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Some notes and a photo on building a high quality,
automatically indexed mechanism for operating a turntable.
J.Banner photo & text
 This is the drive
for the turntable that the Saskatoon Railroad Modellers use on their HO-scale
exhibit at the Saskatoon Western Development Museum. It is seen here from
below the table. Gear motor (A) turns an indexing disk (B) via a
rubber tired friction wheel at (C). The PVC indexing disk, which
is about 10" in diameter, is coupled via a vertical shaft to the
16" turntable bridge which sits in a pit cut in the table above the disk.
The shaft turns in a long brass bushing set in the bottom of the pit.
A microswitch (D) with arm and wheel (E) is mounted next to the
disk. When the bridge rails are aligned with the approach rails,
the microswitch wheel drops into one of the semicircular notches (F-F).
This mechanically indexes the bridge and cuts off the power to the motor.
There is a tension spring attached to the microswitch arm, visible at (E).
The other end of this spring is attached to a length of mono filament nylon
fishing line which runs over a series of pulleys and down inside a wall
to a lever at a convenient location on the fascia of the lower level of
the layout. Pulling the lever disengages the microswitch from the indexing
disk and also turns on the power to the motor. In operation, the
lever is held until the bridge approaches the desired track, then released
so that the microswitch wheel drops into the next notch, stopping and indexing
the bridge. A cross wired DPDT switch reverses the polarity of the
applied power to reverse the direction of the bridge.
For maximum reliability, a split pit rail was not used. Rather,
all the bridge wheels where connected together and wired to one bridge
rail and the centre shaft was wired to the other bridge rail. While
this scheme increases reliability by providing more pickup points on the
pit rail, it normally requires a reversing switch to control bridge track
polarity, depending on which way the bridge is aligned. Such a reversing
switch seemed to defeat some of the ease of operation we were trying to
achieve on this layout.
Our solution was to paint the indexing wheel half black, half white
then position a reflective optical sensor at (G). An LED in
this sensor beams infrared light at the surface of the disk and a photo
diode in the sensor detects the reflected infrared light only if the white
section of the disk is above the sensor. When the photo diode detects
light, it activates a transistor which in turn pulls in a relay which reverses
the polarity of the bridge rails. This worked well with both dc and
later with DCC. With DCC an alternative would have been to use an
automatic reversing module like the one sold by MRC.
Also seen in the photo are the wire to the pit rail (H) the wire to
the central shaft (J) and the spring loaded carbon brush which transfers
the power to the shaft (K).
This, and a similar mechanism installed on the author's home layout
have been in operation for a number of years. The author's version
uses a split rail in the pit and occasionally suffers from interruptions
of the bridge power, but the version pictured here has been satisfactory
in all respects. |
