LED Headlights for Micro Scalextric Cars

Micro Scalextric is a smaller version of the popular slot car toy from Hornby Hobbies. Micro Scalextric uses 1:64 scale cars, powered by 9V. Though the Micro Scalextric cars are beautifully detailed models, they are not available with some of the cosmetic additions of the larger Scalextric models, like headlights and brake lights. This project modifies a Micro Scalextric car to add LED headlights, powered from the same track that supplies the car's electric motor. This Instructable assumes that you are competent at soldering - there is some very delicate work to be done.

This project is for "version 2" of the Micro Scalextric system, introduced in 2019. The general principles should be the same for other versions of Scalextric or brands of slot car, but you would need to be very sure of power supply voltages and polarity to proceed.

Parts

(datasheets for the regulator and LEDs are in the links)

1. Tiny white LEDs. I used these tiny 1.6 x 1.2mm LEDs: https://www.digikey.com/en/products/detail/qt-brightek-qtb/QBLP617-IW5/6051821
2. 5V voltage regulator. I used an LP2950: https://www.digikey.com/en/products/detail/texas-instruments/LP2950ACZ-5-0-NOPB/148221

The following parts may be substituted for what you have on hand.

1. 440 ohm resistor x2
2. 2.2 uF capacitor, 15V minimum
3. 1 uF capacitor, 15V minimum
4. Small piece of prototype board, "Veroboard" or similar
5. Fine insulated wire to solder to your LEDs
6. stranded insulated wire to join your circuit to the car's electrical pickup
7. Clear hot glue
8. Soldering kit

Micro Scalextric cars are powered by 9 volts. The hand controllers (throttles) for the cars are a simple resistor in series with the track that reduces the voltage seen by the car's motor. Therefore our circuit needs to cope with a range of voltage, up to a maximum of 9V.

The LEDs I used require 2.8 volts at a current of 5 milliamps. To drive an LED from a DC voltage, we usually use a resistor to reduce the voltage seen by the LED, and to limit the current that the LED can draw. But our supply voltage will vary depending on the position of the throttle. So, if we choose a resistor expecting 9V, our LEDs will be dimmer than expected at anything less than full throttle. If we choose a resistor to give us bright LEDs at part-throttle, we run the risk of burning out the LEDs at full throttle.

So, we need a way to supply a stable voltage whenever the car is moving, regardless of the position of the throttle. For this project we can use a simple linear regulator. The regulator we have chosen will accept any voltage from about 5.5 V to 30 V and give us 5 V on its output pin. Then we use a resistor to drop the voltage to 2.8 V at 5 mA for each LED. We have two LEDs, and while it is tempting to just use one resistor of appropriate specification to power the two LEDs in parallel, this is usually a bad idea. Each LED is slightly different, due to manufacturing tolerances. Driving them in parallel from one resistor will result in one LED receiving more current than the other, and therefore one LED will burn out much faster. As soon as one LED burns out, the other will receive double its rated current, and therefore it will burn out soon after. Resistors are cheap, so just use one for each LED.

To calculate the resistor, we use Ohm's Law V = IR, where V is voltage, I is current in amps and R is resistance in ohms.

We have 5 V coming out of the regulator, and we need 2.8 V for the LED. Therefore we need to drop 2.2 V. Each LED needs 5 milli amps, which is 0.005 amps. Now we can rearrange the formula to make R the subject of the equation: R = V/I. Dividing 2.2 volts by 0.005 A gives us 440 ohms. So our target resistance is 440 ohm. I like to increase the resistance by 10 - 20%, to ensure that the LEDs aren't close to their current limit. In this case, I already had some 1000 ohm resistors in stock, so I used two of these in parallel per LED, to give a total resistance of 500 ohms per LED. A schematic of our design is in the pictures, and the Kicad design files used to draw the schematic are also attached. You will need to rename the file from .txt to .kicad_sch in order to open it.

The circuit above is pretty simple, so I have assembled it on a piece of prototyping board, also known as "stripboard" or "Veroboard." If you are very careful you could probably wire it up without any board at all, in a style known as "dead-bug" or "circuit sculpture." If there is enough interest I may design a little custom printed circuit board to facilitate this step, but as you will see in later steps, the tricky part is soldering the LEDs, not the rest of the circuit. Unfortunately I didn't get a clear photo of the finished circuit board, but hopefully the annotations make it clear.

This is the fun part! It's also the part where one slip can ruin the appearance of you car, so work carefully and think about each cut or hole before you make it. For example, I used a drill bit held in my fingers, instead of a power drill. A power drill would be faster, but much easier to slip and ruin the car in a moment.

The main idea is to drill a hole in the non-functioning moulded headlights of the plastic car body, then fill the hole with a little tiny bit of hot glue to act as a light pipe and lens for the LEDs

1. Make a pilot hole. I used a bent paperclip, heated with a lighter to carefully poke through the plastic in the middle of the headlight.
2. Drill the hole. For this Aston Martin car, a 2mm drill bit was the right size to completely remove each painted-on headlight.
3. From the rear (inside the car body) slowly squeeze a small drop of hot glue through each drilled hole. The glue should form a pleasing lens shape that protrudes slightly out the front of the hole.

This is the fiddly part. I used thin 30 gauge "wire wrap wire" to attach the LEDs I dabbed each metal surface with a flux pen before soldering. Remember that LEDs are polarised - if you put them in the wrong way around you won't break anything, but they won't illuminate. The LEDs I used have a marking on the back in green to indicate their polarity. After you have soldered both wires, it is a good idea to mark one of the wires to remind you of the polarity after the LEDs are glued in place, in case you can no longer see the marking on the back of the LED.

1. Use another spot of hot glue to secure each LED behind the holes you drilled earlier.
2. Solder the fine headlight wires to the output of each resistor pair.
3. Test your circuit from a 9V battery or power supply to make sure both headlights are working
4. Solder power wires from your circuit board to the pickup that supplies the motor. I added a drop of hot glue here after soldering, to provide some mechanical stability.
5. Glue the circuit board out of the way in the top of the plastic car body. You can see where I found space in the car I used.
6. Carefully re assemble the car, taking care not to pinch any wires or foul the moving parts.

So far the circuit seems to work well - the brightness of the headlights remains the same at part-throttle as with the trigger squeezed 100%.

I haven't done any heat calculations, but the whole circuit should only dissipate a fraction of a watt, much less than the motor, so I am hoping the airflow through the moving car will be enough to keep everything cool.

Each car will be a little bit different, and may require different LEDs to fit. If different LEDs are used, you will need to do the calculations for the resistors again.

If there is enough interest in this project, I may do a small production run of PCBs to make the job a bit easier. The LEDs would still be up to you, because each car will have slightly different requirements.

A custom PCB would also make it more practical to add an extra channel for some nice red tail lights, if the host car is suitable...