Interactive electronics resource
Traffic Light Kit Instructions
Build, understand and experiment with the Traffic Light Kit. This resource brings the original kit material into the current MitchElectronics format with the schematic, circuit explanation, component and PCB information, construction guidance, troubleshooting and practical ideas where available.
Introduction
Traffic lights are an essential part of modern life as they help direct traffic and prevent accidents. However, those who have a passion for dioramas and miniature models will often find it difficult to find traffic lights that follow the British traffic light sequence whereby red is followed by red/amber, then green, then amber. The MitchElectronics Traffic Light Kit does exactly just this and as such is an excellent addition to any model train or city display.
Schematic
SCHEMATIC (Light)
How does the traffIC light work?
The traffic light kit reproduces the traffic light sequence of common UK traffic lights that include a red, amber, and green light. The circuit is made up of a 555 astable oscillator, a 4017 Johnson counter, a pattern generator, and the output LEDs.
The first stage in the circuit is the 555 astable oscillator which is made up of components R1, R2, C2, and C3. The resistors R1 and R2 along with capacitor C2 provide the timing for the 555 oscillator and for this circuit, the 555 astable oscillator has an output frequency of 0.27Hz.
The second stage is a 4017 Johnson counter which is incremented every time the output of the 555 goes from low to high. Since the period of the 555 square wave output is 4 seconds (frequency of 0.27Hz), the 4017 is incremented once every 4 seconds. This long time is used to make the traffic light stay at each light stage for a reasonable amount of time (for traffic to get through). Output Q8 (ninth output as it is 0-based) of the 4017 counter is connected to its reset pin so that when the counter reaches the 8th count it resets the system.
The third stage is the pattern generator which may sound complex but is in fact very simple. The pattern generator takes the current count output of the 4017 and then turns on the needed LEDs on the traffic light. Common British traffic lights have a very specific light sequence (shown below) and to make this happen on our LEDs each output is connected to a diode which feeds the output to the needed LEDs. You will also notice that multiple outputs are combined to increase the length of time a state is shown for. For example, Red and Green are used for 3 counts while the amber and red + amber states are only one count long.
Each traffic light circuit can drive a single traffic, so if more traffic lights are to be controlled by the same circuit then drivers will need to be used. Drivers can be made out of a wide range of circuits including transistors, op-amps, and dedicated LED driver ICs. This kit does not have the lights directly mounted to the controller circuit but instead on a separate PCB. This allows greater flexibility so that you can add a different light sequence, multiple lights, and other circuits.
Project ideas
Model Railway
Considering that this kit mimics the British traffic light pattern, it is an excellent addition to model railways. In fact, the output of the controller could even be used to automatically open and close gates at car crossings. Simply connect the output of the amber light to a Raspberry Pico microcontroller which then controls a servo to open and close a barrier.
Try it yourself
Circuit simulation
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What you need
Component List
| Component | Quantity | PCB Reference |
|---|---|---|
| 16 DIP Socket | 1 | U1 |
| 8 DIP Socket | 1 | IC1 |
| 4017 | 1 | U1 |
| 555 | 1 | IC1 |
| 10K Resistor | 1 | R1 |
| 22K Resistor | 1 | R2 |
| 100nF Capacitors | 2 | C1, C3 |
| 100uF Capacitor | 1 | C2 |
| 1N4148 | 9 | D1 to D9 |
| 4 Way Pin Socket | 1 | J1 |
| PP3 Connector | 1 | BT1 |
| Controller PCB | 1 | — |
| 680R Resistors | 3 | R1, R2, R3 (Light PCB) |
| Red LED | 1 | D1 (Light PCB) |
| Yellow LED | 1 | D2 (Light PCB) |
| Green LED | 1 | D3 (Light PCB) |
| 4 Way Pin Header | 1 | J1 (Light PCB) |
| Light PCB | 1 | — |
Inspect the board
Interactive BOM
Board reference
PCB & assembly
The PCB silkscreen and component references should be checked against the component list before soldering each part.
Before applying power
- Check every component against its PCB reference.
- Confirm the orientation of all polarised components and ICs.
- Inspect for solder bridges, unsoldered pads and clipped leads that could cause a short.
- Check that no loose wire or solder debris remains on the board.
Build with confidence
Construction tips
Recommended build order
A reliable way to assemble Traffic Light Kit is to work from the lowest-profile components to the tallest. This keeps the PCB easy to access while you solder.
- Fit resistors, links and other low-profile components first.
- Fit small capacitors and diodes, checking polarity where applicable.
- Fit IC sockets and small semiconductors, observing the orientation markings.
- Fit larger capacitors, potentiometers, switches and other controls.
- Fit LEDs, connectors and the remaining taller components.
- Insert socketed ICs only after soldering around the socket is complete.
- Inspect every joint and check for solder bridges before applying power.
Electronics construction guidance
If you are new to kit construction, use the Soldering Guide alongside these instructions. Identify each component before fitting it and compare its reference with the component list and PCB silkscreen.
When it does not work
Troubleshooting
Nothing happens when power is applied
- Confirm the supply is connected to the correct input and with the correct polarity.
- Check that ICs, diodes, LEDs, transistors and electrolytic capacitors are fitted in the correct orientation.
- Compare component values and positions against the component list and PCB reference.
- Inspect for missed joints, dry joints and accidental solder bridges.
The circuit powers up but does not behave as expected
This kit is intended for british-sequence traffic light controller. If the output is stuck, unstable or outside the expected behaviour, use the schematic to trace the circuit a stage at a time rather than replacing several parts at once.
Check the components around the part of the circuit responsible for the output or timing first. A misplaced resistor, reversed semiconductor or poor connection can allow a circuit to power up while preventing it from operating correctly.
The circuit works intermittently
Intermittent behaviour is often caused by a marginal solder joint, a loose connector or a component lead that has not been fully soldered. Gently inspect the board with power removed and reflow any joint that looks dull, cracked or incomplete.
If the fault remains, compare the assembled board with the schematic and PCB reference one connection at a time.
Ready to test and experiment?
Using the RESET input on the 4017, it’s possible to combine multiple traffic light controllers to create junctions that have synchronised lights
Once the board is working, compare its behaviour with the schematic and the explanation above. Try changing only one input, control or permitted component value at a time so you can clearly see what effect that change has on the circuit.