The Mazda will be lit up using LED lighting, similar to the lighting in the Ferrari 250 GTO. The headlights, indicators, tail/brake lights, rain light and rooftop warning light will all work. I will put a row of switches on the display base so that spectators can play with the lights.
In this post are some details about how the lighting is done. The electronics are mostly re-used from my earlier build; I am republishing this post into the rebuild episodes. First off, some basics:
- I am using SMD (Surface Mounted Device) LEDs, which are tiny and fit almost anywhere. I use a mix of sizes 0603 (0.8mm x 1.6mm) and 0402 (0.5mm x 1.0mm), in various colours. I buy my LEDs with thin wires already attached.
- Each SMD has to be connected to the power supply (9V in my case) with a ‘ballast’ resistor to limit the current in the circuit, otherwise the LED will burn out instantly. The size of the resistor can be varied to alter the current in the LED, and thus adjust its brightness. Multiple LEDs can be strung together in one circuit, so they will light up together (for example, two headlights). [Actually, you can reliably only string so many together like this; eventually the combined voltage drop in the string of LEDs will exceed the powering voltage you have in the circuit and none of them will light up. I try to limit things to just two LEDs in series.]
- I am using a commercial LED flasher unit (designed for model railway crossing signals) to get the indicators and other lights to flash. After some tinkering, it seems like this unit likes every individual LED to be on its own output circuit. The unit already has resistance in the LED outputs, but I add one to each output to set the brightness.
So, more specifcally, I have created a schematic for the full lighting setup, available on the Documentation page. Here’s how it will work:
- Switch 1 will work the headlights and tail/brake lights. With some magic, it allows me to have three positions: OFF, LO and HI. The ‘LO’ circuit runs the outer two headlights, plus both tailights. The ‘HI’ circuit lights up the two inner headlights. On the HI position of the switch, both LO and HI circuits turn on.
- Switch 2 lights up the tail/brake lights as brake lights, which means they will be brighter than when they are being used as tailights. Because of this dual use of the tail/brake lights, a ‘barrier diode’ is needed on the Switch 1 / LO circuit. This trickery is needed because the Switch 1 / LO circuit effectively joins the headlights and tail/brake lights together… without the diode, when Switch 2 turns the brake lights on, the headlights would turn on (dimly) as well.
- Switch 5 turns either the left or right indicators on. They flash, and the three indicators on each side flash in unison. Actually, Switch 5 does a couple of things – it sends power to the flasher unit and connects the left or right output circuit. This way the flasher unit doesn’t run the battery down when the lights are off.
- Switch 6 turns the hazards (and flasher unit) on, which causes all left and right indicators to flash, all six in unison. There are a couple more diodes in the hazard circuit as it effectively joins the left and right circuits… without the diodes, when one side flashes, the other side would also flash (dimly).
- Switch 3 turns on the rain light (and flasher unit). The rain light will flash opposite to the indicators when they are on, which I believe to be per the Group C technical rules (grin).
- Switch 4 turns on the alert light on the roof. I still haven’t found out what this light is for on the real car.
- All the wires to the LEDs in the car are connected to terminals on a custom PCB which carries all the ballast resistors, circuit connections and diodes. This lives with the battery, the flasher unit and a little PCB for the hazard circuit (later designs have this integrated into the main PCB) in a void cut into the display base. Here are some pictures of the PCBs and other display base components:
- In my documentation, I have laid out how the wires will be routed from the display base, up into the car and out to the LEDS. Generally speaking, the wires are hidden inside the model and won’t be seen; wiring for the lights in the tail section is seen and somewhat models the real wiring harness.
- One other piece of magic is the connection to the front cover, which is removable, and on which is mounted the headlights and two indicators. On the real car, there is a plug on the cover which connects to a socket located the left side under the front of the windscreen. I have replaced the modelled socket with a working 6-pin socket at scale size (about 3mm diameter) which will connect to a working 6-pin plug on the front cover, which will route power to the forward lights. I am also making up an extension lead to power the front cover when it is off the car.
You can see the plug and socket assembled into the partially constructed wiring harness, which is sitting on the pin board in these photos. These photos of of my earlier build; for the new build I have split the wiring into two sections (front and rear of the car) which simplifies assembly of the body/engine/tail to the floor/side pods.
Jason's Mazda 787B Build 