Scratch built turntable & control

Discussion in 'Track Planning' started by BigJim, Nov 7, 2007.

  1. BigJim

    BigJim Member

    Nov 19, 2006
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    Not sure if this is the right sub-forum but I am starting on a project to scratch build a turntable with optical sensing to stop the rotation.

    I will be using a 12" beer tray and an HO 85' ME deck girder bridge (with a little bashing) for the top part. Drive wheel, drive mechanics & motor control will be scratch built.

    I have heard that the inertia of the rotating turntable can cause problems with "overrun" so I am going to have two different sensors, one for CW and one for CCW.

    I have tested an IR LED/ IR detection diode combination using a 10 Meg pull up resistor to the input of a CMOS nand gate to detect the on/off state and this works fine.

    My plan is to have one pair of detectors with a "blade" corresponding to each turntable track. If the "overrun" is about the same at each track location I should be able to set the distance once to match the blade width and have it work for all of the blades. The drawing shows one "block" but it will have to be split to allow for adjustment.
    Plan is to lay the tracks last to match the stop "position". You look like the world's best shot if you shoot the arrows first then paint the targets.

    Control will be two push buttons (CW & CCW). Holding one button down will run the turntable at full speed in the selected direction. Letting the button up will change to slow speed and stop when the detector for that direction is covered. The speeds will be set by a variable duty cycle square wave motor drive. (see circuit below) Pushing a button sets its flip-flop and switches to the high speed source. Letting it up immediately selects low speed but the flip-flop stays set (motor on) until a blade covers the sensor for that direction to remove the reset inhibit. When the blade covers the sensor AND the button is up the motor stops.

    The electronics part is working but I have some questions on the mechanical aspects.

    Anyone have a recommendation for the drive style. I have seen small rubber wheels driving the outside of a plywood wheel, belt drives, gear drives, worm gear drives, etc. Anybody with any experience with any of these?

    Best RPM for fast & slow speeds? Source for a good motor and gear reduction box?

    Plan is to have a roller micro switch with 1/2 of the drive wheel raised so the micro switch is on for 1/2 a revolution. This will drive a relay to set the track polarity. Track power will run up the middle of the shaft. Will a 1/4" audio jack mounted in the center of the shaft work OK or do I need to plan on using ring comutators?

    Any other suggestions?

    The below circuit fits very nice on a medium project board. Total parts cost is less than $10.00.

  2. TinGoat

    TinGoat Ignorant know it all

    Aug 29, 2002
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    Turn Table Control

    Hi Big Jim,

    The 1/4" phono plug and jack should work fine.

    Track polarity can be controlled with a roller micro switch or you can use a DCC auto-reverser unit.

    You can even use optical detectors for polarity.

    There's more than one way to skin a cat and here's a couple of ideas that you could incorporate into your design.

    I would imagine that intertia could cause problems. I think that inertia would change depending on the weight of the locomotive on the turn table too... wall1 wall1

    One method of control is to use positive mechanical stops.

    You can see how it is done on the Saskatoon Railroad Modellers webpage here.

    Another method is to use a slow motion stall motor and just align the tracks by eye. There's a description Here.

    I've even heard of people using the motorized and indexed Atlas turn tables underneath the layout with a shaft running up to a nicer looking turntable on deck, or dressing it up to look like a covered pit turn table.

    For motor speed, slow is the way to go... :thumb: :thumb: :thumb: