. Although the components and wiring layout makes shorting pretty unlikely, heat-shrink tubing was used pretty extensively, just in case.

Distributing the components like this really did help with the fit. The only tight fit was with the Gold Cap and resistor-- the magnet pocket forces anything in that slot behind the cockpit to be located either to the far right or left. Surprisingly, the switch fit perfectly, both in depth and width. The tabs needed to be folded down, but even after wiring, there wasn't a hint of bulging, binding, or compression.

Fly's cars can be tough to reassemble even if you don't add junk to the innards. That's because the motor leads don't have an clear path to the guide, and the tight fit between the chassis and cab often pinches at least one of the wires. This pins the wire, and unless you make sure that guide can rotate freely, you may have to loosen screws and try again. I suspect that the wires may have been intended to be threaded through the channel between the seats, but the way the part is cast doesn't produce enough of an opening. That's easy to fix with a Dremel, and while it adds another thing to align during reassembly, the wires don't get pinched. I also cut a groove leading from the motor mount to help align the wires, but I don't think it's needed.

 

I'm pleased with the way the home-made circuit performs. Unlike the Ninco circuit, the lights come on the moment you apply power. The downside is that the moment you remove track power, the circuit runs off of the Gold Cap, which produces the drop in lighting intensity that I mentioned earlier. Still, the change in output isn't really a distraction while you race the car. After power is cut, the Gold Cap runs the headlights and taillights for quite a while. Eventually, the headlights go out, but the taillights glow for a loooong time after that. Evidently, the Ninco circuit's lights runs entirely off of their Gold Cap, which explains why the lights don't go on until the Gold Cap is charged-- I tried to analyze their circuit, but it's on a double-sided circuit board with printing, which makes it hard to follow traces.

The Ninco circuit is clearly superior though, and a good value. Although components (including LEDs) in the home-made circuit are approximately 1/4 to 1/3 of the cost of the Ninco product, it's a much simpler circuit with far fewer components: The Ninco circuit has 4 LEDs, 12 resistors, 3 transistors, 3 diodes, 1 zener diode, 1 capacitor and the Gold Cap-- all fitting on a rectangular circuitboard not that much larger than the LM7805 voltage regulator! Besides having the braking function, the Ninco circuit also handles low voltage levels better. The home-made circuit doesn't work very well in a pace car, since pace cars run on a constant, but lower voltage. This may or may not be enough to turn on the lights. I wasn't even aware of this until I ran it as a pace car-- when I drive a slot car, I don't drive slowly, at a constant speed. However, the Ninco circuit handles that with no problem.

The main virtue of the home-made light circuit is that it gives basic, off-the-track (a.k.a. "permanent") lighting on a budget. This may be just the ticket if you've got lots of cars you want to light and don't want to spend a lot of money on a higher-quality product. It's also a lot of fun to make, and especially gratifying when it works!

I think the tinted LEDs came off well, and seem in the ballpark of Le Mans photos I've seen. Maybe I'm just being too anal about this, but I was concerned with how they'd look when the lights were off. Indeed, the bottom left pic shows how dorky they look with the reddish lightbulbs (but I can get over that). However, the typical viewing angle is more like the picture at the top, showing the light reflection off the gold powder. This tends to obscure the view inside the lens, so they don't draw attention as much as they might. I'm not sure that I'd want to do that to all the cars I put lights in, but as I said, this was a practice run for the Salzburg #23 car.

I think the in/out switch is a good addition since it gives the car another "mode", if you don't want to run lights during the daylight hours. The switch cuts power to the lighting circuit, but doesn't cut the lights out of the circuit, so the lights gradually grow dimmer as the Gold Cap slowly empties... The taillights seem to take an eternity to drain, so it's taillights only for quite a few laps around the track.

--05/14/06

(PART 1) LIGHTING A FLY PORSCHE 917K
(PART 3) DEFILING COLLECTIBLES

 


So many collectibles to defile, so little time...