Rear Lights on modern image U.K. freight trains.

 

 

Note that the smaller pictures can be enlarged by clicking on the images. (except the lamp on the right)

 

Introduction:

The rear warning lights on today's U.K. freight trains are typically battery powered red LED technology lamps that flash on and off approximately 120 times per minute (or twice per second).  This webpage explores simple techniques to model operating rear warning lights and illustrates several examples applied to the  rear of container flat cars. The solutions are designed for operation on DCC layouts only.

Rear warning light of the type commonly encountered on modern freight trains

 

Keeping it simple!:

I did not want to use a dedicated DCC decoder for this purpose, so the objective was to find a simple discrete component approach. The initial idea was to use a flashing red LED powered from the DCC track signal via a bridge rectifier. This is a cheap solution, with parts available from Maplin plus a flashing red LED sourced from Bromsgrove Models. The main disadvantage is that the only flashing red LED types I have found on the web are 3mm round top cylindrical devices, which are really too large for 00 gauge applications.

Prototype circuit diagram:

An unacceptable solution:

A first prototype, fitted with a 3mm diameter flashing red LED, shown below, illustrated just how bad the large LED looked. The flashing rate was correct, but clearly a solution with a more realistic looking rear lamp was needed.

Dapol "Spinewagon" fitted with a 3mm diameter LED rear light.......NOT a very realistic solution!

 

 

 

Towards a better solution:

Experiments with the flashing LED showed that it could be used as a simple timing circuit to make a smaller ordinary chip LED flash at the same rate.

So, if a more convincing model of the rear light can be made, incorporating a small chip LED, then this LED can be flashed at the correct rate using the circuit above.

 

  Taking a closer look at the standard UK rear light unit:

Rear warning light close up

 

Side view of light

 

Rear light removed

 

Back view of rear light showing mounting slot

 

Drawing of the real Dorman rear light, copied from their webpage and re-dimensioned to OO scale.

 

 

Hopper car showing the rear light in action

More Details on the essential parts of the circuit

1) The Wheel Contacts

The DCC pulsed AC supply can be used to power the rear light system. A set of contacts needs to be fitted to the rear bogie of the final wagon in the train. Phosphor bronze strips made by Albion Alloys can be obtained from good model shops, made specifically for this purpose. On the prototype Dapol Spinewagon shown below, the contact strips are held in place with plasticard, super-glued to the bogie frame, and bear against the rear of the wheels.

Rear bogie before addition of wheel contacts

Modified bogie and rear platform

 

2) The Bridge Rectifier

A full wave rectifier produces a peak voltage of approx. 14 Volts DC when connected to the track on my Dynamis DCC system.  This can be made from 4 discrete diodes or a miniature bridge rectifier module. I used a bridge from Maplin for the first prototype, which is a bit on the large side, but smaller types are available from DCC Supplies.

3) Series isolating resistor

220 ohm resistor to prevent excessive loading of the DCC supply.

4) Reservoir capacitor

A small capacitor of circa 10uF is needed on the output of the bridge rectifier, to maintain the voltage throughout the DCC supply waveform. (Without this, an erratic flashing frequency is produced).

5) Flashing LED

Although the flashing Red LED is now painted over and hidden from view, it provides the essential timing circuit to drive an ordinary red LED. The 3mm type from Bromsgrove Models works well.

6) Resistors

The flashing LED draws current when in both the on and the off state. The resistors ensure that the voltage across the small rear light LED is low enough to keep it "off"  during the off phase of the flashing LED cycle. The value of the resistors, controls the brightness of the rear light LED in its "on" state.

7) The rear light LED

A small 0603 size red chip LED was used for the rear light illumination. This was incorporated into a model of the standard UK trains Dorman rear light.

 

1/76 scale model of the Dorman Rear Lamp:

The OO scale model is made from 1mm plasticard and an 0603 size red chip LED.

After soldering the wires to the chip LED, the device was fitted in a groove filed in the rear of the lamp body, with the LED light window facing forward in the centre of the 1.5mm hole. Super glue was used to hold the LED and wires in place. "Glue 'n' Glaze" was used to fill the hole above the LED, forming the lamp lens.

The red LED light is so bright that it makes the white body of the entire rear light glow. To prevent this, the body was coated in opaque black paint after assembly (except of course for the lens area). When dry, the black was over-painted with semi-matt white paint and the red rectangle below the lens was added. Finally, a thin coat of red water colour paint was applied to the lens surface, providing the correct "off" colour. The red water colour paint still transmits most of the red LED light when in the "on" state.

 

 

Fitting the solution to the Dapol Spinewagon:

The 3mm flashing LED was painted overall in opaque black and installed, hidden in the space under the rear platform, with the resistors and capacitor. The Dorman rear light model was mounted on a small plasti-card bracket above the wagon's rear platform. The first model used a cylindrical bridge rectifier mounted above the rear platform. Subsequent models used smaller bridges, mounted under the platform with the other components.

 

The circuit with flashing LED

now hidden under the platform

The new rear light active

(photo with light levels low enough not to saturate)

Rear of the Freightliner container train showing the new rear light (before I discovered that red water colour paint can be used successfully to colour the lens).

 

Rear of a GBRF container train showing the second new rear light. (The light is really red but is saturating the camera)

Showing actual colour of rearlight flash

(The light saturates the camera at normal exposure)

GBRF electrics with smaller bridge rectifier

(All hidden under rear platform)

 
Fitting the solution to the Bachmann container flat:

The rear bogie of the Bachmann container flat can be modified in a similar way to the Dapol bogie. To reduce the chance of interference between the pick-up feed wires and the coupler mechanism, the rear coupler was removed. As no hidden space is available on the rear of the Bachmann flat, the electronic components are hidden in the container. Two thin wires are routed from the container rear to the rear light bracket.

 

Unmodified rear bogie

 

 

Electrical pick-up wires fitted and coupler removed

(before painting the plasticard black)

 

Plenty of room for the parts this time!

 

Third new rear light

 

Circuit fitted and 3mm flashing LED painted black

Container top fitted and rear light flashing away

 

Conclusions:

The use of a flashing LED as a pulse generator for a smaller chip LED was a useful and completely unexpected outcome to this exercise. The end result is a simple low cost circuit driving a scale chip-LED equipped rear warning light with the correct flashing properties and powered entirely via the DCC track waveform. (With accuracy only limited by modelling skills!) The prototype model light looks brighter than the real thing, particularly when seen in daylight, but the intensity could be adjusted by a suitable change to the resistor values if a dimmer light is preferred. (e.g. change the 2k2 to 4k7). Or of course: apply a thicker coat of red water colour paint to the lens!

 

A more traditional approach using an Astable multivibrator:

Where there is plenty of room to accommodate a few more parts, e.g. in a container or tanker wagon..... This arrangement enables a correctly timed flash frequency, combined with a brief on period, to simulate the strobe flash produced by the real rear lights. The following circuit is copied from the Class 60 Zimo decoder web page, where the parts are housed in the tank of an Amoco bogie oil tanker wagon.

 

The circuit was optimised for the on and off times of the flashing sequence. The resultant circuit diagram is shown below:

(The two diodes in series with the transistor emitters prevent reverse breakdown of the base emitter junctions when the base voltage shoots up to maximum, which could otherwise blow-up the transistors.)

The on period is determined by the 0.22uF capacitance multiplied by the 220k resistor.

The off period is determined by the 0.22uF capacitance multiplied by the 2.5M resistor.

The frequency is approx 2 flashes per second and the flash on period is short to simulate a strobe.

 

For the tanker application, the circuit was built into a shape that could be slid into an aperture cut in the underside of the tank.

Circuit fixed to a plasticard substrate. The 0.22uF caps are huge....I'll use small electrolytics & lower value resistors next time!

 

The plasticard is glued to the aperture edge. (A quick slice by the Stanley knife will facilitate removal if repairs are ever needed.)

 

Phosphor bronze wheel contact strips super-glued on to plasticard supports fixed to the rear bogie, provided the DCC supply voltage.

All complete and working well! (Difficult to catch the flash on camera though.)

 

Equivalent astable circuits can be designed around an opamp or a 555 timer, with a single timing capacitor. (A large remaining stock of high gain PNP transistors determined my route.) It was also a trip down memory lane, as this was the very first electronics circuit I ever soldered together, for a physics project at Thorne Grammar School in the early 1960s.

 

Using the DCC Concepts Flashing Rear light:

DCC Concepts supply a modern image flashing rear light kit with sufficient parts for 3 applications. The lamp mouldings are very good, a big improvement on my crude plasticard efforts. The kit also includes the flashing electronics module and a Hall Effect magnet detector which acts is used to activate the unit via 3 included magnets. However, there were a few problems:

1) Although the polarity of the wires in the rear light assembly is defined, the polarity of the corresponding flashing module connections is not. (Orange wire = positive and grey wire = negative.)

2) No advice is given on how to keep the module on in the absence of the Hall device if magnetic activation is not required. (Connect module purple and yellow wires together.)

3) The spring wheel contacts provided are not appropriate for UK wheelsets where both wheels are isolated from the axle. (Wheel wipers required.)

4) The flasher unit looses the plot whenever the briefest brown-out occurs, producing an irregular flashing rate. (A heavy ballast weight above the pick-up bogie or an external diode bridge, isolating resistor and large capacitor, solves this problem.)

 

Supplier & Enthusiast website links:

 

Hattons of Liverpool A first rate mail order company for RTR models.
Trains 4 U (Peterborough) A good (almost local) source of Bachmann and Dapol product. (My source for the Dapol flat cars, containers & phosphor bronze wheel contact strip.)
Bromsgrove Models Excellent source of DCC decoders and specialist LED devices for this type of project. Unfortunately the web sales business has now closed.
DCC Supplies DCC specialist supplier based in Worcestershire. My source for bridge rectifier, LEDs and other DCC supplies!
TCS (Train Control Systems) A U.S. decoder company, with good programming data and advice on their website.
Maplin A convenient local source for electronic components such as resistors and transistors

The photos of real freight train wagons and rear lights were taken at Ely in November 2009.    

 

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