Simple PhotoCell Timing System
(S.P.T.S.)
OPTIONAL EXTERNAL START LIGHTS

(c) Bryan Brown & Art McEwen 1997-1999
All rights reserved, not to be copied without permission.

F1 style external starting lights are OPTIONAL and are NOT a requirement of SPTS. This is a project that you can do anytime after you get SPTS up and running.

External lights add another level of realism to your circuit. They also allow the drivers to concentrate 100% on the track without ever having to refer to the computer screen during the heat of battle. Wiring up external lights is a fun project and helps to expand your limited knowledge of electronics (at least it did in my case). It also offers yet another opportunity to dazzle family and friends with your technical wizardy.

All Scuderia McEwen International programs, with a starting procedure, use the computer screen as the primary display for the starting lights. If you choose to wire up external lights, the starting lights on the computer screen can be deactivated in the PARM.TXT file, ensuring that all drivers must watch the external lights.

SMI programs try to be faithful to the current F1 procedure for starting. Namely, there are five red lights which go on, one at a time with a random period of .5 to 3.5 seconds when all lights stay on. The race begins the instant all five lights go out. SMI programs also emit ascending beeps with each light and a loud beep to mark the actual start.

The five external start lights can be either LED's (Light Emitting Diodes) or 12 volt lights. I have tried both and found that LED's were not as bright as the 12 volt lights. They need to be visible from anywhere in the race room. Accordingly, I decided to go with 12 volt lights and a separate external power supply. LED's do have the advantage of being simpler to wire and do not require an external power supply. It's your chocie. Both wiring diagrams are included.

All electrical components are of the most common type and can be obtained at any Radio Shack or equivalent shop.

Wiring and Components


External lights require six additional wires to the DB25 connector. The 12 volt lights require transistors for switching and diodes to protect your computer port. They also require an external 12 volt power source, as the computer port supplies only a limited amount of 5 volt current. The LED's require only resistors to limit the current and use the 5 volt current supplied by the computer port.


Additional DB25 Pin Wiring for External Lights
DB25 PINExternalLane Light
Pin 2 Start Light 1 Pin 10 Lane
Pin 3 Start Light 2 -
Pin 4 Start Light 3 Pin 13 Lane
Pin 5 Start Light 4 -
Pin 6 Start Light 5 Pin 12 Lane
Pin 25 Common Ground -

For 12 Volt Start Lights:

Click Here for Wiring Diagram for 12 Volt Start Lights

The power source provides the current to illuminate the five light bulbs. A separate power source (from your track supply) ensures that the performance of your slot cars is never affected by this accessory. It should be 12 volts in order to provide the best illumination. All you need is one extra power source, it will drive all five lights.

The light bulbs do not have to of any particular type or size. I used Radio Shack's 272-332B but they have many other types, including Mini-Lights 272-1099. The five lights should be identical and all should be red (to be faithful to F1).

The transistors are of the most common generic NPN type. The 5 volt current from the computer causes the transistor to switch "on and off", thereby allowing the 12 volt current to flow to the starting lights. They came in a package of 15.

The resistors limit the amount of 5 volt current that flows from the computer to the transistor in order to prevent the transistor from overheating and blowing up. A 4.7K resistor has Yellow/Violet/Red color bands.

The silicon switching diodes are also of the most commom type. They only allow the current to flow in one direction and thereby protect your computer port in the event of a short in the 12 volt circuit. This prevents 12 volts from ever being fed back into the port which will certainly damage your computer. I bought a package of 50 of them, enough to last me a lifetime (?).


Bill of Materials
Component QtyRadio Shack
12v Power Supply 1 any
12v Lights (red) 5 272-332B
Generic NPN Transistors 5 276-1617
4.7K Resistors 5 271-1330
Silicone Switching Diodes10 276-1122

For LED Start Lights:

Click Here for Wiring Diagram of LED Start Lights

The power source is the computer. It supplies 5 volts. No external power source is needed.

The Light Emitting Diodes can be almost any type, but should all be the same and they should be red. You could use Radio Shack's standard red LED 276-041. The positive wire must go to the anode and the cathode leads to the ground. If you reverse the connection, you will get no light.

The resistors limit the amount of current going to the LED. Without a resistor or with a resistor of too little resistance, the LED's will instantly burn out. The size of the resistor determines the brightness of the LED. Your computer may supply more power than mine so you may have to experiment with resistors to get the correct intensity. Start with a higher resitance and work down. I settled on 47 ohms resistors (Yellow/Violet/Black) for each LED. I bought a "value pack" of about 100 various resistors.


Bill of Materials
Component QtyRadio Shack
LEDs (red) 5 276-041
47 ohm Resistors 5 various


You Can Also Wire In Pit Signalers and "Tex" Watkins, Automated Flagman


If you really want to impress your friends, see the How To section on Bryan's page and you can include Pit Signalmen and an Automated Chequered Flagman. All the signals to these devices are built into the SPTS programs. There's nothing like rounding the last corner of the race and speeding by your miniature flagman who is programmed to wave at exactly the right time.
Click Here for Summary of External Light Usage in SPTS


Is That All They Do, Just Start The Race?


No way! After you have gone to all the trouble of wiring up the lights, they had better do more than just that. They also indicate the end of the race or session when all five lights turn on.

Not only that, when a race is in progress, one light is assigned to each lane and acts as a "lane light".

In the timing programs, DINO, ENZO and MONTY, the lane light indicates that the lap just completed was the fastest lap of the session for that lane. This is the equivalent of the flashing Large Number Display.

In MARIO, the lane light indicates the fastest lap overall in the race.

In LEMANS and BERNIE, the lane light indicates that the fuel tank is nearing empty in that lane, that is, it has 5 gallons or less in it. When your lane light comes on, it's time to pit!

Make sure you are using the current version of all programs to ensure that you have the external start light code!

Building Your Circuit Board


Building the external start lights circuitry (five circuits for five lights) requires that you keep the various electronic components neat, separate and firmly attached to something. Probably the best solution is to buy a Grid Board or MultiPurpose Board from your local electronics supply shop and assemble the various bits and pieces with it. These boards are basically plastic sheets with hundreds of holes in them. You slip the transistors, resistors etc. thru the holes and solder them on the underside.

This was too easy for me. I made my own custom board using a scrap of non conductive vinyl house siding (which I picked up at the world famous Yarker Dump) and drilled holes at the required places. I made one template and repeated it five times. I then inserted the components, soldered them up and secured them with a small dab of 5 minute epoxy in each of the drill holes. It works great, it's neat and best of all, it's cheap!!!

In order to test the circuitry before installing it on the track, I used the program SPP20.EXE to mouse click data bits 2-6, "ON" and "OFF", one at a time. This allowed me to check the operation of the circuit before I crawled under my track. This utility program is supplied with the "advanced" software kit.

Special thanks go to my internet friends who helped me with the circuitry and politely answered all of my dumb questions. Thanks for sharing!!


Last Updated Jul21/2000

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