Amplified AUX
Contains its own output stage. LEDs, suitable lamps, relays, digital couplers and other loads may be connected directly only within the decoder manufacturer's permitted individual and total current limits.
Digital Lighting Function Workshop
Connect · Check · Dim · Map
Configure Lighting Outputs | AUX, LEDs & Mapping
Headlights, tail lights, cab lighting, engine-room lighting, high beam, shunting lights and interior lighting turn a digitised locomotive into a much more realistic model.
The order is crucial: first check the output type, permitted load and vehicle circuit board. Then connect the LED with suitable current limiting. Only afterwards should the AUX output, function button, direction, dimming and lighting effect be programmed.
AUX
switchable function output
LOGIC
control signal instead of power output
PWM
dimming and lighting effects
F0–Fxx
free assignment through mapping
Before Connecting
Amplified or Logic Level?
Always identify the output type first →
LED
Limit the Current
Check the series resistor and actual voltage →
Function Button
Assign the Output Freely
Map the function button and direction afterwards →
Output Overloaded?
Switch Off Immediately
Check current consumption and wiring →
Not Every Lighting Output Is Electrically the Same
What Does Configuring Lighting Outputs Mean?
In addition to its motor output, a vehicle decoder can provide several electrical connections for lighting and auxiliary functions. Conventional front and rear lighting outputs are often already configured for direction-dependent headlights. Additional connections can be called AUX1, AUX2, FA1, FA2, A1 or similar, depending on the manufacturer.
The electrical name alone does not tell you what may safely be connected. An amplified function output contains its own output stage and can directly switch a suitable load within the manufacturer's specified current limits. A logic-level output, by contrast, supplies only a control signal for a suitable vehicle circuit board, transistor stage or other compatible electronics.
The next step is function mapping . It determines which function button activates which output and under which operating conditions. F0 can therefore control the standard headlights, F1 the cab lighting, F6 a high-beam function and another button white shunting lights at both ends.
Output effects come afterwards. Many modern decoders can dim lighting, fade it in and out, flash it or simulate particular lamp effects. These functions are manufacturer-specific. Never copy a mapping or dimming CV from an unrelated decoder without checking the correct instructions.
Never Connect an LED Directly to a Decoder Output Without Checking the Circuit
Possible Exception
The Vehicle Circuit Board Already Provides Suitable Current Limiting.
The decoder output voltage, LED forward voltage and existing vehicle circuit must be known. Modern LEDs often need only a few milliamps for a convincing model-railway brightness. When in doubt, begin with a higher series resistance and lower brightness.
Distinguish the Electrical Connections Correctly
Power Output
Control Signal
Standard Lighting
Data Bus
Movement
Return Connection
AUX, Logic Level, SUSI and Special Outputs
Logic-Level AUX
Does not provide normal load current. It is intended, for example, to control a suitable vehicle circuit board or an additional transistor or amplifier stage.
Front / Rear Lighting
Usually configured for direction-dependent headlights. Function mapping can nevertheless change the assignment, brightness and operating conditions.
SUSI
SUSI is intended for communication with compatible auxiliary modules and can also provide module power where specified. Its contacts must not automatically be treated as normal load-bearing function outputs.
Servo and Special Outputs
Pantographs, moving doors, smoke generators and digital couplers can require special signal forms, operating times or protective circuits.
Common Positive
Many wired decoders provide a separate common positive connection for function loads. Vehicle circuit boards can nevertheless use different internal arrangements, so the pin assignment must always be checked.
| Connection | Typical Application | Direct Load? | Check First |
|---|---|---|---|
| Amplified AUX | LED, lamp, relay, digital coupler | within the decoder limits | individual and total current |
| Logic-Level AUX | control input on a vehicle circuit board | no | signal level and amplifier stage |
| SUSI | sound and auxiliary modules | compatible module only | pin assignment and power supply |
| Servo | pantograph, doors, moving function | only as specified | signal, power supply and end positions |
| Loudspeaker | sound decoder | compatible loudspeaker only | impedance and power rating |
Plan Before Soldering
LED Resistor Calculator and Personal Lighting Function Planner
The calculator provides a technical reference value for one LED or several LEDs connected in series. The lighting planner then documents which AUX output should receive which function, direction condition and function button.
Calculate an LED Series Resistor
Output voltage minus the combined LED forward voltage gives the voltage that must be dropped across the resistor.
Reference Value: 2.6 kΩ
The next higher common E12 value is 2.7 kΩ.
Create a Personal Lighting Plan
The plan is stored only in the local browser and does not change any settings on the locomotive or decoder.
Workshop Rule
Test the electrical output first. Add mapping, dimming and special effects one at a time afterwards.
From Headlights to a Complete Lighting Arrangement
Eight Useful Lighting Functions for Digital Locomotives
Headlights
Direction-dependent white front lighting remains the basic function. Reduce the brightness so that individual LEDs do not resemble excessively bright modern car headlights.
Tail Lights
Make red tail lights separately switchable so that they can be turned off when the locomotive is hauling a train.
Cab Lighting
Ideally illuminate only the active cab and preferably only while stationary or at very low speed.
High Beam
Can be implemented as a second brightness level on the same output or with separate LEDs and an additional output.
Shunting Lights
Depending on the prototype, switch white lights at both ends or another appropriate shunting signal together with shunting mode.
Engine-Room Lighting
Subtle internal lighting can add convincing depth to modern electric locomotives and large diesel locomotives.
Interior Lighting
Function decoders are especially useful for control cars, multiple units and coaches whose lighting should be operated independently of the locomotive.
Digital Coupler
This is not a normal lighting load. Output capacity, pulse duration and protective functions must explicitly match the coupler being used.
Measure First, Map Afterwards
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Configure Lighting Outputs Safely in Ten Steps
Identify the Decoder
Record the family, product number, interface and firmware.
Check the Vehicle Circuit Board
Identify existing resistors, transistor stages and pin assignments.
Identify the Output Type
Clearly distinguish amplified AUX outputs from logic-level outputs.
Determine the Load
Compare the load current with the individual and total decoder limits.
Protect the LED
Check polarity and provide suitable current limiting.
Test One Function
Connect and test only one new lighting function at a time.
Map the Output
Assign the function button, direction and any additional conditions.
Reduce the Brightness
Begin with a low brightness setting and adjust it visually.
Add an Effect
Add high beam, fading, flashing or a timed function only afterwards.
Save the Project
Document function buttons, AUX assignments and the completed decoder project.
Function Mapping by Decoder Family
Manufacturers of Decoders and Lighting Control
Output names, mapping tables, electrical limits and programming procedures differ between decoder families. Always check the exact decoder installed in the vehicle.
Lighting and Auxiliary Functions Inside the Vehicle
Current Locomotive and Function Decoders for Lighting Control
ESU · H0 · Function Decoder
LokPilot 5 Fx DCC
PluX22 function decoder with ten amplified function outputs and two additional logic-level outputs for extensive lighting and auxiliary functions.
View Product →
PIKO · Next18 · Function
PSD XP 5.1 F
Function decoder with ten function outputs, including six load-bearing outputs and four logic-level outputs, plus flexible mapping up to F68.
View Product →
Uhlenbrock · 6 Outputs
76800 Function Decoder
Six outputs with switching, dimming, flashing, fade-in and fade-out functions for coaches and functional models.
View Product →
ESU · H0 · PluX22
LokPilot 5 DCC
Locomotive decoder for vehicles requiring extensive lighting, mapping and additional function control.
View Product →
Märklin · H0 · mfx
LokDecoder mLD3
Multi-protocol retrofit decoder with numerous digitally switchable functions for suitable Märklin, Trix and other H0 conversions.
View Product →
Functionality Meets Installation Space
Lighting Outputs from Z Gauge to G Gauge
| Gauge | Typical Interfaces | Lighting Consideration |
|---|---|---|
| Z | direct wiring / specialist circuit board | minimal installation space and very small LEDs |
| N | Next18, NEM 651 | logic-level outputs on vehicle circuit boards are often particularly relevant |
| TT / TTe | Next18, PluX12, PluX16 | compact function mapping with limited installation space |
| H0 / H0e / H0m | PluX22, 21MTC, Next18, NEM 652 | widest range of separately controllable lighting and AUX functions |
| 0 | circuit board, wires, screw terminals | cab, engine-room lighting and digital couplers are often controlled separately |
| 1 / G | large-scale decoder / screw terminals | higher load currents and numerous auxiliary functions |
When a Lamp Does Not Behave as Planned
Common Problems with AUX Outputs, LEDs and Function Mapping
The LED Remains Completely Dark
Check polarity, series resistor, output type, common return, function mapping and the vehicle circuit board. A logic-level output cannot power an LED like an amplified AUX output.
The LED Is Extremely Bright
Check the current limiting immediately. Decoder dimming does not replace an electrically required series resistor.
The Output Works Only with the Body Removed
Check for trapped wires, a short circuit against the chassis or unwanted mechanical contact with the body shell.
The Light Operates in the Wrong Direction
Check the direction condition in the function mapping before rewiring the vehicle.
Cab Lighting Remains On While Moving
On compatible decoders, add a stationary or speed condition to the function mapping.
The Decoder Becomes Hot After the Conversion
Switch off track power immediately and check output currents, wiring and possible short circuits. Do not continue testing with higher dimming values.
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From a Working Output to a Convincing Lighting Arrangement
Function Mapping Comes After the Electrical Test
Once all lighting outputs work correctly electrically, configure the function buttons, directions and operating conditions. Only then add dimming, fade-in and fade-out times and special effects. This makes it much easier to determine whether a fault is electrical or purely a programming issue.
The Interface Name Does Not Automatically Describe Every Lighting Function
Check the Vehicle Circuit Board as Well
The Decoder and Vehicle Circuit Board Form One Electrical System.
With interfaces such as Next18, 21MTC and PluX, individual connections can be amplified, switched or used for specialist functions by the vehicle circuit board. Always consider the decoder documentation and vehicle documentation together.
Frequently Asked Questions from the Lighting Function Workshop
FAQ About Lighting Outputs, AUX and LEDs
What Is an AUX Output on a Locomotive Decoder?
AUX describes an additional function output on a vehicle decoder. Suitable outputs can switch cab lighting, tail lights, interior lighting, digital couplers or other compatible loads.
What Is the Difference Between an Amplified AUX and a Logic-Level Output?
An amplified AUX contains an output stage and can switch a suitable load within its specified current limits. A logic-level output supplies only a control signal for compatible electronics.
Can an LED Be Connected Directly to a Decoder?
Only when suitable current limiting is already present. If the vehicle circuit board does not contain an appropriate resistor or current-limiting circuit, the LED must be connected with suitable electrical protection.
Can LED Brightness Be Dimmed with the Decoder?
Many modern decoders allow function outputs to be dimmed using PWM. Programmed dimming does not replace an electrically required series resistor.
What Is Function Mapping?
Function mapping assigns function buttons to physical decoder outputs and, on advanced decoders, to operating conditions such as direction, stationary operation or other logical functions.
Can the Red Tail Lights Be Switched Off Separately?
Yes, provided that the vehicle circuit board and decoder provide separate or independently controllable lighting circuits.
How Can Cab Lighting Be Controlled Realistically?
Ideally, both cabs are controlled separately. Suitable decoders can additionally use the direction of travel and stationary operation as mapping conditions.
Can the Same Output Be Assigned to Two Function Buttons?
Advanced mapping systems can allow complex assignments and several triggers. The available combinations depend on the exact decoder family.
Why Does AUX3 or AUX4 Sometimes Not Work Directly with an LED?
On some decoder and interface variants, particular AUX connections are implemented as logic-level outputs. A suitable amplifier stage is then required before connecting a normal electrical load.
How Much Current Can a Function Output Supply?
The permitted current is decoder-specific. Both the maximum load of each individual output and the decoder's total function-output load must be observed.
Why Does a Decoder Become Hot After a Lighting Conversion?
Possible causes include a short circuit, overload or an electrically incorrect connection. Switch off track power immediately and check the wiring and output loads.
Which Decoder Is Suitable for a Driving Trailer?
A function decoder without a regulated motor output is often ideal. The number and type of outputs, interface, current capacity and digital protocol must suit the vehicle.
Lighting Outputs Working Correctly? Now Link the Functions Intelligently.
The comprehensive Model Railway Guide covers decoders, function mapping, digital command stations, interfaces, motor control, sound, electrical systems and many other topics.