A Simple Reverser

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One relay, two microswitches and a few power supply components make a simple reversing circuit to send an engine, a train or a street car back and forth on its track. A street car scurries back and forth on a city street.  A switcher trundles to and fro on an industrial spur with a box car in tow.  A mine motor slips in and out of an adit with a string of ore cars.  These are all possible with a simple circuit which reverses the polarity of the rails each time the vehicle reaches the end of the track.  In the circuit presented here, the rail polarity is reversed by a relay, and the state of the relay (pulled in or dropped out) is reversed by switches at each end of the track.   Both the circuit and the train can be powered by the usual train set power pack with variable d.c. and fixed a.c. outputs.  Other than that, all that may be required is one or two small components, depending on the relay used.   The circuit looks something like this:     The relay originally used in the above circuit is a 3PDT with an eleven pin tube base and a 24 volt d.c. coil.  It was purchased used and had probably been salvaged from a vending machine, jukebox, pinball game, or something similar.  This relay, shown at right, comes with a clear plastic case about 2" high.  It can be plugged into an 11 pin tube socket.   Your relay could be different - it could have a coil rated at 12 volts d.c. or 12 volts a.c., and might require a different socket or no socket at all.  Any of these will work provided the relay  is a 3 Pole Double Throw (3PDT) relay.  This just means that the relay has 3 or more poles (sets of contacts), and each pole is double throw (that is, each pole closes one circuit when the relay is pulled in and closes another when the relay is dropped out.) The relay is pulled in by the train hitting microswitch #1 and closing the normally open (N.O.) contacts.  This applies about 21 volts to the 24 volt coil, which is enough to operate the relay.  Pulling in the top two sets of contacts in the diagram (pins 1,4,3 and 11,8,9) reverses the polarity of the rails.  Pulling in the bottom set of contacts (pins 5,6,7) applies power to the relay coil independent of microswitch #1, but through the normally closed (N.C.) contacts of microswitch #2.  This allows the relay to stay pulled in as the train moves away from microswitch #1 and travels toward microswitch #2.  When the train reaches #2, power to the relay coil is interrupted as the N.C. contacts of microswitch #2 open.  The relay drops out, the track polarity returns to its original state, and the train moves again toward microswitch #1.     The microswitches can be any brand.  "Microswitch" is sort of like "Aspirin" - a trade name that has become generic.  Microswitch, the company, was the first to make and market push button switches that require only tiny movements to make them work.  Then everyone else horned in.  The bodies of the three microswitches shown at right are 3/4", 1" and 2" high.  More importantly, they all have lever arms attached to operate the switch proper.  The length of the arm as well as the characteristics of the switch itself determine how much pressure is required throw the switch and how much overtravel the switch has.  Overtravel is the amount the arm can still move  after the switch is thrown.  For our application, this is the amount the train's momentum carries it after the switch is thrown.  We can increase over travel by lengthening the arm, perhaps by soldering or gluing on a strip of thin brass.  Lengthening the arm also has the effect of reducing the mechanical force the trains must apply to throw the switch, which helps in the smaller gauges.  Lengthening the arm to 6" or more is usually satisfactory. Normally, microswitches are installed vertically under the ends of the track, one at each end, with the arms poking up through slots between the rails.  Small pads of foam rubber glued to the protruding portions of the arms can help protect the paint where the train strikes the arms.  If necessary, the switches can be laid horizontally just above and at right angles to the rails.  While this may require disguising the switches with some sorts of scenery or structures, it may be the only choice if vertical space below the track is lacking. The lower photo at right shows how microswitch terminals are usually marked - C (or COM) for Common, NO.  for Normally Open and N.C. for Normally Closed.    While the third switch shown above has a long  arm and a nice soft touch, it is unsuitable for this project as it has only two terminals (the N.O. function is missing.) The other two components, a bridge rectifier and a capacitor, change the 16 volts a.c. from the power pack to d.c. and filter the ripples out to make it pure d.c. at 21 volts.  Getting 21 volts d.c. out of 16 volts a.c. isn't magic.  It is just that the capacitor charges to the peak voltage of the a.c. waveform.  If  your relay is rated 12 volts d.c., just leave out the capacitor.  Your relay will not mind if the d.c. is pulsating, nor will it mind that it is 15 or so volts after losses in the bridge rectifier.  And if you are lucky enough to find a 12 volt a.c. relay with the right contacts, then you can leave out both the filter capacitor and the bridge rectifier.  If you do have to use the bridge rectifier and capacitor, please pay close attention to the markings.  Those "~" on the rectifier are the a.c. connection, the "+" is positive and the "-" is negative.  These must go to the + and - terminals respectively of the capacitor.  Failure to properly match the polarities of the rectifier and the capacitor can lead to a loud bang  and thousands of little flecks of silvery foil floating around the room.  Don't ask how I know. There is a 50% chance that the first time you try to operate this circuit that it will not work.  That would be because there is a 50% chance that the train will go the wrong way the first time you put it on the track.  If you find that your train sits against one of the switches spinning its wheels, then reverse the direction switch on the power pack, and that should be the last time you ever have to touch it.    The finished prototype looked like this.  A screw terminal relay socket, an electrolytic capacitor (yellow arrow), a bridge rectifier (red arrow) and a ten point terminal strip were mounted on a scrap of tinted Acrylic.  The unit was wired according to the diagram above, and the external connections were labelled.    For testing, the micro switches were installed horizontally using lath nails pushed into predrilled holes.  Hammering them in would be an invitation to disaster.  The microswitch lever arms were extended by attaching  six inch lengths of 1/16" O.D. brass tubing using 5 minute epoxy.   The reverser was tested for several hours on the author's work bench before being deemed ready for installation.   The Saskatoon Railroad Modellers have used a variation of this circuit to operate a street car on their Prairie to Parkland display for about 6 years.  The only major difference is the added complication of magnetic detectors at the ends of the track operating the relay via transistor switches.  If you are interested in this or in optical control, we would be more than willing to discuss that as well. Jim Banner, electrical guru and one armed paper hanger Saskatoon Railroad Modellers group