Railway Points Sequencer


Dedicated model rail
enthusiasts using sophisticated train and points controllers often have
the problem that as their layouts get bigger and more complex, the
transformer supplying power to the points does not have enough current
to switch several points at the same time. The actuators in the points
are designed for ac operation so it doesn’t help by rectifying the
supply and adding reservoir capacitors, the coils can overheat and burn
out if they get jammed during their travel (ac operation actually helps
to overcome friction in the mechanism). The circuit shown here solves
this problem by using a sequencer to ensure than only one points
actuator can be active at any point in time.

During operation the controller will switch all the points on one
line at the same time as usual, but the other connection to each coil is
connected to the sequencer unit. This circuit will only allow current
to flow through one coil at a time. The sequencer circuit consists of a
555 timer configured as an astable multivibrator clocking a 4017 Johnson
counter where the ten outputs are used to switch ten triacs in sequence,
enough for ten sets of points. P1 alters the oscillator frequency of
the 555 timer and can be adjusted so that each time interval of the
sequencer is long enough to allow the points to switch.

The switching time varies depending on the type of points but is
typically between 1 s and 1.5 s. Any points that jam during switching
give out a characteristic humming noise in time to the switching
frequency so it makes them easier to find. The eleventh output of the
4017 can be connected to an LED (together with
a series resistor). This will flash to give a visual indication of the
sequencers operation. Power for the circuit is provided by 15 V ac from
the points transformer. The B80C1500 bridge rectifier (80 Vpiv, 1.5 A)
and regulator IC1 produce a stabilised 12 V for the circuit. Current
consumption is only a few milliamps.


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