A Turnout Lock for a 3-D Printer

A project by Ken Hanawalt for the Turtle Creek Railroad.

On the Turtle Creek Railroad layout, at the town of Charleroi, there is a small yard for the Ferry Train branch and also a few mainline industries. In the diagram below, the branch tracks and industries are blue, the mainline tracks are red, Tracks shared by the branch and main are violet, and mainline industries are black. Both main and branch are protected by signals where they go into a tunnel on the right. The turnout marked with a circle in the diagram separates the main and branch areas, and is usually set in "reverse" to guide any mainline train onto the front (mainline) track and to restrain the branch train to the back. The Ferry branch is a turn, coming in from the right and returning to the right.

However, after the ferry train is finished working the town, it needs to use the shared part of the inner main track to reach the turntable before returning through the tunnel. The turnout (marked with a circle) connecting the two areas has a lock on it that prevents it from being turned to the "normal" position unless the lock is opened, and when the lock is opened, the signal at the tunnel for the inner (branch) track turns red to remind mainline train engineers not to exit the town on the branch track.

The lock is a small 3D printed object with a hinged lid that fits over the throwbar of a Peco turnout and prevents the turnout from being changed when the lid is shut. Other brands of turnouts would have to be modified or would require a different cutout. The base of the lock (left part of the diagram shown below) contains a cut out for a Hall-Effect transistor, a cutout for the end of the turnout throwbar, and part of the hinge. The top of the lock (middle part below) contains the other part of the hinge, a cutout for a magnet, and a cutout for the throwbar. The cover for the lock (right part) is glued to the top of the lock when assembled and serves as a handle. The cutout in the bottom part prevents the throwbar from moving toward the lock and the cutout in the top part prevents the throwbar from moving away from the lock unless the lid is open. When the lock is assembled, a straight sewing pin is used to hold the hinge parts together and serve as an axle.

The left photo below shows the lock installed over the turnout throwbar with the lid shut so the turnout cannot be changed. The turnout is in "reverse" position, so a train on the inner (shared) mainline will be directed to the outside main and right hand-tunnel entrance. The right photo shows the lock with the lid open (painted red inside as a warning to the operator). With the lid open, the turnout position can be changed and the track signal at the tunnel entrance will be red.

The Hall-Effect Sensor transistor (HES) illustrated on the left below turns on when a magnet is present. The illustration shows the front of the HES. The back of the HES is flat. The left pin (1) is +5, the middle pin (2) is the ground, and the right pin (3) is the output. When a magnet is present (very near the back of the HES) the transistor turns ON (pin 3 goes low so current can flow into it). The right part of the illustration below shows a test circuit for the HES. NOTE pin 3 is the right pin, not the middle pin. So when the lid of the lock is closed, the magnet in the lid is near the transistor and current can flow to pin 3 from +5 through the 220ohm resistor and the LED, making the LED light. If you build a lock, you will want to connect this pin to your signal circuit where you can control what happens when the lid is opened.

The transistor will fit into the opening in the base of the lock. The pins are bent toward the front and at a right angle to the transistor so the back of the transistor is even with the top of the opening and glued in place. The magnet is glued into the circular opening in the lid. You can see these parts installed in the photo of the lock above. The transistor can be ordered from Amazon or from Electronics123.com using part number SO70375, SEN-14709s, AH1815 (a non-latching, low sensitivity, omnipolar HES) for about $1.50. The magnet is from APEX (1/4” x 1/16” Neodymium) about $0.25 each in quantity. You can download the file TurnoutLock3.stl for the lock parts from a Stereo Lithographic (STL) file for a 3D printer.