There's more than one way to do it. This is my way.
First, a word about my particular setup. I'm using a Lenz Set 02/4, in HO scale, and do not have any separate electrical blocks. Track is Atlas code 83. The benchwork where the turntable goes is 1/4" ash plywood. Roadbed is Woodland Scenics foam. There is currently no scenery in the turntable area.
My need is for just a simple mechanism for turning locomotives, with possibly one or two short engine storage tracks. Nothing fancy required.
Next, some pros and cons of using the Atlas turntable instead of a different brand.
My initial plan was to provide a DCC decoder (the cheapest one I could find) to drive the turntable. This would allow speed and direction control from the handheld just like a locomotive. Other people have implemented this method successfully.
Once I saw how the mechanism on the Atlas table paused at each track, I realized that additional fine position or speed control wasn't needed. I had the idea of running the DC drive motor directly with track power. (I honestly don't remember if I saw it mentioned someplace else or not.) That way I could save US$16 that I would have spent for a decoder.
In a DCC system, the track carries a high-frequency square wave as long as the power is on. (I'm simplifying, and ignoring zero stretching for this discussion.) A bridge rectifier and a capacitor are used to convert this to DC, which is then fed to the motor through a DPDT center-off switch. Here's a circuit diagram:
I mounted the rectifier and capacitor to a terminal strip. This isn't really necessary, but I use terminal strips for everything. I'm not even sure the capacitor is required, but I had one, so I used it. (It's only a 4.7uF, so I don't think it does much good at filtering out the high-frequency noise.) Flip the switch one way for clockwise rotation, the other way for counter-clockwise. Leave it in the center to stop.
Some notes: I made an engineering estimate (better known as SWAG, or Some Wild-Ass Guess) for the current draw of the motor at 0.5A at 12V. I also reduced the output voltage from my LV101 power station from 16V to 13V. (Actual voltage at the motor terminals is about 11V.) Running the motor at more than 12V for a short time probably won't harm it. If you need a higher track voltage, add a voltage divider (two resistors) between the capacitor and the switch. Lowering the track voltage also improved speed control on my locomotives.
I would have liked to use a spring-return center-off toggle switch, but didn't want to actually spend money on one. I ended up with a rocker-type switch that works fine.
Be sure that if you use an electrolytic (tin can) capacitor that the '-' lead on the rectifier is connected to the '-' lead on the capacitor. If you have a flat disc capacitor, don't worry about it.
My motor unit made a godawful screech in one direction when I got it. A tiny drop of oil on the pulley axles and motor bearings helped a lot, but it's still pretty noisy. I don't know if it's feasible to insulate the little shed that houses the motor.
I suspect that there is still some high-frequency signal getting through to the motor. It seems to work well, but may add to the noise. I don't plan to do anything about it.
I won't cover the track wiring here. The instruction sheet included with the turntable addresses that.
After I did all of this, Bruce Petrarca of Litchfield Station sent this much more elegant (using fewer components) variant, which I wish I'd thought of. He suggests a 1N4001 diode and a 10uF 25V capacitor, which should be fine.
Ed Roberts then suggested adding a resistor to lower the voltage on the motor. He says that the Atlas motors work best at 6 to 9 volts DC; I haven't checked it out, but it should reduce noise. I would guess that a 15 ohm, 1 watt or so resistor would be appropriate. Thanks to both for their improvements.
If all this sounds like a neat idea, but you just can't get over the looks of the turntable, there is information on the web (I forget where) about building a nice-looking bridge on top of the Atlas mechanism, which is mounted in a pit. The wiring and movement remain unchanged, and the motor could be mounted differently to hide it under the layout.
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Last updated 06 June 2002