Relay control, instant ON, delayed OFF.

Discussion in 'The Projects Forum' started by MRW1962, Jul 9, 2012.

  1. MRW1962

    Thread Starter New Member

    Jun 29, 2012
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    Hi,

    I am redesigning a circuit that detects trains on sections of 3-rail track in a large model railroad. I take advantage of the fact that the trains run on a constant 18VAC that is applied to the center rail and then to the outside rails. Because the outside rails are in common, I disconnect one rail and run it to my detector. When a train rolls onto my track, the 18VAC from the opposite side now flows through the detector wire to my circuit which is a AC/DC rectifier that puts out 12VDC when on. My current design has no logic, its a simple 12 volt output.

    This is a problem when the train is only the wheels of a car that initially makes intermittent contact with the rails. The output causes the 12VDC relays that control signalling for the track to chatter and the signal indication to fluctuate between green for clear, and red for occupied.

    What I would like to do is use some kind of circuit that would use a very small delay for the output to come ON, and a 1/2 second delay for OFF. As I browse the electronic advice here I see alot of references to the 555 circuit. I wonder if it could be made to automatically reset itself after the 1/2 second to be ready to turn on the output again?

    The ON delay is not necessary, it could be instant ON.

    Thanks in advance.

    - Miles
     
  2. mcasale

    Member

    Jul 18, 2011
    210
    12
    I'm having trouble picturing the circuit. Can you provide a schematic?

    It seems that if one rail is at ground and the "sensing" rail is not, no current will flow through the sense rail.

    What am I missing?
     
  3. #12

    Expert

    Nov 30, 2010
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    Use a bigger capacitor on the input of the 12V regulator.
     
    Last edited: Jul 9, 2012
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    unless the regulator is on the train, which is bounching off the rails. not sure how these things are set up. might need a cap across the relay coil.
     
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  5. #12

    Expert

    Nov 30, 2010
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    The rectifier/regulator gets connected when 2 rails get connected by the presence of the train wheels. The bridge rectifier and regulator will allow quite a lot of current at first contact. My idea is that this first surge can charge a capacitor, probably best to put it at the output of the rectifier. That capacitor can defeat the startup bounciness for a few milliseconds. In addition, its nature would be to cause the output to persist after the original connection is gone and not going to resume.

    18 Vrms will rectify to 25.45V minus 2 diodes = ~ 24 volts DC.
    The first filter cap will charge to 24 volts and supply the 12V regulator until the voltage falls to about 15VDC. 9V x C/load will tell the persistence time.
     
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  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    isn't there some sort of signal present in the 18VAC wave as well? I think I remember reading that in another train thread. not sure if/how that might be involved/effected by changes.
     
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  7. #12

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    I can only work with what was given in the original post because I don't know about the conditions you bring up here.
     
  8. strantor

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  9. SgtWookie

    Expert

    Jul 17, 2007
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    Charging a large cap from the rectifier isn't such a wonderful idea, as it will tend to make the wheels and rails get burnt sections as electrical contact is made and broken. It would be better to use a relatively high impedance as a load/trigger to start the timing; the DC source can be made available constantly from the other rail or the transformer.
     
  10. #12

    Expert

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    Wet blanket. If he goes high tech he won't get to enjoy the smell of vaporized steel :(
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Yeah, but replacement track and new wheels are kind of pricey nowadays. :(

    It would be nice to know what exactly our original poster is dealing with; I'm only aware of O gauge and 027 gauge having three rails and operating on AC at up to ~18v or so. I used to have some Lionel stuff from back in the 1950's; the train's transformer had a secondary winding with a slider across the windings; you could easily change the output voltage (and thus the speed of the train) by moving a control arm that moved in an arc.

    I'm aware that HO trains are now using a fixed voltage DC supply and have electronic controls for the engine's speed. I was not aware of O gauge and 027 having fixed VAC with electronic controls. It would be nice to know some more details.
     
  12. MRW1962

    Thread Starter New Member

    Jun 29, 2012
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    Hello!
    The track power is for Command Control, it uses 18VAC and handheld remote controls for the individual trains. The 2 outside rails are in common, so even without one, the locos get everything they need from the middle rail and the one remaining outside rail.

    Attached is a schematic for the current design in use.
     
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