Vines Garden Railroad Workshop Articles

       

Out And Back Again: A Simple Automatic Reversing Unit

The OEM reversing unit originally installed on the trolley line at the Vines Garden Railroad would work for a few months and then, for no known reason, ceased to function without warning. After being returned to the manufacturer two or three times it was decided to build a simple circuit based upon the 555 timer. This was to be used as a backup for the OEM unit when it was out of commission. There are numerous electronic circuits to be found on the Internet where, if you are comfortable in using a soldering iron, you can build a delay circuit for a relatively low cost.

In this instance we looked at some of the circuits suggested by Rob Paisley at:
http://home.cogeco.ca/~rpaisley4/CircuitIndex.html

The LM555 timer chip is a very versatile and reliable device. With the addition of a few components and a relay to handle the anticipated current this will provide a very simple reversing unit for use with any scale model railroad equipment.

First, one should determine the length of track and the time required for the trolley or locomotive to travel the complete length, wait for a period of time, then reverse to repeat the cycle. In this instance the length of track is fifty feet from end to end. The time of travel including the stop time can be set from approximately 75 seconds to 150 seconds. Actual time of travel will depend on the track voltage (speed of the trolley).

The stop positions are determined by two diodes connected across two gaps in one rail. The two diodes must be wired to the rail in the same polarity with the Power Feed connected in between. The end of track, or stop section, being of sufficient length to fit the complete train.


While it is possible to cut a gap in one section of the rail and connect the diode across this gap. It is preferred to use an OEM re-railer similar to the Aristo-Craft ART-11350. The housing both conceals the diode and keeps the rail fixed from moving.

For the delay unit itself, all of the components can be installed within a plastic project box fitted with power connectors, fuse holder, potentiometer (for adjustable time delay) and two LED indicator lights. A simple 12 volt supply can be included using a 7812 regulator. In this instance a surplus 12 VDC 500mA regulated wall adaptor supplied power for the delay circuit.



C1 100 MFD 50 WVDC (272-1044)   R3, R4 Resistor 1K 1/2W (271-1118)
C2 0.01 MFD 50 WVDC (272-1065)   RLY1 DPDT 12VDC 5A (275-249)
D1 Diode 1N4001 (276-1101)   T1 NPN 2N3053 (276-2030)
F1 Fuse 5A. (270-1011)   LM555 Timer (276-1718)
LED1 Red LED (276-041)
LED2 Green LED (276-022)
R1, R5 Resistor 10K 1/2W (271-1126)
R2A Resistor 1M. 1/2 W (271-1134)
R2B Potentiometer 1 M. (271-211)

Additional items required to mount components:

Project Box 5 x 2½ x 2 (270-1803)
Fuse Holder 3AG (270-367)
Knob for Potentiometer (274-415)


Miscellaneous:

Perfboard (276-1395)
8 Pin DIP IC Socket (276-1995)
Binding Posts (274-661)
Connector for AC - DC Regulated Wall Adapter. Type K (274-1565)
Regulated AC - DC Regulated Wall Adapter. 12 VDC 500mA. (273-357)


Please Note:

The Radio Shack part numbers are provided for reference only where individual components may be more readily obtained from other electronic suppliers. As an example we used two 220 MFD capacitors for C1, taken from our own box of parts, to come up with the 75 - 150 second delay. Other delay times may be calculated from Rob Paisley’s web site.

The LM555 is capable of supplying 200mA at the output of Pin 3. In most cases this will be sufficient to operate the relay coil RLY1. However some heavy-duty relay coils may exceed this value. The switching transistor T1 is included to handle current up to 700mA. The LED’s are added to monitor operation and aid in adjusting the timing at a remote location.

Some G Scale power supplies and controllers can deliver more than 20 amps of current during operation. In the event of a short circuit at the track the contacts of the relay and the wiring from the Power Input out to the Track could be overloaded. On one occasion this did occur when the trolley jumped the track, short-circuited and burnt out the relay contacts and the wiring within the delay unit. The fuse F1 was added for future protection. In retrospect, this could have been one reason why the original reversing unit repeatedly failed!

This reversing unit has now been in operation for five years. Except for the incident of the short circuit it has remained in operation every weekend during the summer months without any problem. The original unit is now kept on stand-bye duty in case of an emergency.

Norman Allum - 6/24/2010