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Related Topics Programming Decoders Digital Decoders Decoder Finder Choose a Locomotive Decoder Choose a Sound Decoder Compare Interfaces Digital Command Stations Digitising Model Railways
Connect and assign lighting safely

Configure Lighting Outputs: AUX, Mapping & LEDs

Headlights, tail lights, cab lighting, engine-room lighting, high beams, shunting lights and interior lighting can be controlled individually with modern decoders. This guide explains output types, electrical limits, series resistors, function mapping, dimming and lighting effects.

Amplified output or logic level? Check the permitted load Never connect an LED without checking it
Configure the Outputs Calculate the Resistor Explore Decoders
ESU 53451 LokProgrammer for configuring lighting outputs PIKO 56517 SmartDecoder XP F with configurable lighting outputs
The quick explanation

What does configuring lighting outputs mean?

A decoder contains electrical outputs that switch lamps, LEDs, cab lights, tail lights, digital couplers and other loads. The two conventional lighting outputs are often described as front light and rear light or as AUX0. Additional outputs may be called AUX1, AUX2, FA1, A1 or similar, depending on the manufacturer.

Configuration involves more than simply switching a light on or off. Function mapping determines which function button activates an output, in which direction it operates and whether additional conditions apply. The brightness, fade-in and fade-out behaviour, flashing patterns and timed sequences can then be adjusted.

The output type is essential. An amplified output can directly switch a suitable load within its specified limits. A logic-level output supplies only a control signal and may require a vehicle circuit board or an additional amplifier circuit. SUSI, servo and loudspeaker connections are not freely loadable lighting outputs.

Electrical safety information
Check the output type and permitted load before connecting anything

An LED must be operated with suitable current limiting unless this is already provided on the vehicle circuit board. Do not exceed either the permitted current of an individual output or the decoder's total output load.

Manufacturer ranges for decoders and lighting control

Manufacturers offering extensive function mapping

Names, electrical limits, CV ranges and programming procedures differ between decoder families. The manufacturer pages provide access to suitable decoders, command stations and programmers.

Do not confuse electrically different connections

Understand amplified outputs, logic levels and SUSI

An interface may provide more connections than the decoder can switch through amplified outputs. The vehicle circuit board and decoder must therefore always be considered together.

Front and rear lighting

The conventional direction-dependent lighting outputs are often assigned to front headlights and rear lights. Function mapping can nevertheless assign them to different buttons and conditions.

Amplified AUX outputs

An amplified output contains its own output stage. It can directly switch a load within the specified individual and total current limits.

Logic-level outputs

Logic outputs provide only a control signal. They are intended for suitable vehicle circuit boards, transistor stages or other circuits explicitly designed for logic-level control.

SUSI interface

SUSI transfers data and power to compatible auxiliary modules. Its contacts must not automatically be treated like normal AUX outputs.

Servo and special outputs

Certain decoders can control servos, smoke generators or digital couplers. Special connection and timing parameters apply to these loads.

Common return connection

On many wired decoders, a separate common positive connection is provided. Vehicle circuit boards may nevertheless use different internal wiring arrangements.

1

Identify the output

Compare the pin assignment, wire colour and circuit-board labels in both sets of instructions.

2

Measure the load

Determine the current required by the LED, lamp or auxiliary function.

3

Add suitable protection

Connect an LED with correct polarity and suitable current limiting.

4

Map the output afterwards

Test the output at low brightness before assigning it to a function button.

Output or connection Typical application Direct connection? Most important check
Amplified AUX LED, incandescent lamp, relay or coupler Within the manufacturer's limits Individual current and total decoder load
Logic-level AUX Control input on a vehicle circuit board Not as a normal power output Signal level and required amplifier stage
SUSI Sound or auxiliary module Compatible modules only Pin assignment and module power supply
Servo output Pantograph, door or moving function Only as specified in the instructions Power supply, signal and end positions
Loudspeaker Sound reproduction Compatible loudspeaker only Impedance and power rating
Energy-storage module Bridging brief power interruptions Designated connection only Charging circuit and capacitance
Planning aid for individual LEDs

Calculate an LED series resistor for a lighting output

The series resistor limits the current flowing through the LED. The calculation requires the output voltage, LED forward voltage, number of LEDs connected in series and required operating current.

The actual output voltage may depend on the track voltage, decoder, vehicle circuit board and return connection. When in doubt, use a higher resistance and begin with a low brightness setting. Modern LEDs often require considerably less current than their theoretical maximum rating to produce a convincing result.

If the vehicle circuit board already includes current limiting, do not add another connection path without checking the circuit. Series and parallel arrangements must be planned separately when several LEDs are used.

LED Resistor Calculator

Approximate value for one LED or a series-connected group.

Calculated Reference Value
Calculated resistance 2,600 Ω
Recommended standard value 2.7 kΩ
Power dissipation 0.06 W
Resistor power rating at least 0.25 W

This calculator is a planning aid only. The decoder, vehicle and LED data sheets and the voltage actually present in the circuit remain decisive.

From the electrical connection to the function button

Configure lighting outputs in ten steps

1

Identify the decoder family

Determine the manufacturer, product number, firmware and interface version. Different variants of a 21MTC decoder may provide different output configurations.

2

Identify the vehicle circuit board

Check which pins are already amplified, dimmed or connected through series resistors. The interface standard alone does not describe every vehicle function.

3

Check the output type

Clearly distinguish between amplified AUX, logic level, SUSI and servo connections. A logic output must not be loaded like a normal power output.

4

Determine the current requirement

Consider loads both individually and together. Headlights, cab lighting and interior lighting may be switched on simultaneously.

5

Complete the wiring without track power

Insulate the wires, keep soldering times short and prevent short circuits to the chassis. Never connect the decoder to the command station while soldering.

6

Test the output at low brightness

Activate the output with a low brightness setting. Check the polarity, temperature, flickering and any unintended interaction with other circuits.

7

Assign a function button

Decide whether F0, F1 or a higher function button switches the output. Avoid duplicate assignments involving sound or shunting mode.

8

Add direction and operating conditions

Headlights and tail lights can depend on the direction of travel. Cab lighting can additionally depend on the vehicle being stationary or on the selected direction.

9

Configure the brightness and effect

Reduce the brightness before activating soft start, flashing, firebox flicker, fluorescent-tube effects or other suitable lighting modes.

10

Save the project and test every operating state

Test every combination of direction, stationary operation, analogue operation, function buttons and braking sections. Then save the completed project.

Link buttons, outputs and operating conditions

Function mapping for realistic lighting arrangements

Basic mapping connects a function button to an output. Advanced mapping may additionally consider the direction of travel, stationary operation, speed range, logical conditions or several outputs that must operate together.

Required function Possible button Condition Suitable configuration
Headlights F0 Direction-dependent Alternate between the front and rear lighting outputs
Red tail lights F1 or F2 Each end of the vehicle controlled separately Allow each end to be disabled when hauling a train
Shunting lights F3 Both directions Combine white lights at both ends with shunting mode
Cab lighting F5 and F6 Relevant end only, preferably while stationary Dim the lighting and disable it automatically while moving
High beam F7 Additional to the normal headlights Use a second dimming value or an additional output
Engine-room lighting F8 Independent of direction Use subtle dimming and optional timing
Firebox flicker Linked to sound or a separate function button Steam locomotive and operating state Use an irregular effect supported by the decoder
Interior lighting Any available function button Entire train or individual coaches Use a warm colour and keep the brightness well below maximum
Lighting functions in every model railway scale

Important considerations for H0, TT, N, Z, 0, 1 and G

Gauge Typical interfaces Special challenge
Z gauge Direct wiring and very small decoders Minimal installation space, sensitive LEDs and limited heat dissipation
N gauge Next18, NEM 651 and micro decoders Compact circuit boards and a limited number of amplified outputs
TT gauge PluX12, Next18 and NEM 651 Vehicle-specific circuit boards and different decoder assignments
H0 gauge NEM 652, 21MTC, PluX16, PluX22 and Next18 The widest variety of amplified and logic-level outputs
H0e / H0m Micro decoder, Next18 or wired connection Limited space for the decoder, resistors and energy-storage module
Gauge 0 PluX22, 21MTC, screw terminals or solder connections Numerous lighting functions and higher load currents
Gauges 1 / G Large-scale circuit boards and high-output decoders Smoke units, servos, powerful lamps and high total loads
Narrow down faults systematically

Why is a lighting output not working correctly?

The LED remains completely dark

Check the polarity, series resistor, common return and function mapping. A logic-level output may additionally require an amplifier stage.

The LED is extremely dim

Check the dimming value, resistance, output type and vehicle circuit board. Do not confuse a logic output with an amplified AUX output.

The output switches in the wrong direction

Check the direction condition in the function mapping. The motor or lighting wiring may also have been reversed.

The light flickers while the vehicle is moving

Check the current collection, contact springs and return connection. An energy-storage module can provide support but cannot replace reliable electrical contacts.

The decoder becomes warm or switches off

Switch off the track power immediately. Check the individual and total current, wiring, short circuits and permitted output load.

Several functions respond together

Check the mapping rows, conditions and vehicle circuit board. Some models connect several LEDs together at the factory.

The cab lighting remains on while moving

Add a stationary or speed condition if the decoder supports advanced function mapping.

An incandescent lamp is too bright after conversion

Dim the output and check the permitted lamp voltage. An unsuitable lamp may overheat and damage parts of the vehicle.

The programmer displays a different output name

Compare the interface version and decoder project. AUX labels on the vehicle circuit board do not necessarily match those displayed by the software.

Suitable digital technology at Modellbahnshop

Programmers, function decoders and lighting by gauge

Before selecting a product, compare the interface, output type, number of amplified outputs, logic levels, individual current, total load, digital protocol and installation dimensions.

Configuration and diagnostics

Programming and testing devices

Control cars, multiple units and illuminated coaches

Function decoders and interior lighting for N, TT and H0

Locomotive lighting with several functions

Locomotive decoders for H0 gauge

Compact lighting control

Decoders for TT, N, Z, H0e and H0m

Numerous outputs and larger loads

Decoders for Gauges 0, 1 and G

Add useful digital functions

Related guides to decoders and vehicle electronics

Basic configuration

Programming decoders

Configure addresses, CVs, driving behaviour, lighting, mapping and sound.

Open guide → Pin assignment

Compare decoder interfaces

Understand the differences between Next18, PluX, 21MTC, NEM 651 and NEM 652.

Open comparison → Buying guide

Choose a locomotive decoder

Find a decoder by interface, motor current and function outputs.

Open buying guide → Lighting and sound

Choose a sound decoder

Combine the sound project, loudspeaker and switchable lighting functions.

Open buying guide → Retrofitting

Digitise a model railway

Add driving, lighting and auxiliary functions to analogue vehicles.

Open digitisation guide → Coaches and layouts

Model railway lighting

Discover interior lighting, lighting strips and suitable retrofit sets.

View lighting →
Frequently asked questions from the digital workshop

FAQ about lighting outputs and function mapping

What is an AUX output on a locomotive decoder?

AUX describes an additional function output. Depending on the decoder, it may be amplified or provided as a logic-level output and can control lighting, a coupler or a vehicle circuit board.

What is the difference between an amplified and logic-level output?

An amplified output contains a power output stage. A logic-level output provides only a control signal and requires a suitable amplifier circuit for many loads.

Does every LED require a series resistor?

An LED requires current limiting unless this is already integrated into the vehicle circuit board or lighting assembly. The required value depends on the voltage, LED and desired current.

Can an LED be connected directly to a logic output?

Only when the manufacturer explicitly permits this application and the current and voltage are suitable. Logic outputs must not be treated as equivalent to amplified AUX outputs.

What does function mapping mean?

Function mapping assigns function buttons and operating conditions to physical outputs or logical decoder functions.

How can headlights or cab lighting be dimmed?

The brightness value assigned to the relevant output is reduced. The responsible CV or software setting differs between decoder families.

Why does the cab lighting remain on while moving?

The function has been assigned to a button but not to a stationary or direction condition. Advanced mapping can add this dependency.

Can white and red lights be switched separately?

Yes, provided the vehicle circuit board and decoder offer enough separate outputs and each lighting circuit is electrically accessible.

Why does an LED flash unintentionally?

Possible causes include an active lighting effect, poor current collection, incorrect wiring or an overloaded output circuit.

How many LEDs can one output supply?

This depends on the circuit, LED current and permitted load of the individual output. The decoder's maximum total load must also be observed.

Can function mapping be programmed on the main track?

Many DCC decoders support Programming on Main. A manufacturer-specific programmer is usually clearer when editing complex mapping tables.

Why do AUX3 and AUX4 not work directly?

On certain interfaces or decoder variants, these outputs are available only as logic-level signals. The vehicle circuit board must then contain a suitable amplifier stage.

Realistic lighting begins with the correct output

Further decoder guides, interface comparisons, instructions and product recommendations are available in the central model railway guide.

Open the Model Railway Guide