Traffic Light Circuit Diagram: Timer-Based Sequence and PLC Control Wiring Explained

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A traffic light circuit diagram shows how red, amber, and green signal lamps are sequenced using a timer circuit or PLC output module to control a safe and predictable light cycle.

Traffic light control circuits range from simple discrete-component timer designs (suitable for educational and small intersection applications) to complex PLC-based or microcontroller-managed systems coordinating multiple signal heads, pedestrian push-buttons, inductive loop detectors, and adaptive traffic management. This reference covers both approaches.

In a basic discrete-timer traffic light circuit, the sequence is generated by a 555 timer IC (or CD4017 decade counter driven by a 555 clock) that steps through output states, each activating a different lamp relay or transistor output. A three-output sequence (red, amber, green) for a simple two-phase junction requires at minimum six lamp states (R, R+A, G, A, R, repeat) distributed across two signal heads. Simple one-junction educational circuits often use a 555 timer to clock a decade counter (CD4017 or CD4022) whose outputs in sequence activate the correct lamp combination via transistors. Timing of each phase is determined by the RC network on the 555 clock and optionally by separate RC circuits on each output stage.

In industrial and infrastructure PLC-based systems, each signal lamp (or lamp group — a PLC output may drive an LED lamp driver or a solid-state relay that switches the mains-voltage signal head). The PLC programme contains the sequence logic: timers, counters, and conditional logic for pedestrian phases, emergency vehicle preemption, and failure mode detection. The control circuit diagram for a PLC system shows the input/output wiring — power supplies, output module connections to lamp drivers, input connections from push-buttons and loop detectors — while the programme itself contains the timing logic.

A standard UK full traffic signal sequence is: red → red+amber (simultaneous, 2 seconds) → green → amber → red. The red+amber phase is unique to UK and some other national standards; USA signals omit it, going directly from red to green. LED signal heads are now near-universal in modern installations — they use vastly less power than incandescent and have built-in monitoring for lamp failure detection.

For any public infrastructure application, traffic signal control must comply with applicable national standards (UK: TSRGD, IAN 68 Series; USA: MUTCD, NEMA TS-2 or NEMA TS-1 standards).

How to wire traffic light circuit diagram

Define the sequence and phase timing Write out the complete sequence of lamp states for each signal head. For a simple two-phase junction with two signal heads (A and B), list every state: Head A red / Head B green → Head A red / Head B amber → Head A red+amber / Head B red → Head A green / Head B red → etc. Assign a duration in seconds to each state. This defines the logic before touching any hardware.
Select the control method: timer IC or PLC For educational or demonstration purposes, a 555-clock / CD4017 counter discrete circuit at 5–12 V DC is appropriate. For industrial or infrastructure applications, a safety-rated PLC with appropriate I/O modules is required. The control method determines all subsequent component and wiring decisions.
Design the power supply circuit For a discrete educational circuit: a regulated 5 V or 12 V DC supply from a mains adapter or bench supply. For mains-connected signal heads (real infrastructure): a dedicated controller cabinet power supply with battery backup, surge protection, and isolation transformers. Mains-connected systems must comply with applicable wiring standards and are not suitable for DIY construction.
Wire the 555 timer clock stage (educational circuit) Configure the 555 in astable mode: connect pins 2 and 6 together (trigger/threshold), pin 4 (reset) and pin 8 (VCC) to supply positive, pin 1 (GND) to supply negative. Connect timing resistors R1 (pin 8 to pin 7) and R2 (pin 7 to pins 2/6) and capacitor C1 (pins 2/6 to GND). The oscillation frequency f ≈ 1.44 / ((R1 + 2×R2) × C1). Adjust for the desired clock period.
Wire the CD4017 decade counter outputs to lamp drivers (educational circuit) Connect the 555 output (pin 3) to the CD4017 clock input (pin 14). Connect the CD4017 enable (pin 13) to GND, reset (pin 15) to the output that marks the end of one complete cycle (allowing the counter to reset and repeat). Connect each output pin to the base of an NPN transistor via a base resistor; collector to the LED or lamp; emitter to GND. Multiple outputs can drive the same lamp to implement simultaneous states (e.g. red+amber).
Implement the PLC programme for controlled systems In PLC ladder logic: create timer instructions (TON — on-delay timers) for each phase duration. Use set/reset coil instructions or sequencer function blocks to step through output states. Wire physical outputs to signal lamp relays or solid-state lamp drivers. Implement a failure mode output (flashing amber) triggered by a watchdog timer that resets only when the programme is running correctly.
Test the sequence and verify all lamp states Power up the circuit and observe one complete cycle. Verify that no two conflicting outputs (e.g., green on head A and green on head B simultaneously in a cross-junction) are ever active at the same time. For PLC systems, use the online monitoring display to watch timer and output states in real time. Document the verified timing for the commissioning record.

Specifications

555 timer supply voltage range	4.5–16 V DC (bipolar 555); 2–18 V DC (CMOS TLC555)
CD4017 supply voltage range	3–15 V DC
555 astable frequency formula	f = 1.44 / ((R1 + 2×R2) × C1) where R in Ω, C in Farads
Standard UK amber phase duration	3 seconds (red to green: 2 s red+amber; green to red: 3 s amber)
Standard USA yellow (amber) phase duration	3–5 seconds, calculated per approach speed: Y = t + v/(2a + 2gG) — MUTCD guidance
LED forward voltage (typical, red/amber)	1.8–2.2 V at 20 mA
LED forward voltage (typical, green, AlInGaP)	2.0–2.4 V at 20 mA
Infrastructure signal head lamp power (LED)	Typically 8–15 W per signal aspect (LED module)

Safety warnings

Educational traffic light circuits at 5–12 V DC are safe low-voltage projects. However, real traffic signal heads and infrastructure controllers operate at mains voltage (120 V AC or 230 V AC). Mains-connected traffic signal systems must be designed, installed, and maintained only by qualified electricians and road infrastructure engineers in compliance with applicable national standards. Unauthorised interference with public traffic signals is illegal in all jurisdictions.
In any circuit where two signal heads must never show a conflicting green simultaneously, implement both software/programme interlocks and hardware fail-safe outputs. A failed output card or programme fault in a PLC must cause a safe fallback state (flashing amber), not an arbitrary output combination. Use safety-rated PLCs and safety I/O modules for public infrastructure.
In mains-connected signal head installations, all wiring must comply with the relevant electrical installation standard (BS 7671 in the UK, NEC/NFPA 70 in the USA, IEC 60364 internationally) including appropriate protection from water ingress, UV exposure, and mechanical damage. Cabinet installations must be weatherproof and properly earthed.
Never modify or attempt to connect to roadway traffic signal equipment without proper authorisation from the relevant highway authority. Traffic signal tampering poses direct risk to road users and carries serious legal penalties.
Any educational circuit built on a breadboard should use a low-voltage DC power supply (5–12 V) from a mains-isolated regulated adapter. Do not connect any mains voltage to a breadboard prototype circuit.

Tools needed

Breadboard and jumper wires (for educational circuit prototyping)
Digital multimeter (DC voltage, resistance, diode test)
Oscilloscope (for verifying 555 timer output frequency and duty cycle)
Regulated DC bench power supply (5–12 V, current limited)
Logic analyser or LED probe (for tracing CD4017 output states)
PLC programming software and USB/serial programming cable (for PLC-based designs)

Common mistakes

Failing to reset the CD4017 counter at the correct output — if pin 15 (reset) is not wired to the output corresponding to the end of the required cycle, the counter steps through all ten outputs including unused ones, producing incorrect lamp states.
Designing the circuit without an interlock that prevents conflicting green signals on crossing approaches — this is the most critical safety error in any traffic light logic design.
Using a single 555 timer stage to drive all six lamps without a counter — a single 555 can only alternate between two states (on/off), which is insufficient for a multi-colour sequence requiring six or more distinct states.
Connecting LED lamps without current-limiting resistors — running LEDs at full supply voltage without resistance protection destroys them rapidly. Calculate resistor value: R = (Vsupply − Vf) / If.
Choosing a phase duration that is too short — a 1-second green phase followed by a 1-second amber does not give vehicle drivers or pedestrians sufficient time to react. Even in educational circuits, use realistic timing (5–30 seconds per phase) to demonstrate realistic behaviour.

Troubleshooting

All lamps remain off after power-up: Cause: 555 timer is not oscillating, or power supply is absent. CD4017 pin 15 (reset) may be held high, preventing counting. Fix: Measure the supply voltage at the IC power pins. Check the 555 output (pin 3) with a multimeter in DC mode — it should toggle. If stuck high or low, check the RC timing components and verify VCC and GND connections. Verify CD4017 pin 15 is connected only to the reset trigger output, not permanently tied to VCC.
Lamps cycle through some states but skip others or the sequence is incorrect: Cause: CD4017 outputs are wired to the wrong lamp drivers, or the reset is triggered at the wrong count, causing the counter to skip outputs Fix: Map the CD4017 output pins (Q0 through Q9) to the wiring in the circuit. Verify which pin corresponds to which lamp state using a multimeter or oscilloscope in sequence. Rewire the base resistors of the transistor drivers to the correct counter output pins. Confirm the reset pin connection is at the correct terminal output.
One lamp (e.g. amber) is always off: Cause: Transistor driver for that lamp is faulty, the LED is installed backwards, the current-limiting resistor is open, or the CD4017 output for that state is not connected Fix: Isolate the suspect lamp circuit. Apply the correct base drive voltage directly to the transistor base (via a resistor) and observe whether the LED lights. If not, test the LED polarity and the transistor. If the LED and transistor work manually, trace back to the counter output connection.

Frequently asked questions

What is the standard traffic light colour sequence in the UK and USA?

UK sequence: red → red+amber (2 seconds) → green → amber (3 seconds) → red. USA sequence: red → green → amber (yellow, 3–5 seconds depending on approach speed) → red. The red+amber phase exists in UK and several European countries to warn drivers to prepare to move; it is absent in the USA and many other countries.

How does a 555 timer circuit generate a traffic light sequence?

A 555 timer configured in astable mode generates a clock pulse train. This clock drives a CD4017 decade counter IC, whose ten outputs activate sequentially on each clock pulse. By connecting outputs 0–9 to lamp transistors (or relay drivers) in the correct combination — some outputs driving red, others green, others amber — the required sequence repeats continuously. Timing is set by the 555's RC components.

What is a PLC I/O module in a traffic light context?

A PLC (Programmable Logic Controller) digital output module switches DC or AC outputs under programme control. In a traffic light application, each output channel connects to one signal lamp or lamp driver circuit. The PLC ladder logic or function block programme contains timer instructions that hold each output on for the required phase duration before stepping to the next phase. Inputs may include pedestrian push-buttons and vehicle detectors.

What happens to a traffic light system if power fails?

Modern traffic signal controllers enter a defined failure mode on power loss — typically flashing amber at all heads (or flashing red in some jurisdictions) to indicate that the automatic control is inactive and drivers should treat the junction as an uncontrolled crossing. When power is restored, the controller performs a startup sequence before resuming normal operation to ensure all heads display a safe initial state.

Can I build a working traffic light circuit for educational purposes?

Yes — a low-voltage educational traffic light circuit using a 555 timer, CD4017 counter, small signal transistors, and 5 V LEDs is a well-established electronics learning project. Such a circuit operates at safe low voltage (5–12 V DC) and involves no mains wiring. It demonstrates the principles of sequence timing without the complexity or safety hazards of an actual traffic signal installation.

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