Overview

I developed this after my nephews got a Scalextric Sport slot car racing set for Christmas.  The set included a lap timer and counter but one thing that was missing was a way to start a race.  Having someone say "3-2-1-Go" wasn't really doing the job, especially as it was easy to go just after "1" and before "Go"  This works in a similar way to the lights used in F1 motor racing with five lights illuminating sequentially, holding for a random time before all lights turn off to signal the race start.  

I put together a battery operated version one afternoon between Christmas and New Year, it took about four hours from concept to completed hardware in a box. [see original] It worked quite well but we still had the issue of 'jump starting' so I decided to develop things further.

I had four key requirements for the design.

• It should work with the standard Scalextric power-base (part C8217), throttles and transformer pack.
• It had to be simple to use.
• Powered from the Scalextric power supply, no extra transformers or batteries.
• Not require any alterations to the Scalextric power-base, throttles or cars.

I know that there's a lot of computerised solutions that will do all kinds of smart things out there, including controlling the start of a race, but not everyone wants or needs that level of complexity.  The device described on this page is easy to use, cheap to build and doesn't require any permanent modifications to the original Scalextric setup, although it can be hard wired in to permanent setup should you wish to do so.

Using the controller is very simple. To start a race simply press the start button.  The controller than waits for both hand throttles to be released before starting the countdown.  During the countdown the LEDs illuminate at 1 second intervals until all five LEDs are on.  The start of race is signaled when the LEDs extinguish after a random delay of between 0 and 3.5 seconds. 

The start controller detects a jump start by sensing the hand throttle; if it's operated during the countdown the controller will either ignore it, display a Jump-Start condition on the LEDs after the countdown completes, or abort the countdown and display Jump-Start condition on the LEDs.  This action is determined by the operating mode which is user configured.

Operation and modes

• Mode 1 - Turn track power off during start sequence, restore power at 'Go' signal.

• Mode 2 - Normal start countdown, no jump start detect.

• Mode 3 - If jump start detected during countdown, track power is cut, start sequence aborts and jump start condition is displayed on LEDs..

• Mode 4 - If jump start detected during countdown, start sequence continues and then one second after 'Go' signal Jump start condition is displayed on the LEDs.

Graphical representation of the four modes are shown in the PDF document here.

If the Controller is used on its own without the PSU described here then modes 1 and 2 behave in the same way. Reference to the track power is also only relevant when used with this PSU.  The Controller still functions in all other respects as described.

Start Race

To start a race countdown, press and release the start button.  LEDs 2 and 4 light if the controller has detected an open throttle. Countdown commences when both throttles are released.

Change Modes

To change modes, press and hold the start button for 3 seconds. When the controller goes to mode change setup a single LED will light indicating the current operating mode.  Repeatedly pressing the start button will cycle through the four modes. When the desired mode is selected press and hold the start button for 3 seconds until the LED turns off.  

Operating modes can be changed at anytime, except when a start is in progress. The mode is saved to non-volatile memory so the controller will start up in the mode that was selected when it was last used.

Construction

Full details of the Race Start Controller with schematics and PCB layouts are provided below. Click on the images for full size view.

Photo of the start controller connected to a standard Scalextric Sport powerbase and throttle.	Close up of the start controller PCB. I've used PCB mounted screw terminal blocks for the connections to the throttle wiring, power and an IDC header for the LED board.  There is no reason why the wiring can't be made directly to the PCB saving the cost of the connectors.	Inside a standard 45ohm Scalextric Sport hand throttle. The inset diagram shows how the throttle is wired.  When the throttle is in the closed position (as shown in the photo) the wiper (black wire) short circuits to the  'brake' contact (red wire).  It is this feature that is used to detect jump-starts.
The wiring and circuit schematics show how to connect the Race Start Controller to your Scalextric Sport powerbase track and hand throttles. If you're building this I assume you have some knowledge of electronics and circuit construction. The electronic side of the circuit operation is fairly self explanatory.	Wiring Schematic click here for PDF version	Race Start Controller Schematic click here for PDF version
A schematic diagram of the internals of the Sport Powerbase track part C8217. You can see from this that the power delivered to the car is unsmoothed DC.   D1/C2 on the controller PCB isolate and smooth this supply which is then fed into voltage regulator VR1 to get a 5 volt output for the controller.   PDF version   A few facts, traps and other information to be aware off during construction and testing. As detailed on the circuit schematic, you must use 1.8Kohm resistors for R6A and R7A and omit R6B and R7B. I recommend powering up the circuit without the PIC installed and using a voltmeter to check that you have something between 4.8 and 5.1 volts between pins 5 and 14 of the IC socket.  If not correct the problem before inserting the PIC. The standard Scalextric Sport Power base transformer pack outputs 16V RMS under full load. Without a load (that is when the cars are stationary) the peak output can be almost 27 volts. Therefore the 220uF capacitor, C2 on the schematic, which is shown rated at 35 volts should not be substituted for a 25 or 16 volt alternative.  The jump start detection works by sensing when the throttle wiper is contacting the brake connection. Some throttles are more sensitive than others where this is concerned. If the controller continually shows a throttle open status when you press the start button, check that both throttles are connecting the wiper to the brake contact in the throttle. Be careful when wiring the 3.5mm jack plugs that the wires and connectors don't short together when you slide the cover on. Ideally use some sleeving over the soldered connections. The wire colours shown are those I've found in a number of Scalextric Sport hand throttles and I've used those in the schematics for ease of reference.  I don't suppose they'll change it, but it's always worth checking.  The Scalextric Sport Powerbase allows either a single transformer to be used or separate transformers for each track.  The start controller will work with either configuration without modification. The start controller circuit uses ~75mA with all five LEDs illuminated and under 5mA with all LEDs off.  Each throttle sense opto-coupler will draw around 10mA continually. The throttle sense circuit works by using the lower forward voltage drop of a normal diode to 'by-pass' the LED in the opto-coupler when the throttle wiper connects to the 'brake' terminal.  This also means that when no throttle is connected, it appears to the controller that the throttle is closed so the circuit can be used with only one throttle The throttle detect circuit works with a common positive brake connection.  The throttle wiper varies the negative supply to the track. It's not how I would have designed it myself, but that's how Scalextric have done it so that's what it has to work with and to be fair it doesn't really make any difference. When I got into building this I bought another Powerbase track section and four sets of hand throttles to pull apart, test and play about with.  The circuit has been used with my test track, the original track the kids got for Christmas and with six different Scalextric hand throttles. I've not had any problems, but if you do I'd be interested to know more - drop me an email. Testing Once the circuit is constructed, connect to the Powerbase as shown in the wiring schematic above.  It's not necessary to have any cars on the track while you're testing the controller. When power is first applied, the LEDs illuminate in a sweeping pattern to indicate it has powered on okay. If you now hold open one of the throttles and then press the start button you should see LEDs 2 and 4 illuminate. Release the throttle and LED 1 should light followed by LEDs 2 to 5 at 1 second intervals.   When all five LEDs are on they will turn off after a random delay of 0 to 3.5 seconds. Repeat steps 1 to 3 with the other hand throttle. If it has worked as described, the circuit is functioning correctly.
PCB Artwork The Controller PCB and LED PCB are done as one layout. You can either cut the artwork and etch the boards separately or etch the board as one and cut the PCB afterwards which is how I do it. If you're not able to make your own PCB here's a prototype pad board layout. You should be able to work out from the schematic what the components are. The layout is similar to the PCB and the connections at the edge are the same as those on the PCB so the wiring schematics can still be followed.		PCB Layout (PDF) Component Overlay (PDF)
Alternate PCB and component kit for this project. You can now buy a PCB and kit of parts for this project.  It uses an alternate PCB design to that shown above. An an additional project page has been added with full construction details.  The assembled board can be substituted for the one shown on the main page here See kit #484 supplemental project page

Parts Listing  

You can buy all the parts needed to build this project from most component suppliers world wide. In the UK you can get everything from Rapid Online and I've included a parts list with their part numbers below.

• All Rapid parts/descriptions correct at 19-Sept-2008.  You should check part# and descriptions are correct when ordering in case I've made a mistake transferring them onto this page.

• The resistors are sold by Rapid in packs of 100 and the ribbon cable is a 30M reel.  You may find it cheaper to buy some or all of the parts from other sources - I make no recommendation.

• You will also needs some short lengths of wire for the interconnections between the PCB and the jack plug and sockets.  Rapid sell a pack of 11 colours x 2M lengths of stranded equipment wire, Part # 01-0108

PIC Firmware

The Microchip PIC 16F627A requires programming with the firmware for the start controller.

The HEX file ready to program into a 16F627A can be downloaded for free from here (right-click Save As).
(a 16F628A can also be used without any modification to the code or hardware)

If you have problems programming the HEX file into the PIC make sure you are using Vpp first programming mode.  For PICkit2 programmers you can find this option under Tools - Use VPP First Program Entry

This code works with the hardware design shown on this page only.  For the code used with the Race Start Controller kit #484 please see the supplemental project page

Use with other slot-car systems

Not having experience with other makes of slot-cars I can only say it works with Scalextric, though I expect it would work with other makes that use a three wire throttle.

If you want to use the circuit with other throttle systems the following information will help.

The throttle detect works as follows. The PIC port pins RB4 and  RB5 detect throttle 1 and throttle 2 respectively.  A high logic level on either pin signals to the PIC firmware that the respective throttle trigger is released (closed) and a low logic level signals the throttle is open (trigger moved).   The 'weak internal pull-up' feature is enabled on the PIC so no external pull-up resistors are needed on these inputs.

Resistors R6A and R7A set the current through the LEDs in the opto-couplers.  Ideally for a given input voltage the peak LED current should not exceed 50mA and the average current ought to be around 10-15mA.  When the throttle triggers are released the resistors are dropping Vin-0.6 volts, the drop across the 1N4148 diode.  This is the highest dissipation condition and you need to make sure that the power dissipation in the resistors does not exceeded their maximum rating.

The PCB layout was specifically designed to take two resistors in parallel which allows standard 0.25 watt resistors to be used where the power dissipation in one resistor alone would exceed 250mW.  

You also need to consider that with transformer packs like that supplied with the Scalextric the off-load output voltage is higher than the voltage stated.

For example.  With the 16 volt AC / 800mA transformer pack supplied with the Scalextrix I measured an offload RMS output of 19 volts (peak 26.8 volts).  Fully loaded the RMS output of 16 volts has a peak of 22.5 volts.

Dual Output PSU

You may have noticed on the controller schematic a 'Power Control' signal from RB0 pin of the PIC.  I've also designed and built a dual output variable voltage power supply for my Scalextric setup. This features a power shutdown input which, when used with the race controller described above, shuts off the power to the track when a jump start is detected. It also has a 'launch control' start mode where power is cut to the track during the start countdown and then restored when the LEDs go out.  

See full PSU project here

Development rig