Turntable

at the Pioneer Stadium annual train show. It had to run forwards then backwards along a shuttle track. Logic inside of it achieved this. (see Model T page). Knowing that this was not the way the prototype ran, I knew that one day something more realistic had to be done and a turntable at one end would help the situation. And rather than tackling two such devices, a balloon loop at the other end was the obvious answer.

A successful hunt through the garage for appropriate wood scraps and electric motors, etc. prompted me to have a go at upgrading the system. It had to be automatic in operation, and thus based around a delay timer, reed switch and magnets, similar to the railcar. This would also have to be modified slightly to fit in with the turntable cycle.

The actual turntable was roughly cut out by wood saw and then made round and true using a sanding disc with a suitable shaft at the centre point of the table.

View of underside of turntable.

The centre bearing was originally only a hole drilled in a piece of aluminium, but after some use (and wear!) this was replaced by a small ball bearing race. The underside of the turntable also has two contacts connected to the reed switch.

The base board with wiper contacts.

The table is supported by three equally spaced old-style curtain rollers running on short axles mounted across holes drilled in the base board. Two wiper contacts connect with the reed switch at each ‘stop’ location.

The electric motor (ex photo copier I think) was ideal, having built-in reduction gearing and a suitable output wheel. Drive to the turntable is via an idler disk, with suitable sized O ring, which is spring loaded to maintain pressure for operation. The direction of rotation is such that under load, the idler tightens. A good idea I thought, except for when things get out of alignment, hence the improved centre bearing.

Reed switch at centre of the table.

The overall operation is as follows:       The railcar arrives at the turntable, passing over a magnet mounted just outside the actual table. This magnet closes the reed switch under the car, which turns off the power to its motor and starts a delay timer (approx. 50secs).

Momentum carries the car onto the table, where a magnet mounted under its front axle closes a reed switch located at the centre point of the table.

The car stops before running off the end of the table. This reed switch starts a 555 delay timer (approx 15secs) after which power is momentarily applied to the second electric motor. This pulse winds a cord up and pulls the latch lever out from the table and also starts the main motor via the micro switch.

 The table rotates 180deg (takes approx 12secs) and stops when the latch lever returns to a home position in one of two recesses in the circumference of the table. These are slots filed into pieces of aluminium plate. The micro switch turns off the motor drive at the same time. After the balance of the original 50secs has expired the car starts again and leaves the turntable passing over the magnet. This is ignored by the car because of a second delay timer (4secs) inhibiting the reed switch circuit.

The ‘Ladies  & Gents’ building conceals the main drive motor and circuit board.

Having now turned the car around, rather than going through all of this construction a second time, I opted for the easier solution of putting a ‘balloon’ loop at the other end of the track. I have made a symmetrical, spring return point for this purpose. The 12 pieces of a 4 foot diameter circle are arranged as shown.

Intermediate station stops for the railcar can be organised by placing more magnets on the track.

Other aspects of the project include, increasing the stop time of the railcar from 12 to 50secs, and changing the logic so that it does not reverse direction. I have fitted a two-way switch inside the railcar to select between the two modes of operation. (shuttle or turntable).

So far the whole setup has been very reliable with few problems.

I do have to adjust the position of the turntable magnet periodically, moving it closer to the table, to compensate for the decrease in railcar speed, and resulting momentum, as the batteries expire.

Go to My Train Layout to see a video clip.

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