Additional lighting functions turn a 4 function decoder to 6+ with this easy to assemble “function enhancement board”. Even if you do NOT need it right now, I'd suggest that its a really good "Learning: experience" that will give you confidence in playing with lighting, so why not have a try - the component cost is low, and its a good, simple project to learn with!!
It seems that no matter how many functions the decoder has, there’s always “one more” that would be nice to have – whether it be cab lights, destination boards or interior lighting or additional marker lamps for EMU / DMU units, or perhaps a smoke unit, but need them all for basic lighting…. This simple “add-on” will give you the best of both worlds. As a simple bonus - full power is always present with DCC so these effects can happen whether the loco is stationary in the yard or at full throttle on the main.
Note – I have reworked the text and drawings considerably, but this is NOT my original work or design. However, it’s been on my PC for a while and I can’t for the life of me remember where I got this excellent circuit – I would like give credit where credit is due so if you do know it’s creator please let me know!
Circuit Description
Looking at the circuit diagram it can be seen that one function controls the cab and marker lights. This function acts like “turning on the ignition”. When it is switched on the class and cab lights come on, bringing the loco to life into life without having to move.
But – we don’t want the cab lights on while the loco is moving, so when the headlight is activated, the circuit switches in two diodes (D1 & D2) to fix this by "grounding" the power to the cab lights when either headlight is on.
The second decoder function controls the high/low level for whichever headlight is currently on by switching a resistor in and out of the headlight circuit. When High Beam is on the transistor is turned on, shorting the resistor and applying full power to the lamps. With the function off the lights are dimmed by the transistor going off switching the series resistor into the circuit.
Now the loco can dim its headlights when approaching a station or waiting in a passing siding. This is also called Rule 17 in the USA and adds a new dimension to operating sessions.
In the Diagram the “End of Train” indicator wiring is drawn as dotted lines. Connected as shown they will be controlled by direction of travel as is done in most commercial models.
Parts List
• This parts list is for H0 & 00 scale – the circuit will of course work in ANY scale however for N / large scale you may need to change LED sizes
• The resistor values will still work, although you may choose to increase or decrease them depending on LED’s chosen
• Any 4 function decoder (F&R light plus 2 more)
• 3mm Golden white LED’s (DCCconcepts part # LED-GW3)
• 0.8mm SMT Golden white LED’s (DCCconcepts part # LED-SM1)
• Miniature red LEDs (2mm lighthouse type or small SMT types)
• 1.1" x 0.8" piece of 0.1" Vero board
• BC327 or similar 800mA PNP Transistor
• D1, D2 1N4148 or similar Small Signal Diodes
• R1, R2, R4, R7 4K7 Resistor
• R9, R10 10K Resistor
• R8 1K0 Resistor
• R5, R6 75R Resistor
• R3 56R Resistor
• Very Thin black wire, or spare decoder wire
Construction
It is sensible to assemble and test the whole lighting system on the bench first. This enables easy light intensity adjustment away from your model.
The brightness of all LEDs & lamps may have to be adjusted to suit the particular brand that you purchase. Simply replace the appropriate resistor higher or lower values to increase brightness.
This is an idea for a simple “LED resistance tester” that costs very little and will allow perfect selection of the right resistor FIRST TIME, EVERY TIME. (I developed this based on an idea Suggested by MAX who models 7mm fine scale modern British Railways in Thailand)
Once everything is assembled and checked, the whole unit can be mounted. Initially, its sensible to use a little more wire than absolutely needed… you can easily shorten to the final length as you add the board to the loco and dress the wires to keep them out of sight. Do keep wires routed clear of motors, and make sure they do not affect truck/bogie swing.
PCB
So that you have an insulated surface to mount on, assemble all the components on the copper track side of the printed circuit board, the opposite side to where they are normally mounted, as shown in the drawing.
CLICK on the image for a larger “Pop-up” image of the PCB.
Be careful with the diode orientation (indicated by the diode's band). Don’t forget that the cab lights should be hidden out of view on the ceiling so as to fill the cab with a soft glow.
TIP: It may be easier to solder the wire ends for the cab lights to a small square of PCB and glue these to the cab side – later, paint a “wiring line” line with special silver conductive paint from the squares of copper to the “glued in place” cab roof LED – this will work well and prevent difficult soldering. When it’s been tested you can over-paint the silver paint with cab interior colour, and it will become invisible.
CLICK HERE to buy conductive silver paint.
Check the fit of the "class light" LED’s in the holes. If the LED is slightly too big, don't worry...LED casings are made from a very forgiving epoxy compound… so they can be filed to be a tight fit in the hole. Note that for loco's with illuminated class numbers the LEDs should possibly be flush with the outside surface so that numbers can be glued on to them. It is important to have both LED’s the same way round. Or they won’t work.
TIP: “Illuminated” class numbers can be printed on transparency film using a Word processor capable of printing clear characters on a black background (for example Microsoft Word). Trim the film to fit the shape of the class light and glue in place. (use a clear varnish OR an adhesive that dries "water clear" for this, and coat the whole "glued" surface with a thin layer for even light transmission)
For the end of train lights use miniature red LEDs. As with the class lights these are also soldered in series, but this time the resistor is soldered between them. Be sure to check the spacing. These are usually soldered between power and the opposite end headlight.
Testing
You can do all tests on a normal 12v DC power pack…Connect +12 Volts to the "Power" connection on the PCB. Touch the power packs ground to the "Ignition" connection. The cab and class lights should light. If they do connect this wire more permanently so that both hands are free.
Use another wire connected to the power pack ground to touch the "Front Headlight" (DFH) connection. The front headlight should light and both cab lights extinguish. If wired as per dotted lines the EOT lights at the rear should light. Leave this connection and connect the wire from the power pack ground to "High Beam" (F2). The front headlight should brighten considerably.
Disconnect the two wires in the above paragraph and repeat using the rear headlight connection (DRH).
Once everything is OK, you are ready to go. Install all lights in your loco and carefully connect and dress each wire (Trim all wiring as short as possible, route it close to the roof and away from motors, gearboxes etc and solder each end (tinning first) to the PCB using the board layout as a guide).
Wire to your decoder and enjoy a lighting experience that YOU have created!
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