Traffic Light Wiring Diagram: How to Read Signal Head Schematics
A traffic light wiring diagram maps every conductor, terminal and controller output. Learn to read the schematic, follow the common and switched conductors, and verify the work before power is applied.
Cable colour code by region
| Region | Red | Yellow | Green | Common |
|---|---|---|---|---|
| North America | Red | Yellow | Brown or Orange | White or Grey |
| Europe (IEC) | Brown | Orange or Yellow | Green/Yellow striped (or red) | Blue |
| China (GB) | Red | Yellow | Green | Blue |
| Australia/NZ (AS/NZS) | Red | Yellow | Green | Black |
1. Why a wiring diagram matters
A wiring diagram is more than a piece of paper: it is the legal record of how the signal system was installed, and it is the fault-finding tool every technician reaches for when something fails. A missing or wrong diagram causes mis-wired signal indications (red on green output, green on red), conflicting greens that the conflict monitor should have caught, difficulty finding the right conductor during maintenance, and disputes about warranty coverage between installer and supplier. Always keep a printed, laminated copy inside the cabinet door, plus an as-built version stored digitally for future reference.
2. Anatomy of a signal head schematic
A 3-aspect vehicle signal head schematic shows three coloured symbols (red, yellow, green), each connected to a switched output, plus one common conductor shared by all three indications. The common is usually drawn as a thicker line or marked explicitly. The controller outputs are labelled R, Y, G and C (or COM).
For a 4-aspect head with arrow: red, yellow, green (ball) on three outputs; red arrow, yellow arrow, green arrow on three more outputs; and common shared across all six. Each output on the controller corresponds to one switched conductor on the cable. The cable schedule lists every conductor by number, colour and function.
3. Common (neutral) vs switched wires
The signal head has one common conductor (often called COM or NEUTRAL) and one switched conductor per indication. The controller closes the circuit by energising the switched conductor while the common remains connected.
Common mistakes: two switched conductors share a common return, which is correct but only if the common gauge matches the total load; the "neutral" is not the same as protective earth, never use earth as the return; and LED signal heads are polarity-sensitive on DC, polarity-insensitive on AC, so confirm the input voltage before connecting. For 110V/220V AC heads, the common is at mains potential (line) and the switched conductor is switched to neutral. Always verify with a tester before touching.
4. Controller terminals and conflict monitoring
The controller has one output terminal per switched conductor and one or more common terminals. A 22-output controller has 22 output terminals plus common. Inside the cabinet, each output passes through a conflict monitor that prevents conflicting greens from energising together.
When reading the diagram: find the output panel (row of terminals labelled R1, Y1, G1, R2, Y2, G2 etc.); find the conflict monitor (separate module that monitors opposing outputs); find the detector rack (input terminals for loops, video, radar); and find the power supply (110V/220V AC input, 12V/24V DC for accessories). Match every wire in the cable schedule to a specific terminal on the output panel. If a wire does not have a terminal, it is either spare (documented and labelled) or an error.
5. Reading 4-aspect, arrow and pedestrian schematics
A 4-aspect head adds a green arrow section. The arrow has its own three outputs (R arrow, Y arrow, G arrow), so a 4-aspect vehicle head uses 6 outputs, not 4. A pedestrian head uses 2 outputs (red man, green man) plus common. Pedestrian push buttons are inputs to the controller, not outputs from the head. Audible and tactile indicators use additional low-voltage outputs. A bicycle signal is similar to a vehicle signal but usually 3 aspects (red, yellow, green bicycle symbol). A bicycle head uses 3 outputs plus common.
When reading combined schematics (vehicle plus arrow plus pedestrian on one pole), follow each head's wiring back to its specific controller output. Cross-wired outputs are the most common commissioning error.
6. Common wiring faults
| Fault | Symptom | How the diagram helps |
|---|---|---|
| Reversed polarity (DC) | Head does not light | Diagram shows + and -, check orientation |
| Mis-routed common | One indication stays lit | Diagram shows shared common, find the break |
| Wrong terminal | Wrong colour appears | Diagram shows the correct R/Y/G terminal list |
| Missing earth | Fails insulation test | Diagram shows earth terminal and conductor size |
| Undersized cable | Voltage drop, dim indication | Diagram shows conductor size per output |
| Unsealed gland | Water ingress | Diagram shows gland spec at cabinet entry |
7. Testing before power
Before energising, work through this checklist: visual inspection (every conductor terminated and labelled at both ends); continuity test (every switched conductor reads correct resistance to its terminal); insulation test (every conductor reads correct resistance to earth); earth continuity (cabinet, pole and signal head earth bonded and tested); conflict monitor self-test (passes with all outputs off); detector inputs (each detector channel responds to a test pulse); lamp test (energise each output in turn, walk to the signal head, confirm the colour); and sequence test (run the controller through a full cycle, confirm each phase). If any test fails, isolate and fix before commissioning. Energising an unverified signal head is unsafe.
Frequently asked questions
How do I read a traffic light wiring diagram?
Start at the controller output panel and follow each conductor to the signal head. Identify the common shared conductor and the switched per-indication conductors. Match each conductor colour and number to the cable schedule. Confirm the head indication (red, yellow, green) is wired to the correct output.
What is the common conductor in a traffic signal?
The common conductor (often labelled COM or C) is the shared return path for every indication in the head. Each indication also has its own switched conductor that the controller energises to light that colour. The common is not the same as protective earth.
Are traffic light wires colour-coded?
Most projects use a regional or project-specific colour code. Common codes include red, yellow, green for the switched conductors and blue, white or black for the common. Always confirm the code in the project specification before you start.
How many wires does a 3-aspect traffic light need?
A 3-aspect LED traffic light typically needs 4 conductors: red, yellow, green and common. If the head has a heater, fan or monitoring sensor, additional conductors are needed. Always check the head data sheet for the exact count.
What is a conflict monitor?
A conflict monitor is a safety device in the controller cabinet that prevents two conflicting green signals from energising at the same time. If a conflict is detected, the monitor forces the intersection into flashing red or all-red until the fault is cleared.
Can I read a wiring diagram without electrical training?
The basics (conductor count, common vs switched, terminal labels) can be learned quickly, but final termination and commissioning should always be done by a qualified electrician following local electrical codes. Use the diagram as a reference, not a substitute for training.
How do I update an old diagram after a field change?
Mark up the as-built drawing with a red pen, then transfer the changes to a clean digital copy. Replace the laminated copy inside the cabinet. Update the master drawing held by the authority and the supplier. Never let field reality drift from the documented drawing.
Relevant GAOQIAO products
These public GAOQIAO catalogue pages are related to the guide. Open them for the representative description, then confirm the latest wiring schedule and configuration before ordering.
- Traffic signal controllers22-output and 56-output controllers with documented terminal layouts.
- 300 mm (12 inch) LED traffic lightsStandard 3-aspect, 4-aspect, arrow and pedestrian heads.
- Wired traffic signal controllers22-output and 56-output controllers matched to the cabinet terminal schedule.
- Traffic signal controller solutionsSelection guide for wired, wireless and solar controller systems.