Perpetual Back and Forth Circuit

Discussion in 'The Projects Forum' started by Jeauxm, May 17, 2013.

  1. Jeauxm

    Thread Starter Member

    May 8, 2013
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    Perpetual Back and Forth Circuit

    I am trying to design a perpetual back & forth circuit to run a 12 volt trolley on a single HO track. I actually created a design of a circuit 3 years ago and eventually built it. It did NOT work - hence my appearance here!

    Any help with this hobby obsession of mine will be greatly appreciated.

    I will start with the requirements and then give what I have attempted along with photos attached (as PDFs).

    Requirements :
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    A) DC Only (no DCC) - track polarity will be reversed to reverse trolley (Street Car if in Nawlins).

    B) Track will be 2 rails and can be any length - it will have a "dead end" on each end (this is NOT a loop).

    C) Power applied to track using only 2 wires (no other wires connected to track).

    D) There will be a gap on the starboard side rail at both ends of the track. This will be bridged with a diode which will stop the trolley until the polarity is reversed.

    E) Voltage applied to the two wires will be nominal 12 volts (or more if using HO power pack and speed setting).

    F) Trolley should travel past the gap and stop. After current consumption has stopped and waiting a specific time, the polarity should be reversed.

    G) Trolley will travel in the opposite direction and once past the other gap will stop. After current consumption has stopped and waiting a specific time, the polarity should be reversed.

    H) Some time delay is needed once trolley has stopped - do not want to throw the motor in full reverse when running (this is noted in steps F and G).

    I) This will run indefinitely until the power is removed from the circuit (turned off).
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    I have built a test track (see attached pictures) which implements a manual version using a double-poll double-throw knife switch (wired as cross-over). When the switch is dropped to the left - the trolley travels to one end and past the diode gap at which point it stops. If the switch is flipped to the right - the trolley travels in the opposite direction until the other diode gap is passed. This test board test track proves the concept of the diode gaps and polarity reversing. It has a two wire terminal strip so the "new" reversing circuit can be plugged in (and replacing the knife switch).

    Attachments :
    Perpetual B&F.pdf - original design with suggestion of substitution of optoisolators. This was actually built (not with optoisolators) and when power applied - it buzzed like an electric razor. I believe the problem is in the 2 latching relays - they may be flipping each other back and forth.

    Double Latching Relay for Reversing.pdf - this is my logical view of how I intended the 2 latching relays to operate the polarity reversal. This part needs some delay or isolation - I have not bread boarded this piece but I think it is a problem.

    Test Track.pdf - contains pictures of the test track and trolley along with the gaps and diodes. When the trolley passes a gap and stops, the switch is thrown and the trolley travels in the opposite direction until it hits the other gap.

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    Here is what I think I need : 3 major portions of the circuit.

    1st is the reversing portion (see Double Latching Relay for Reversing.pdf) which fired from a momentary voltage. I wanted to do this with 2 coil latching relays (I bought a bunch from D.K. - see the B&G picture for part numbers).

    2nd is the current sensing portion. I got the original idea from neighbor's model railroad layout. He used a bridge rectifier and Christmas lights to trigger a photocell. I substituted LED's (one for each direction) based on findings from the web. My original design did not account for any delay. The photocell was supposed to pull the SPDT relay in when the trolley was running. The absence of the current would make the LED's go out causing the relay to fall and send the momentary voltage to the latching relays. This would cause the polarity to switch, the trolley to run, the LED's to light pulling the SPDT relay on. This whole thing was "kinda" electro-mechanical. I think a 555 timer might be better suited (more on that in 3rd part). I also showed some optoisolators on my original drawing (with part numbers). I bought a bunch of these on my last Digikey order. I got the DC and AC versions - The AC version will detect in both directions. The DC versions are for future block detection activities in another layout. I would like the circuit to use those if possible (instead of the photo cell and lights).

    3rd part is to "Invert" the absence of current on the tracks in order to "fire" the 2 latching relays. From what I have found trolling the websites - 555 timer circuits may be the trick for this. I don't know enough about interfacing multiples of these but I have read up on the different implementation favors of the 555 timers. I also figured this is where I might be able to add some timing and delay.

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    I have thrown a lot out there for my first post hoping to be as detailed as possible. Please ask for clarifications if needed.

    I have never posted to a forum before but after reading through tons of posts and the great answers some of your members provided - I couldn't resist.

    My profession is software applications design. I have done some dabbling in electronics over the years.

    Thanks in advance for any help your members can provide to me in figuring this out.

    Joe
     
    Last edited: May 17, 2013
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    Can you please post the schematics and photos etc as images? ie GIF/PNG/JPG etc.

    PDF is not the preferred format for images as the files are larger, it requires a PDF viewer to open and PDFs can contain trojans and auto web downloads etc. :)
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
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    Well here is what i don't understand,

    1) What is an "HO track"
    2) "Starboard" is a Nautical or Aeronautical term for right side, and your tram is on rails!
    3) Where is "Nawlins"
     
    Last edited: May 17, 2013
  4. MrChips

    Moderator

    Oct 2, 2009
    12,442
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    1) What is an "HO track" - model railroad gauge of 1:87, track gauge (width) 16.5mm

    2) "Starboard" is a Nautical or Aeronautical ,term and your tram is on rails! - right side for you landlubbers

    3) Where is "Nawlins" - New Orleans, Louisiana, USA
     
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  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    I'm a model railroader too and I understand the concepts. Actually the standard for polarity is that when a track runs east-west "If the south rail is positive the train will run eastbound".

    Forget this optoisolator stuff. All you need is a relay wired to take the place of the knife switch, plus electronics to drive the relay. I don't see any need to sense the location of the trolley, as long as its speed is more or less consistent and you don't care exactly what the dwell time at the end of the run is.

    So you'd have an oscillator (everyone will say use a 555) that drives the relay with a 50% duty cycle where time between reversals is always longer than it takes the train to make the journey. So when the relay is set for eastbound travel, trolley goes east until it hits the dead section, trolley pauses until the relay reverses for westbound travel, trolley starts up and runs to west end of track, pause and repeat.

    HO is "Half O", the most common scale for modeling, 87:1. It's a crazy mix of units, 3.5mm to the foot.
     
  6. Jeauxm

    Thread Starter Member

    May 8, 2013
    32
    0
    Sorry about the PDFs. I thought I read in the posting rules that PDF was the preferred format. I will extract the pictures and post each individually.

    I have posted two images - but get Token security errors (I think the file sizes are too big). I will post in different replies.
     
  7. Jeauxm

    Thread Starter Member

    May 8, 2013
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    HO is one of the many modelling scales used in model railroading. It means Half of O-scale (HO).

    Starboard - I used this Nautical term to indicate a particular side of the track - facing forward on a boat (in this case trolley) it is always the right side - if you were to turn around and face the rear - the Starboard side does not change. Note: Trolleys don't really have a front or back - they are same on both ends. Bad choice on my part.

    Nawlins - that's the way folks in New Orleans say "New Orleans" !!! The Streetcar used in the pictures is a model of the Desire Street Streetcar. Only found in New Orleans - hence the reference. I think they made a movie way back in 1951 with Marlon Brando called "A Streetcar Named Desire".

    I am having problems posting the JPGs - I will have to do it in multiple replies because of the size.. Please bear with me.
     
    Last edited: May 17, 2013
  8. Jeauxm

    Thread Starter Member

    May 8, 2013
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    Good definitions.......Thanks
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Try using PNG's, those usually work for larger images.
     
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  10. Jeauxm

    Thread Starter Member

    May 8, 2013
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    Here are the JPG images from the TEST Track PDF - one member preferred JPG over the PDF. I had to Shrink the image sizes to be able to post pictures. See the PDF in my earlier post for the FULL size (better quality).
     
  11. Jeauxm

    Thread Starter Member

    May 8, 2013
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    Hey John P - thanks for the quick update. I actually considered doing just that (using a timer set longer than the travel time to flip-flop a DPDT relay).

    That IS still an option for the simple back and forth. I didn't want to give up on my challenge of getting me original circuit polished up. The reason is that I intend to use the current detection in my main layout to control other blocks and turnouts. For example - a simple reversing loop consisting of three distinct blocks.

    So I broke my circuit into the 3 main components with the intent of learning how those interface or integrate into each other. Of course I need to get double latching relays to play nice first using a trigger.

    I appreciate the model railroader's insight and your help. I just want to take the more difficult road. I guess I had other intentions for these parts of the circuit that I didn't disclose as for fear of muddying up the waters (another nautical term !!)[​IMG].

    I will actually breadboard a 555 timer to flip-flop a DPDT relay this weekend just to get the practice of calculating the timing with different values. That's the cool thing about my testtrack - I just disconnect the knife switch and route the output of my breadboard in place. But I am not giving up on the more complex solution.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    If you wanted to go all electronic you could use a MOSFET H-bridge circuit (4 MOSFETS) to control the direction of the current through the rails. That would avoid the mechanical noise and limited life of mechanical relays.
     
  13. Jeauxm

    Thread Starter Member

    May 8, 2013
    32
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    I am not sure what that is. I will do some research into MOSFET H-Bridge. Can you explain what parts of my original design that would replace ?

    Thanks, Joe
     
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  15. crutschow

    Expert

    Mar 14, 2008
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    You can replace all the relays with MOSFETs.
     
  16. Jeauxm

    Thread Starter Member

    May 8, 2013
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    Can anyone help me figure out how to get my original circuit to work ? I appreciate the input about going totally electronic but I want to keep the circuit I made and use the 12 volt supply. If I start using transistors don't I have to have different voltages ?
     
  17. Jeauxm

    Thread Starter Member

    May 8, 2013
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  18. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You can use transistors/mosfets with 12vdc.

    To use what you've got why not put a detector at each end instead of the middle, using the bridge with the AC opto and non-latching relay. It looks like that relay is sensitive enough (11.7ma) that the AC opto just might be able to drive it directly. Add a protection diode across the relay coil. Also add a small value 10-22 ohm resistor in series with the opto led's.

    Just an idea.. gotta go for now.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    Yes it will work on 12V, just apply 12V where the 9V is shown. And yes, 1a and 1b are the inputs to control the direction of the voltage applied to the load (Coil 1, which would be the tracks in your case). When 1a is high (12V) and 1b is low (0V or Gnd) the right side of the load is positive and the left side is negative. When 1a is low and 1b is high the load voltage is reversed. If 1a and 1b are both low (or both high) then the voltage across the load is zero.
     
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