I've been working on the controls for the turntable. Originally I was going to use a simple manual control but my dark side won out with a more involved design. I'm using an Arduino Mega 2560 microprocessor to control an EasyDriver board running a NEMA 17 stepper motor. The Arduino also controls a 4x4 matrix keypad, a Nokia 5110 LCD display and some switches and LEDs. Total price of the system is right around $75 which includes everything mentioned above.
The motor has 400 steps/revolution and has a 20 tooth pulley driving a 60 tooth pulley on the turntable shaft. The pulleys are connected with a GT2 timing belt. With the pulleys 3:1 ratio we get 1200 full steps/revolution. The EasyDriver has the ability to run 1/8 steps/revolution so in that mode we get 9600 steps per full turn of the bridge and very smooth motion. I'm also using the AccellStepper library which provides smooth acceleration and deceleration curves for the stepper motor. When the bridge is driving from one track to another the motion is very cool. It starts off slow and smoothly ramps up to walking speed (Armstrong turntable). As it approaches the new position it smoothly slows down and stops.
When you first give power to the system the turntable begins rotating CCW until it trips an IR detector mounted beneath the pit. This sets the home position that all tracks are referenced to. To operate the bridge you enter a track number on the keypad. I have 16 tracks around the pit so the numbers are 0 - 15. If you want to turn your locomotive 180 degrees you enter the track # and "A" and then hit # to enter. The LCD displays current track position, next destination and provides a "moving" statement while the bridge is turning. There is a switch for reset and one for emergency stop and LEDs for "Home Position" and "Emergency Stop".
The controls will mount on a 5" x 3" panel on the layout fascia with most of the electronics hidden behind it.
Thanks Si. I've been having a good time with this but it's turned into the never-ending project. I think I have at least two months into it so far although much of that was waiting for parts and time-outs for beer runs. Still have to finish up the pit and control panel before the final installation. Thanks again for your encouragement.