Subject:
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Re: "No Contact" Homebrew Train Sensor for Mindstorms
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Newsgroups:
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lugnet.trains, lugnet.robotics
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Date:
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Thu, 31 May 2001 19:21:01 GMT
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Viewed:
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1511 times
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In lugnet.trains, Mario Ferrari writes:
> Hi Chris, thanks for sharing this.
>
> I had the same positive results with reed sensors in my automated rail
> station. The station actually detects the incoming train through the well
> known light sensor + lamp method, but I wanted a way to distinguish
> passenger trains from freight trains, so a couple of months ago I asked for
> some help to John Barnes and Sergio Lorenzetti. During a short discussion
> where we examined many interesting ideas (including bar codes, transponders
> and other sophisticated stuff), Sergio suggested to use the good old reed
> switches and a small permanent magnet on the train. So did I and... it
> works! In my set up the reed switch is incapsulated into a standard brick
> and looks like a very standard Lego sensor with its cable. That brick is
> incorporated into the platform of the station and connected to the (hidden)
> RCX. I placed a Lego magnet on the side of the passenger trains, just below
> the car plate. When a train triggers the reed contact the RCX knows that
> train must stop at the station.
> I still continue to detect all the trains with the light sensor, because the
> RCX also controls semaphores and insulated sections of tracks to separate
> trains, but just now from your post I realize I can use reed sensors at rail
> level for this purpose too :-)
>
> Ciao
> Mario
>
> "Chris Phillips" <chris.phillips@computerboards.com> wrote:
> > As has already been reported by others, I built an automated railroad
> > crossing for the NELUG train display at last weekend's Greenberg show. I
> > wanted to take a moment to describe the "homebrew" sensor that I used to
> > detect an approaching train so that others might benefit in their own • layouts.
> >
> > I originally tried several techniques to detect the presence of a train,
> > including the touch sensor, the light sensor opposite a light source, and
> > the good old infrared "radar" technique that has been well-documented on • the
> > robotics forum in the past. I also considered using Vision Command, but I
> > didn't want to have a PC in the loop if I could avoid it. None of these
> > techniques was very reliable: I couldn't find a good way to trip a touch
> > sensor without derailing the train, and the other techniques required a • lot
> > of highly visible and unsightly "plumbing" somewhere along the track. I • had
> > pretty much abandoned the idea of using an RCX to detect a train before I
> > remembered an idea that I had awhile back but never tried, due to my
> > semi-irrational aversion to non-LEGO solutions.
> >
> > I used reed switches which are small switches encased in glass that close
> > whenever a magnetic field is present. I simply chopped a long electrical
> > connector in half, (ouch!) soldered the reed switch to the bleeding end, • and
> > connected it to an RCX sensor input which was configured as a touch • sensor.
> > Voilla! The RCX could easily detect the presence of a magnetic field as • if
> > a touch sensor were being pressed.
> >
> > I then placed the touch sensor _lengthwise_ along the track, and found • that
> > the magnetic couplers between the train cars were strong enough to trip • the
> > reed switch. (At first, I placed the reed switch perpendicular to the
> > track, but this didn't work due to the alignment of the magnetic field • from
> > the train magnets.) It was then a simple task to program the RCX to close
> > the drop-gates and turn on the signal lights at the crossing.
> >
> > I went to Radio Shack to buy the reed switches. The 'Shack used to sell
> > reed switches in a pack of four, but they don't any more. Instead, I • looked
> > at their home security stuff and found a sensor/magnet pair for $4 that I
> > was able to extract the reed switch from. If I had more time+patience, I
> > would've gone to a larger electronics store and bought a pack of reed
> > switches instead. It would've been a lot cheaper, and also I broke 2 out • of
> > 4 switches during the extraction process -- they are _very_ delicate!
> >
> > At first, I didn't think the train magnets were strong enough to trip the
> > reed switch, so I stuck one of the Radio Shack magnets onto a 2x4 plate
> > (using the double-sided tape supplied with the magnet) and attached it to
> > the underside of the locomotive. This worked great. But later, we • removed
> > the non-LEGO magnet from the train and found that it worked anyway. I • think
> > the reason this didn't work the first time I tried it was that I had the
> > reed switch oriented perpendicular to the track, because it fit between • the
> > ties better that way. However, I soon discovered that even the more
> > powerful magnet would not trigger the reed switch unless the N-S poles • were
> > aligned lengthwise with the switch, and by then we had already attached • the
> > stronger magnet to the train. We turned the magnet and reed switch
> > lengthwise because we wanted to hide the offending non-LEGO magnet
> > completely on the underside of the loco; otherwise we might never have
> > discovered that the LEGO magnets alone will do the job! So I must • emphasize
> > that, although it is tempting to lay the reed switch between two ties, it
> > will only work with the train coupler magnets if the switch is lying
> > lengthwise along the track. Otherwise, you'll need to add extra magnets
> > that are aligned side-to-side in order to trip the switch.
> >
> > For the show, I simply used the RCX to close the gates for a fixed length • of
> > time whenever it sensed the train coming. This worked OK, as long as the
> > speed and length of the train remained constant. But for our next show
> > (which I have now been tasked with creating several more crossings for) I
> > will improve the software to sense each train coupling as it passes to
> > determine how long to keep the gates down. I also plan to use multiple • reed
> > switches at each crossing in order to detect a train approaching from • either
> > direction, and also to detect the actual speed of the train on-the-fly.
> >
> > Anyway, I was amazed at how well this technique worked once I figured out
> > the alignment thing. Maybe LEGO will add a reed switch sensor to the
> > Mindstorms product line so that I won't have to feel guilty about using a
> > non-LEGO component in our otherwise pure-LEGO train display. Here's
> hoping...
Hi everyone.
My name is Tony Hedglen and I’m very interested in automated Lego railroads.
I am a member of Michlug and we are one of the newer clubs. We have done one
train show so far and we won second place. I read these posts on sensors and
I think it’s awesome. I’ve been searching for some time now on how to
automate Lego trains. This is the first I seen someone talk about the
subject. I wold like to learn more. I have no idea on how mindstorms work
and I have basic electrical knowledge. Could someone please help me out on
this subject? Thanks.
Tony
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Message has 2 Replies:
 | | Re: "No Contact" Homebrew Train Sensor for Mindstorms
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| (...) Hi Tony - I haven't had a Mindstorms for long, but I've already built an automated level crossing. The Mindstorms is based around the Lego 'RCX'. This is a large brick, about the size of the old 4.5V battery boxes, but with some very special (...) (23 years ago, 31-May-01, to lugnet.trains)
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