The timing project

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JohnW
Jaded Figurehead
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Joined: Fri Apr 09, 2004 10:12 am
Location: High Desert, California

The timing project

Post by JohnW » Fri Dec 01, 2006 4:26 pm

Here's the deal.

I need some help finding something on the 'net. So all you Google nuts, Jeeves Junkies, or any other search engine smart asses. Here's the task.

I need to find a timer that starts, when a car leaves a ramp and stops when it crosses the finish line. I need the whole rig to be under $200 if possible. I would PREFER it to NOT need something added to the car to do this, as the extra weight might skew the results.

What am I doing ?

A guy I know at work has his kids into Soapbox Derby racing. Last season, he kicked butt, but, this season he can't make it past the second round. He wants to be able to do private runs and adjust the car to figure out why. So, he wants a timer that will start as the car comes off the ramp and stops when the run is done. It needs to be at least a 1/100 sec accuracy. I suppose a system with an in car unit and magnetic strips on the road would be OK,if that's all we can come up with.

Someone told me that Kart timers might do this, but most of them are based on a single point to start and stop for a complete lap. I need more of a drag race sort of thing.

If I can't find this, I have sort of committed to building one. I have a scematic to go from , but it's the electronic eyes and getting them to start and stop the clock that is the problem. I don't know how accurate they are. Some are run by pulsing the LED at 60 cycles, which would be only 1/60 sec accuracy, and I'd have to come up with some digital logic circuit to replace the start/stop button.

So...I don't think it is a Mission Impossible task, but....you never know.
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Knightrousoxide

Post by Knightrousoxide » Fri Dec 08, 2006 5:04 am

Two laser pointers, two photo diodes and two transistors should do the trick.

Set the laser pointers to shine directly into the photo diodes, which powers a transitor that connects to the timing circuit you found already. When the car passes through the laser beam, the photo diode will turn off, alerting your timing circuit and startin the clock. then when the car passes the final laser/photo diode, it does the same thing but stops the timer...

EDIT: I'll think more into it when I'm awake.. :P

JohnW
Jaded Figurehead
Posts: 13266
Joined: Fri Apr 09, 2004 10:12 am
Location: High Desert, California

Post by JohnW » Fri Dec 08, 2006 10:08 am

I keep finding setups that us a premade stopwatch as the timer, but I can't find any reference, on the stop watch that it has any external input ports. These watches are accurate and already built, of course, and cost $19-$75. So, if I COULD use one of them, rather than building from scratch, that would save a lot of time.

I like the laser pointer idea. They don't cost much and run on batteries.

Keeping them lined up, on the photo diode, make take some care.
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JohnW
Jaded Figurehead
Posts: 13266
Joined: Fri Apr 09, 2004 10:12 am
Location: High Desert, California

Post by JohnW » Fri Dec 08, 2006 4:13 pm

The stop watches of choice seem to be Robic brand. I found a system that was designed for barrel riding that used one set of infared sensors and a "modified" Robic SC505 stop watch. So...I called Robic. The watch is about $30 and is accurate down to 1/1000 of a second. They don't have an external switch method, but they know people "hot wire" the switches to adapt them to various uses. So..rather than build one, I think tinkering on a known good one would be better. Now to get the logic for the lasers figured out. I think it would be a NAND gate of some sort.
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F1Addict
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Post by F1Addict » Fri Dec 08, 2006 10:46 pm

It looks like $375 is about the cheapest OTS system you're going to find. These folks have a pretty nice system, probably one you've already researched, but fits all of the criteria you listed other than price.

JohnW
Jaded Figurehead
Posts: 13266
Joined: Fri Apr 09, 2004 10:12 am
Location: High Desert, California

Post by JohnW » Fri Dec 15, 2006 4:20 pm

Yep. That was the closest one. They use wireless to connect the units.

I talked to the guy, and the normal course for Soapbox Derby is 800 feet. So...will the output of a simple photo diode with a transistor to bump the signal make it that far ? He has mentioned that it doesn't need to be THAT long for his test runs, but I figure if it is designed for that distance, all bases will be covered.

Also, thinking about how these will work, I think I need something that will be triggered on a negative edge transition. The lights will be on, until the car breaks the beam and then it will go off. Once the car is past, it will go back up to on. I don't want the watch to stop during this condition, as the full run will be in between the start lights off->on transition and the end going on->off. So, if it start the watch on a negative trasition at the first light and stop it on a negative transition of the last light, I should be good.
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