On Thu, 15 Dec 2005 16:40:46 GMT, Ignoramus8558
wrote:

We were playing with my son last night and made a toy railroad
gate. It would open and close based on a relay signal from a momentary
switch that is activated when a train comes near the railroad
intersection.

Thought about a reflective opto sensor? Easy enough to put 3M
reflector tape or gloss/metallic paint on the bottom of the rolling
stock.

http://news.thomasnet.com/fullstory/458963 would be a nice starting
point, just stick them between the tracks.

It worked, kind of.

My problem is that the DC motor (pulled from a demolished HP X-Y
recorder) would overheat from being under power constantly.

I would like to find a little 12 VDC motor that could survive a locked
rotor condition indefinitely, or some other simple solution. A little
solenoid with a dual coil (pulling and holding) also comes to mind. I
need something that can be bought for a few bucks at most or made
easily, and I am out of ideas.

Wouldn't it be easier to rig limit switches on the arm motor that
turn off the motor power at end of travel, rather than figure out a
way to cut current and have it sit stalled on a physical stop?

I'd have to sit down with a pencil and figure it out, but there
should be a simple and elegant way to open the motor power in 'Up'
when it hits the 'Up Limit' and take that switch out of the picture
when the 'Down' relay kicks in, all using relay logic. You won't need
any sort of braking or hold action if the crossing arm is properly
balanced neutral. Probably take a 3PDT or 4PDT relay to reverse motor
polarity and select the right limit switch. And you rig the crossing
lights so they run unless the arm is in 'Up limit' mode

And if you want to go into r.c.m overkill mode... ;-)

You can use slot opto sensors for the limits, a few TTL gate chips
to start and stop the motors, and a 555 or two for timing.

You'd want to add a time delay circuit of some sort so the gate
delays down a second or two for the warning lights, then holds down
while the crossing is fouled but comes back up automatically after the
crossing is clear - if this was a real railroad you could count on
track loop detection to see if the crossing was fouled, but not on
H.O. And even if you could, it would only see the engine, not the
rolling stock - no shorted tracks caused by steel axles and wheels.

And if you space the crossing trigger sensors out for realistic
advance time but where they can't see the train the whole time, you
might need four train sense switches or more - put two right at the
edges of the grade crossing so they'll see the rolling stock if it's
running slowly or stops.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

In article ,
Ignoramus8558 wrote:

I think that sticking a resistor in series is the easiest solution
(and the one that I have done after receiving advice and it works).

Even better if you can find the right bulb is to use a bulb as the
resistor, the current inrush gives the motor higher starting torque and
the hot resistance actys as a current limiter.

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