Traffic light without MCU

Kids wanted a working traffic light for one of their games. I took it as a good reason to hide in the workshop and produced one showed on the video below.

This is super-simple project, why to write a post about it, you may ask? Initial temptation was to grab some small MCU (or even Arduino as that is basically ready to use for such project). But I have opted for old-fashion way without using an MCU and rather some combination logic.

As you can see, there are 3 main sections:

• shared output section with transistors driving the color LEDs
• single NE555 generating pulses for yellow warning mode
• another NE555 driving counter to run a “program” for a full cycle

You can think about the CD4017 counter as being a step counter (program counter), which points into lookup table to resolve into desired LEDs state. Lookup table is realized with set of signal diodes, which just pass 1 when needed (and not allow current to pass back to gate and/or drive other LEDs). Length of the step is determined by NE555 output frequency.

Programming table is simple:

Step counter / Pin active	Active LEDs
Q0	red
Q1	red
Q2	red
Q3	red
Q4	red + yellow
Q5	green
Q6	green
Q7	green
Q8	green
Q9	yellow

Wiring of diodes directly follows the table above and connects counter output pins Q* to appropriate transistors’ base to drive LEDs. I have not designed PCB at the end as construction was simple and one-off.