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LEDs on Model Railroad

Discussion in 'Electronic Basics' started by Joseph Sroka-10.2.8, Apr 12, 2005.

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  1. I have an N-gauge train with some remote-controlled switch tracks. This
    is an older style train, not the DCC type.

    I would like to add red and green LEDs to the switch tracks. The switch
    tracks are operated from the "accessories" terminals of a small power
    supply which are nominally 17 V AC, (I measured it open-circuit at 17.7 V

    The switch tracks are operated by pressing momentarily on a momentary SPDT

    I'd like a simple way to hook up the LEDs either at the remote location or
    at the SPDT switch. Since power to the switch track is applied only
    momentarily, I suppose that some sort of latching IC would do it.

    The 17 V AC is part of the complication, it seems.

    For reference, the switch tracks are Atlas #'s 2580 and 2581; the power
    supply is Model Rectifier Corp. Railine 370N.

    The switch tracks are apparently solenoid powered.

    The power for the tracks only is 15 V DC. I wouldn't want to power the
    LEDs from that terminal pair because it turns off when the train is
    stopped, and includes some sort of (pulse width?) speed control.

    Anybody here ever do this stuff?


    --- Joe
  2. Lord Garth

    Lord Garth Guest

    You're correct Joe, you need a device known as a flip-flop to latch to
    lamp status. Additionally, you'll need a clean and steady source of
    DC to power the device and a small circuit to clean the signal(s) that
    moves the solenoid.

    Tell us about the solenoid, does it activate a rotary mechanism that
    alternately moves the track? What I'm asking is if the solenoid
    is pulsed or whether you supply it one polarity to route the train onto
    the first track and then the other polarity to route the train onto the
    other track? I'm guessing the latter case based on the SPDT switch.
    If so, you can use an inverse parallel set of LEDs and a limit resistor
    to indicate which way the train will route.

    An actual link to your Atlas track switches would be most helpful.
    I also have no clue what a DCC style train is although I have seen
    N gauge models.
  3. It is a *pair* of solenoids. No rotary mechanism. A momentary pulse is
    sent to one or the other solenoid to switch the tracks. No continuous
    power is sent to the switch track.

    The web site I visited is, but it was no help.

    I have a couple of guides/handbooks from about 1979 which indicate that
    these switch tracks are just a pair of solenoids and a straight-line
    motion that moves the track back and forth.

    DCC is what I *don't* have. It's a digital system for controlling
    multiple locomotives on the same tracks.

    My locomotive control is apparently just a rheostat controlling about 15
    volts of DC, which is probably just full-wave rectified. I doubt if it is
    even filtered.

    I read somewhere that these switch tracks can be operated on around 15 V
    of either AC or DC. So maybe running them from some small DC supply would
    make it simplest to connect in an IC flip-flop.

    --- Joe
  4. Lord Garth

    Lord Garth Guest


    Okay Joe, thanks for the clarifications. What you need to do is to continue
    connect the solenoids the way you do currently. It is clear that the
    are not effected by the throttle position, that is good.

    The easiest way I can think of is to tap a line off of each solenoid and
    this to be compatible with the flip-flop. The solenoids are run by an
    voltage so you should determine if it is AC or DC first and then what
    Once this is known, the signal is made to operate the LED portion of an
    isolator. The reason for the opto is so we don't have to share a ground
    the train supply and the flip-flop.

    You will need an SR flip-flop. The Set & Reset inputs will be driven by the
    receiver side of the opto. You will also need to construct a (typically) 5
    DC power supply that will run the optos, the flip-flops and the LEDs. You
    want to include a power on clear for the flip-flops but they will be okay
    one cycle of the track switch.

    Lets hear what your solenoids are being driven by, AC or DC and what
    Once this is known, then we can make it run the LED side of the opto.
  5. Dave

    Dave Guest

    Hi Joe,
    What I used on my HO switches to indicate track position was to glue a
    small magnet to plastic lever on the switch machine actuator. I then
    placed a reed switch so it is transferred when the magnet moves close
    to it. I use the 17 V AC and a bridge rectifier and current limiting
    resistors to power the LEDs. This method does not require any filtering
    of the power supply.
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