Designing a Race Track Circuit Diagram for 3 Cars

[ladydarkness]

Alright I need help drawing the diagram... I know what I'm doing and a lot of the information but I need help with the diagram.

A three car race track has sensors in each lane at the finish line. As the cars cross the finish line their respective sensors respond and send a signal to a digital circut. The three "finish" signals occur in the same order that the cars finish in. The digital circut has a 3x3 matrix of nine LED's which indicate the placement of each car.



You're hired by the race track committee to design and build a digital circut that will operate as described above.

The LED indicators must be laid out as shown on the picture.
http://img135.imageshack.us/img135/595/racetrackproblemkr7.jpg

A push button resets the circut and extinguishes all lit LED's.





Design Hints:

*The signals from the track sensors are 10mS HIGH pulses. One pulse per race.

*Each LED is lit by an R.S. flip-flop.

*A two bit binary counter will start at zero and be incremented every time any track pulse occurs.

*Gating circuts are advised

*remember, each track signal only appears for every race



I figured that I will need:

3 monostable multivibrators

97474 RS Flip-Flops

1 7476 gating chip



so just so you don't think that I'm just trying to get pple to d my homework... I need helping drawing this.. I know what I am talking about but when it comes to putting it on paper I appear to be useless...

I've been trying to figure this out with logic and common sense but this is what I think..

I would suppose from the selection of chips needed that what I should build a matrix of 3 x 3 FF's. What I would do is to use the 7476 as a 2 bit counter, representing the placement. For each placement 0 - 2, you must decode this to a data bit for the placement row. Each car clock all 3 FF's in its column, reading the placement, and increment the counter.

Ths way I clock the current position into the FF's for each car.

Decoding the counter to make a running '1' is fairly easy just AND the /Q's for the first(placement 1), and then use Q1 and Q2 for the other 2(placement 2 and 3).

The only thing I need to do then is to combine the 3 clocks into one signal for incrementing the counter.

and I think the one shots are for Cleaning up the car signals and shortening them.

 

[berkeman]

Well, there are several real-world issues with the problem statement, including potential ties and FF metastability. But ignoring those for a moment, is there maybe an easier way to do this than with counters? Like, if the first pulse in turns on the respective FF and enables the 2nd row for writing. Then the 2nd pulse writes the 2nd row FF and enables the 3rd row for writing. Reset clears all FFs and enables the first row for lighting.

Can you sketch out some logic that would perform the function in that way? Then after sketching it that way, can you see any potential problems with that approach? If so, what improvements can you come up with.