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Model train railway project w/o pic

Phiber

Sep 14, 2012
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we were making a project, model train railway, without the use of microcontrollers and PIC,
I would just like to ask, what can I use for the lever that goes down when the train is near and goes back up again after the train passes.
any ideas for what I can use for that? thanks
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Are you referring to a "Crossing Gate"?

Chris
 

BobK

Jan 5, 2010
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A servo and a '555 with a switch to switch between two duty cycles.

Bob
 

Phiber

Sep 14, 2012
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@CDRIVE
- yes. im referring to the crossing gate

@BobL
- you mean two 555 timer? one for going up and another for going down?
 

BobK

Jan 5, 2010
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A servo will hold a particular angle depending on the length of pulses that are sent to it. You would need to send one pulse length for the up position and another for the down position. A single 555 timer could be used, to provide these two pulse widths by switching in a different resitor to change it. If you needed to control it electronically. a JFET could be used in place of the switch.

Here is an example of a 555 circuit to control a servo.

http://www.hvlabs.com/555servo.html

In your case, you could replace the pot with a fixed resistor to give one position and another resistor in series or parallel, switched in and out to set the other position.

Bob
 

Phiber

Sep 14, 2012
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if the pulse width for 0deg = 1.5ms and 90deg = 2.5ms,
will it go back to 0degrees if I sent a 1.5ms pulse width if it is in the 90deg position?
is that right Mr. Bob?

sorry its my first time using servo.
 

BobK

Jan 5, 2010
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The standard pulse width for servos is 1.0 to 2.0 ms pulses repeated every 20ms.

Whatever pulse width you are sending, it will go to that angle, so, the answer to your question is yes.

Edit: Removed part about sending no pulses. The servo will just shut off if you do that.

Bob
 
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CDRIVE

Hauling 10' pipe on a Trek Shift3
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I'm a bit confused here. I've never worked with servo motor design but I thought I understood their basic use, which is to reposition the armature between 3 positions and do this rapidly. Rapidly is where I'm getting lost here because the last thing you want on a RR Crossing Gate is rapid movement. The primary goal of all model railroad design is realism. I've seen memory wire used for gates. I've also seen conventional brushed and brush-less motors using PWM and gearing but not servos.

Am I all wet here? Can servos be made to move slowly?

Chris
 

BobK

Jan 5, 2010
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Actaully, they can, but the circuit I suggested would not do that. You are right, I did not think about the fact that it must move slowly. A servo will position anywere in between two extremes, proportional to the pulse with. So to move it slowly you would have to slowly increase or descrease the pulse width.

The way to do this without a micro would be to use a voltage controlled constant current source for charging to get the pulse width, and apply a ramp to it. The constant current source is nothing fancier than a common collector transistor with a resistor on the emitter side and the base being the control voltage. But this sitll way more complicated than my first idea.

The ramp could be created by charging, discharging a capacitor.

A micro would be far easier at this point.

Or, an alternate solution is just to use a low RPM gear motor and limit switches.

Bob
 
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CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Actaully, they can, but the circuit I suggested would not do that. You are right, I did not think about the fact that it must move slowly. A servo will position anywere in between two extremes, proportional to the pulse with. So to move it slowly you would have to slowly increase or descrease the pulse width.

Bob

Wow! Talk about not seeing the forest through the trees. I'm embarrassed to say that I should have known that or at least surmised it.

Chris
 

Phiber

Sep 14, 2012
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one more question sir. if a reed switch goes back to an open position after the magnet in front of the train moves away. what can I use in order to continue the process of sending voltage to my 555 timer if the circuit will be opened?

thanks
 

Bluejets

Oct 5, 2014
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Actaully, they can, but the circuit I suggested would not do that. You are right, I did not think about the fact that it must move slowly. A servo will position anywere in between two extremes, proportional to the pulse with. So to move it slowly you would have to slowly increase or descrease the pulse width.

The way to do this without a micro would be to use a voltage controlled constant current source for charging to get the pulse width, and apply a ramp to it. The constant current source is nothing fancier than a common collector transistor with a resistor on the emitter side and the base being the control voltage. But this sitll way more complicated than my first idea.

The ramp could be created by charging, discharging a capacitor.

A micro would be far easier at this point.

Or, an alternate solution is just to use a low RPM gear motor and limit switches.

Bob
Another tack would be to remove the feedback pot and any stops in the servo to convert it's action to a gearmotor. Then gear down the output to the barrier mechanics and fit the feed back pot there. Just a thought. Another would be to use one of the small stepper motors on Ebay similar to a cd carriage drive i.e. linear output via a course screw drive.
 
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eetech00

Nov 17, 2014
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Hi

For what it's worth, a real crossing gate is raised by a motor and held up by the presence of supply voltage. Gravity causes it to lower.
If power is lost, the gate lowers..

eT
 
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