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Level Crossing Gates


Here is a way of constructing electrically operated level crossing gates where a minor road crosses a single line. Only one gate is provided each side of the track so the operation  involves sequential closure of the gates.

The gates are modified Wills gates (4mm scale) covered in K&S 3/64” Diamond Special Shapes mesh to simulate the wire netting covering of the gates. A brass rod is fixed to the stile of each gate by means of soldered pins which fit through two holes drilled into the stile. The rod is a push fit into a collar on the motor gearbox final drive shaft underneath the baseboard.

Each gate is driven by means of a Multi Ratio Gearbox sold by MFA/Como Drills (Part 917D) which offers a number of gear ratios by which the speed of movement of the gates can be chosen in combination with a variable PWM power supply.  The opposite end of the final drive shaft is provided with a disc having a peg near its circumference. The rotational movement of the shaft is limited to 90º by means of microswitches contacted by the peg at each end of its travel in conjunction with the electronics described below.

Level Crossing Gate

The electronic circuit which controls the sequential movement of the gates is shown below. The control panel for the level crossing gates consists of  an on-off switch (not shown) and associated LED and a momentary contact DPDT Switch 1 with associated green and red LEDs to show whether the gates are open or closed to traffic.

The sequence is started by operation of  the momentary contact switch 1 which (a) activates latching relay 1 to change the polarity of the PWM motor supply and (b) activates either relay 2 or relay 3 to trigger the 555 timer. The output from pin 3 of the 555 timer activates relay 4 to close the motor circuit and start gate motor 1. The output pulse is long enough to allow the motor to start its travel and release microswitch M1 which switches SPDT relay 5 to keep power to the gate motor.   When the gate motor has moved the gate through 90º, the peg on the circumference of the disc referred to above switches the two microswitches M3 and M4 to cut power to motor 1 and start motor 2. Microswitches M5 and M6 are then released ready for the return sequence and when motor 2 reaches the end of its travel microswitch M2 switches to operate SPDT relay 5 to cut power to the motor.

The reverse sequence is started by operation of the momentary contact switch in the opposite direction  to reverse polarity of the PWM supply and activate the other of relay 2 and relay 3.

The purpose of the 555 timer is to provide a long enough pulse to allow the motor to start its sequence and release the microswitches without being reliant on the operator keeping the DPDT momentary contact switch pressed for the required time.

The following circuit for a variable PWM supply works fine.  The speed of movement of the gates is adjusted by means of the variable resistors.  See www.eleinmec.com/article.asp?28 for more information.

LevelCrossingGates.mp4

Here is a short clip showing the operation of the gates

© Richard Smith 2012. All rights reserved.