Electronics and the Pinewood Derby

Ever try to judge the final heats of a race on a 4 or 5 lane track by using your eyeballs? Pretty challenging huh? What if you could perform this task flawlessly every time and not have to worry about making the wrong call and be the on the outs with half the parents? Well an electronic lane judge is the answer. These products include all sorts of devices from simple two-lane win-lose indicators to more sophisticated individual timers with computer interfaces and monitoring/charting programs. The prices will span the less than $100 threshold to many hundreds of dollars They are available from a number of mail-order sources and one is even listed in the BSA catalog. Scouters can check in the back of the "Scouting" magazine while we can all check the Internet for additional sources. If you're lucky you might have someone in your group handy enough to build such a unit.

The human eye is a wonderful gift that we use to recognize objects, judge distances, navigate and determine color, size and motion. We rely on it to perform so many tasks we hardly know where to stop. But, sometimes the brain can't register multiple objects quickly moving past a point and recall the order. Too bad we don't have instant replay. It can be nearly impossible to detect the leading car between two Pinewood Derby cars when they are within half an inch of each other. Now, add two more cars between them and another opinion. Like a baseball umpire, the wrong call can put you in an uncomfortable position. Let the electronic lane judge make the calls instead. They can display the place for each car as they cross the finish line.

They are often terms used interchangeably but Electronic Timers differ from Lane Judges in that timers indicate the elapsed time(s) of the cars where lane judges indicate the finish place of each car, 1st, 2nd, 3rd, etc. Timers can be helpful when designing a car or evaluating the lane characteristics of a track. Like which lanes are faster or slower. For a timer to be truly effective it must indicate accurately to at least 0.001 seconds. A resolution of 0.0001 or better is preferred. A car traveling 30+ feet in less than 3 seconds from a standing stop means that the rate of speed averages 10 feet/second and top speeds may exceed 15 feet/second. At 10'/sec (or 120"/sec) the car travels about 1/8" every 1/1000th of a second. You might be surprised at the number of ties a 0.001 second timer will display in the final heats. If you install sensors in your track you may be able to use a laboratory electronic counter and a precision time-base to very accurately display the lane elapsed times. In multiple-lane tracks this can be a pretty elaborate (not to mention expensive) setup. On the cheaper (and more reasonable) side however you could employ solid state event counters that cost $50-$100 each. Some folks have experimented with hand held electronic stopwatches like those used in track and field events. They can be interfaced to your track with some "electronic glue" but usually only indicate to 1/100th of a second.

While a timer is nice for evaluating a car's performance the lane judge is easiest to use for competition events. After all, the concern is who finished in what order not necessarily how fast they were. Yes, you can compare the elapsed times between lanes but that adds a little more work in determining the finish place order. Besides, a lane judge is usually somewhat more economical than timers for each lane.  An electronic lane judge functions by registering the electronic pulses that represent the cars passing the finish line. Every lane has an electronic counter that increments every a time any car crosses the finish line but the counter is halted after the car in that particular lane has finished. In this manner every lane counter reflects the finish place of the car in that lane. Lane judges are commercially available to accommodate tracks from 2 to 8 eight lanes wide. While electronic judges can sometimes show a tie condition it is relatively rare but certainly possible even in an electronic sensing system.

A DIY (Do It Yourself) project some have suggested would have you build a lane judge using an electronic calculator. The theory is that you would electrically connect the numeric keypad digit keys that correspond to the lane numbers to the lane sensor switch. When the car activated the switch the number of that lane would appear on the calculator display. The finish order would read from left to right. There are a couple a bugs in the ointment though. First, there is the problem of "key rollover". Many calculators will 'see' only one key at a time and if two cars come in at the same time only the first will get registered. More sophisticated keyboards or keypads employ 2-key or n-key rollover which overcomes this problem. The next problem involves timing. Keyboards are generally scanned as a matrix of rolls and columns looking for closures between the two. The scanning rate will determine the resolution or window of each polling of the switches. Another nontrivial problem is actually making the connection to the calculator keypad. The flex circuits and membrane contacts can resist efforts to solder to insure reliable connections. If this solution works for you then you've saved lots of dollars. Share your ideas with others.

Car Sensors.

Lane judges and timers require you to adapt a electro-optical or electromechanical sensor to your track to provide an electrical input to the controller's electronic "front end". The "brain" and display unit is often placed over the end of the track and is visible to the audience. An electromechanical switch, optical sensor or light source can be a part of this assembly.

Optical sensors are commonly used in electronic lane judges and timers. They function by "seeing" a very narrow vertical beam of light being interrupted by the car traveling past the finish line. They don't physically touch the car and can't harm it. The optical sensors can be sensitive to sunlight and other bright light environments so make sure you try it out before the race in the environment you will be in during the race.

Photo-detection can be the most reliable and repeatable car detection method but care must be taken in implementing it. Here are a few points to remember when designing this type of system.

Break-switches, mercury switches and other mechanical sensors are also employed as sensors. This class of sensor will use some sort of stiff wire or other low-mass arm hanging in the path of the car. When the car hits the arm a magnet-reed switch or Microswitch is triggered. Mechanical switch-sensors are immune to light related problems but you must make sure they properly set so that close finishing cars are registered properly. Some types of mechanical switches must be reset to their un-triggered position and checked for alignment before each run.

Capacitive mass sensors are not recommended for sensing cars as they can be fooled by the unusual pointed cars or the low-mass front end of a car. If these cars aren't detected soon enough they may not win where they should. Some have also tried using magnets placed on the nose of all cars to trigger a reed switch placed in each of the lane guides at the finish line. This has proved only partially successful as magnet placement plays such a direct role in activating the reed relay.

Computer Systems can help organize a race, run or develop the race chart and drive audience display systems. In some very large Cub Scout district or council-level events computers are used to register the racers, record the check-in process and car specifications like the car weight, verify registration and when networked the racer is "plugged in" to the race computer.

The commercially available software-hardware packages can simplify or eliminate the task of planning race charts and will make the race run more smoothly. Large Packs and even some smaller packs will swear by them.

Whether you're using a commercial software race package or your own application, the overhead projector devices seem to be the easiest to use as they plug into the video output of your computer and let the audience see what you see on the screen. Borrow one from your workplace or rent one. Large electronic displays can be seen in a number of forms. Signboard displays that utilize hundreds of LEDs are used to display race times, racer names, race results and other vital information. Computer screens showing race charts or status information can be displayed on large screens using either color LCD overhead projection displays or projected screen displays. This can help take some the mystery out of the elimination process when you actually show the race results for all to see.

Electrical release mechanisms are convenient when a  race is being timed or controlled by a computer system. They are often used with a timed track where each lane is electronically timed. Some of these tracks employ both a 'set' and 'release' solenoid to electrically control the car restraint gate. These are usually designed into the track at the time of development. Caution must be used in the design as most solenoids have the capacity to easily pinch the fingers of the unaware racer and the electrical supply connections must be isolated.

For those with an adequate budget and lack of time or skills the choice may be obvious. Often however, the youth group doesn't have the financial resources for ready-made tracks and equipment and there's always that someone ready to accept the creative challenge.

This author has no direct experience with any of the vendors producing the items offered in the back of "Scouting " and "Boy's Life" but I have heard nothing bad about them either. The systems available from these folks vary in their design and application. I would recommend that you get as much information as possible so that you adequately match the track, the electronics and the dollars available. Remember, you'll still have to adapt the electronics to your track to varying degrees.

For those that would like to take a look at 'rolling your own' I'll show you some ideas you can use directly or adapt to your own use. There's nothing magic about any of this but you have to be comfortable with my assumed level of your   electronics ability. I can offer no design services, kits, or parts beyond the information presented here. I will make reference to common parts where possible and indicate where they can be found.

Two-lane tracks can use a first place only circuit electronic finish indicator most naturally. The need may be somewhat reduced but even here the cars can finish close in those final heats when the fastest cars are running head-to-head. In 3 and 4 lane tracks a first place finish indicator has value as a tiebreaker. Some racing elimination techniques care only about the first place car no matter how many lanes you use. Be careful here because it is important to also know your overall second, third etc.

The electrical theory behind most of the circuits in this class of is pretty simple. An LED, SCR and even Neon lamps are current devices. That is, they trigger at some specific voltage level and go into conduction and will remain in conduction until the current source is removed. Where there are multiple branches in a circuit sharing a limited current source only one device will have enough current to function. The first device to 'fire' uses the current and subsequent triggers to devices on other branches have no effect. In this way the first car to trigger the respective lane circuit gets the win. This concept can be easily expanded to any number of lanes.

This circuit is an example of an a two-lane, first place indicators. Click the title to invoke an Acrobat graphic schematic view.

Click here to view simple winner detector.

Lane judges are much more complicated than the first place only indicators.  Follow the theory of operation using the simplified block diagram shown below.

Track Sensor. Beginning on the left side of the diagram there are track sensor blocks  that represent the actual photo or electromechanical switches on the track. Their output is brought into the controller where it is 'cleaned-up'.

Conditioner and Single Shot. The analog signals from the photo sensor is passed through a Schmitt trigger or an op amp circuit to give it a discrete or digital appearance. Switch inputs are de-bounced. Both types of signals are converted to their proper logic level for the rest of the digital circuitry. This block diagram also represents the generation of a pulse that signals the presence of a car. Some simple circuits will use a "one-shot" mono-stable multivibrator like a 74LS123 or variant while more sophisticated circuits use clocked circuitry. The output of this block is an extremely short logic pulse that may be no more than a microsecond in length. That's 1/1000000 or (0.000001) of a second.

The 'OR' Gate. In the center of the diagram is an 'OR' function that passes the narrow pulses generated by all the conditioner/one-shot blocks. This function insures that all cars generating pulses are presented to the counter for each lane.

Latch and Delay Function. Here's the key element to the design. The counter circuit block will count every clock pulse that's presented to it unless the counter is disabled or reset. We want the all counters to increment each time a car crosses the finishes line unless the car in a particular lane has already crossed the finish line. For instance, if the car in Lane 1 comes in first then the count pulse will increment all the counters and then after a short delay the output of the latch/delay circuit will disable the counter circuitry for that lane. The delay insures that the count gets incremented before the counter is disabled. Other cars that come across the finish line will not increment the counter for Lane 1. The same will follow for the rest of the lanes. The latched condition reflecting the car finish will remain until the "RESET" condition is imposed.

Counter Circuit. The counter function can be accomplished with a number of different integrated circuits but they will perform the function of an 'up-counter'. When reset at the beginning of a run the counters will be at a zero count with all outputs at a logic "0". The "DISABLE" input is not imposed and the counter is free to accept all input pulses. On each pulse input the counter is clocked up one count. This will occur until the "DISABLE" input is imposed after which the pulses are ignored. The counter will hold the binary representation of the count that relates to the place of the car. This logic output remains until the "RESET" is imposed.

The Display System. Each lane has some sort of indicator that can differentiate the finishing order of the car that raced in that lane. The indicator might be multiple color Light Emitting Diodes (LEDs ), several LEDs that count the finish place or a 7-segment display that displays numbers. The best way to display the lane status is to show the numeric finish order. The counter will have 3 or 4 outputs that represent  a Binary Coded Decimal (BCD) number for the display system. Three lines are significant for this example but a fourth line has to be accounted for, in addition. While there are single displays that are capable of accepting the BCD input and displaying a number they tend to be too expensive for this type of application. The typical BCD to 7 segment display IC costs less than a dollar, is easy to use and will drive most any common 7 segment LED. You just need to determine the type of display you have or want to use, common anode or common cathode. To add a more professional touch and avoid observer confusion the display is blanked until the car actually passes the finish line. Notice the enable input from the latch circuit.

The actual schematic for the implemented 4 Lane Judge is available as a Acrobat file.  This schematic employs clocked logic avoiding use of one-shot circuits and other RC time dependent components. While it takes a few more gates to implement this circuit can be reduced to just a few components with the use of a gate array for those with those tools available to them. In addition, the clock times can be adjusted to produce a variable tie window. The slower the clock the more lightly that a tie is possible. The pdf files were updated 17 January 2005. Errors in the 555 clock timing value changes have been incorporated.

Click here to view PhotoSensor Schematic.

Click here to view RaceMaster IV schematic.

Click here to view Print-Friendly RaceMaster IV Bill of Materials.

Bill of Materials for Four-Lane Electronic Lane Judge

A/R = As Required. The quantities or need for this part may vary depending on your implementation.† These resistor networks may be replaced with 28 discrete resistors. These are Digikey part number 220KQBK-ND at .28 each. The parts listed here are based on catalog part numbers from Digi-Key corp, www.digikey.com 1-800-3444539. The part numbers are linked to the Digikey web site.

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