SparkFun Forums

[dgzilber]

you could have a very narrow conductive strip of material, connected to a sensor which reads either voltage or current

each car can have a light, spring loaded sliding contact which gently drags on the ground, connected to a coin cell battery and resistor. each car can have either a different voltage from the coin cell or a different resistor value.

anyways the sensor would read the voltage or current value and instantly know what car has passed.

advantages:
lightweight
simple to implement
battery will last practically forever
if it works, would be very accurate: the thinner you make the path on the ground, the smaller the chances of two cars sending an interfering signal. If you wanted to go further, you could even implement a system where, if a non-expected measurement comes through, a relay is sent to make a high speed digital camera take a picture.

the only possible problem i can foresee is instrument inertia; how fast a reliable reading can be obtained. I think, however, since resolution is not important for a few cars, this is a minor problem and can be solved with any typical high speed a/d converter on a microcontroller

anyways just a thought

[Sputnik]

A simple solution would be to use IR.

Make a module to be fitted to each car , assign and ID to each (even using remote codes would work).

limit the output of the IR transmitters and narrow the beam so it will not "trigger" early.

You could even use 2 IR receivers placed a certain distance apart and calculate speed of each car going over the line and get a more precise lap time.

Or you could put the transmitter and receiver pair on the track side
and use some sort of "bar code" like label on the cars for it to read. This would be trickier but certainly doable.

IR seems like the cheapest and easiest solution to me.

[VStar650CL]

What if the cars are neck-and-neck? A single set of IR receivers will have the same problem Philba pointed out with RFID... interference with each other.

The advantage to having the track trigger the car, rather than the other way 'round, is that the outgoing signals from each car can be guaranteed unique by something as simple as different xbees or different frequencies.

PS - I still think the xbees would be great, you could could have your lap-count and car telemetry as well.

[Sputnik]

What if the cars are neck-and-neck? A single set of IR receivers will have the same problem Philba pointed out with RFID... interference with each other.

The advantage to having the track trigger the car, rather than the other way 'round, is that the outgoing signals from each car can be guaranteed unique by something as simple as different xbees or different frequencies.

PS - I still think the xbees would be great, you could could have your lap-count and car telemetry as well.

I dont think this would be an issue if done properly. Neck and neck will not be THAT close. Especially if you used the two receivers as stated.

An easy solution to this would be to position the transmitters at diferent locations on the cars. One car on the front, one car on the rear etc. Any discrepancy can be calculated out in software.

You could have telemetry with IR as well.

Very doable!!

[VStar650CL]

I dont think this would be an issue if done properly. Neck and neck will not be THAT close. Especially if you used the two receivers as stated.

An easy solution to this would be to position the transmitters at diferent locations on the cars. One car on the front, one car on the rear etc. Any discrepancy can be calculated out in software.

I dunno, that's an awful lot of kluje-ing to get a signal that a strong magnet and a reed switch can do with dirt-reliability. And xbees... well, we almost all use them for something or other already.

[Sputnik]

I dont think this would be an issue if done properly. Neck and neck will not be THAT close. Especially if you used the two receivers as stated.

An easy solution to this would be to position the transmitters at diferent locations on the cars. One car on the front, one car on the rear etc. Any discrepancy can be calculated out in software.

I dunno, that's an awful lot of kluje-ing to get a signal that a strong magnet and a reed switch can do with dirt-reliability. And xbees... well, we almost all use them for something or other already.

Yes, it is, your right. Same goes for the Xbee solution which the originator does not have (I think). You may have Xbbe stuff, but you I would presume you have no intention of implementing this so its irrelevant.

The problem with a solution such as the reed switch and magnet is being able to hit that spot on every time and wouldnt lend itself well to multiple cars.

The IR solution would still be easier to implement than the Xbee and way cheaper. You could probably get the IR solution working for say $25 (maybe cheaper) with a few small 12F pics and IR leds on the cars, maybe a 18F2550 on the RX side to handle USB CDC to a PC, and a little experimentation.

The Xbee solution would definitely work, its just way overkill and costly for this application, which is rather simple and is being made more complicated than it needs to be.

The solutions are essentially the same, just different transmitters, and would work equally well, just different ways of going about it.

[paulrich]

You guys are great and I am learning stuff lol, xbee is mostly to give me a project this is a nice hobby to get into i think.


Thanks All

[VStar650CL]

The problem with a solution such as the reed switch and magnet is being able to hit that spot on every time and wouldnt lend itself well to multiple cars.

I don't think you read the whole thread. Back on page 2, Monk wrote:

1) Use a strip of that flexible magntic strip stuff, what fridges use, across the track in various places if you wish...
2) Whats wrong with using reed switches? Far more immune from the brushed/brush-less motors in the cars than hall effect sensors...

Some very strong strip magnets have been available since the mid-80's, and it could easily span the whole lap line. I'm just following Ockham's Razor, I think it is the most elegant solution.

[VStar650CL]

If you guys want try mag strip, you can even try it cheap... Skycraft has these for $2.50 ea, flexible rare-earth. They're 7" squares, but they can be scissor-cut. You'd need 5-6 of them to make a 10-foot x ~2" strip. I'm buying a couple myself, just because... never know when I might need to hang my daughter on the 'fridge for a few hours.

http://skycraftsurplus.com/index.asp?Pa ... rodID=1864

[Sputnik]

If you guys want try mag strip, you can even try it cheap... Skycraft has these for $2.50 ea, flexible rare-earth. They're 7" squares, but they can be scissor-cut. You'd need 5-6 of them to make a 10-foot x ~2" strip. I'm buying a couple myself, just because... never know when I might need to hang my daughter on the 'fridge for a few hours.

http://skycraftsurplus.com/index.asp?Pa ... rodID=1864

That is a nice find!!! Wonder just how strong they are?

In your post before this. I thought you were referring to the switch being in/on the track and magnet on the car so yes, a strip would do it.

[VStar650CL]

Those suckers are Neodymium Iron Boron. I was joking about my daughter, but if I wasn't, one sheet of that stuff could probably hang her there for life!

Enjoy, glad I could help.

[Sputnik]

Those suckers are Neodymium Iron Boron. I was joking about my daughter, but if I wasn't, one sheet of that stuff could probably hang her there for life!

Enjoy, glad I could help.

Yea, I see. Its rare to see flexible neodymium maxnets. I may just get some to see. Ive had a few things come along that I could have used this for.

Even being NdFeB, they still cant be all that strong being in the "rubber" carrier.

Well see!!!

[VStar650CL]

PS - Sorry if what Monk and I said wasn't clear. Whatever mechanism you use for transmission, I think that's the real trick to making it accurate and consistent... have the track trigger hardware in the car, rather than the car triggering hardware on the sideline. Aside from maybe-issues like sight-angles, your IR idea would probably serve as well as anything else. The mechanism for that part won't matter, as long as the link and the trigger are both reliable.

[VStar650CL]

Oh, yeah, PPS - Did you notice those mags were in the marine section? No lie, I bet they're for temporarily patching cracks in steel boats.

[Sputnik]

Oh, yeah, PPS - Did you notice those mags were in the marine section? No lie, I bet they're for temporarily patching cracks in steel boats.

That is quite possible!!