Complete instructions on how to make an automatic indexing model railroad turntable.
This indexing system neither uses an Arduino board nor stepper motor. It uses a standard 12v DC motor controlled by a specially designed circuit detailed below.
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- 12 volt sensor relay
- 12 volt DPDT relay with 2 amp minimum contacts
- 12 volt DPDT relay wired for latching
- 220 ohm 1 watt resistor
- 1 k resistor x2
- Rotary switch – number of positions equal to or greater than bridge exit points
- FC51 x 2
- 940nm IR emitters – one for every bridge exit point
- DPDT toggle switch
- Momentary Contact Button Switch
- 2n3904 x2
- 1n914 x2
The Turntable Control Circuit
The image to the left shows the circuit
It ignores the drive mechanism. The drive mechanism is covered in detail at Turntable Drive Mechanism.
This circuit was designed by Melvin Perry. Here is a link to his blog. It is filled with excellent information.
Adding The Components To The Turntable
This is what Melvin Perry said:
” I mounted a IR LED Sensor (removed from the FC-51s) under each end of the bridge pointing to the wall of the pit. I also removed the Emitters from the FC-51s as they are not needed.Â I paralleled the two sensors and routed the wires through the slip-ring assembly to the FC-51 detector mounted under the pit. I built my slip-ring because I couldn’t find any that would work 20 years ago when I started my CMR Turntable project.Â That is not the case today, there are a bunch of very nice small slip-rings available on eBay now.
I drilled 1/16″ holes in the wall of the pit at the same level as the emitters mounted to the bridge. A hole centered under each track, you only have to be close to center as the tracks can be tweaked a bit later for perfect alignment. I purchased the 5mm 940nm IR emitters LEDs off eBay
Behind each hole I mounted a LED Emitter to the wall pointed through the holes to the inside of the pit. When the sensor LED sees the Emitter through the hole it triggers the FC-51 module and pulls in the sensor relay turning off the turntable motor. My turntable uses a Dayton gear motor which is kinda unique, it has a 12 volt motor with a 7189:1 gear ratio. The 12 volt motor will work as a dynamic break when the motor wires are shorted together essentially stopping the turntable instantly. I wired the motor relay to short the windings when the motor is off.
There are a lot of guys on the forum using micro computers for indexing and even though I’m into Arduinos now I think my indexing system works better than if it was controlled by computer, super accurate no adjustment needed. It has never failed me since day one.
The only critical thing is the LED sensors location under the bridge, they must be identical in location, within 1/64″ or better for the bridge position to be accurate on both ends. ”
By Melvin Perry from MyModel Railroad Turntable. – Excellent tutorial
The image of the turntable wiring diagram was made by Gary Granai
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