Tapiola Parish Model Railway Club


Masa built the "Truman", or Tr2 class "Russian decapod" on Piko underframe, using plastics. After finishing that project he was ready to try underframe construction, to build a Hv3 loco.

I (Pekka) told him, that I first want him to build the locomotive depot, and then go into loco scratchbuilding. Hence he has been building the turntable for the depot.

Photos on this page are courtesy Opa, Antti, Olli and Pekka


The mechanical construction of the table is a design of Mr Koskela: ex. 1/2 in. HP tape drive unit's tension arm assembly acts as the load bearing unit. The arm has two tapped holes drilled, and inverted U-channel aluminium profile is attached to it in order to form the solid base for the turntable bridge. The turntable pit is a separate plywood sheet and the bearing unit is attached to this. The plywood is then fixed to the layout base with long screws having nyloc nuts as stoppers and wing nuts for fixing. This way we can adjust the vertical plane of the turntable to match the layout. Ring rail is only cosmetic, but it will later act as feeder to the track on the bridge. The pit edges are fixed to the layout cover and the joint between pit bottom edge and plywood bottom of the pit is concealed befind ring rail base. The pit edge does not reach to the bottom to allow for adjustments.




The bridge top is PCB, glued on top of the aluminium U-channel, and the rails are electrically separated with a jagged insulation made with a minidrill. The line is jagged to prevent bending. The superstructure is brass, soldered together and soldered to the PCB, and later insulated from one side.



As the aluminium U-channel was not dead center (I drilled the holes!), the cosmetic side members (brass) were packed to make the outside look symmetrical. The details are glued on, except the cabin base, which is soldered together.

The cabin is made of scribed polystyrene.


Below the baseboard we have a largish aluminium plate fitted to the axle (there was a screw conveniently at the axle centre). The aluminium plate has a friction drive system to operate the bridge. Microswitches will be fixed to turn the polarity of the bridge tracks (we aim to have the layout built with reliability in mind -- if we loose aux. power we can still run over the bridge and turn it manually as there will be no electronics in track polarity system).

The bridge's electronic indexing was tested for the first time on Sunday 9th July 2001 The idea is based on Teton Line's turntable indexing, but we use only one opto-sensor pair and detect only the other edge of the "vane".

Opto-sensors are usually used to detect if the beam of light is blocked or not. We instead search for the "gray area" in between. The system basically has a window discriminator where one op-amp detects if the beam is blocked, and the other if the beam is un-blocked. On both cases the amount of darkenss required is adustable, so that we also have a situation, where the beam is not blocked and not un-blocked. In this situation the table has reached the correct position. The other op-amp will drive the table clockwise and the other anti-clockwise. Additional electronics provide for slow acceleration of the table, so that whenever the correct position is found, the table will be almost instantly halted but it allways overshoots, so the electronics will slowly accelerate the table again towards the correct position. As the table is stopped from slower speed than before, the table will halt closer and closer to the correct spot and thus due to slow acceleration will not gain speed to pass the correct spot as fast and far as at the previous attempt the table will eventually (after approx. three trials) find the correct spot.

I wished to separate the window position setting from the window size setting.

On the right is a suggestion from Model Electronics Railway Group MERG to make the adjustment easier.

The turntable was shown to public at the Helsinki Club's Xmas exhibition 2001. We added a pin to the aluminium plate that operated a microswitch that changed the track polarity of the table via a relay. The table also has a motorcycle dynamo carbon for current pickup from the ring rail, the other polarity comes via the axle.

The positioning system worked well, it was fun to see the audience reactions when the table halted not quite correctly, but after a brief moment then shifted a bit to actually align the rails.

The positioning system and pickup will be developed further.

Surprisingly identical approach done long before ours -- I understand -- at MIT Model Railroad Club

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