July 7, 2019

Traffic light without MCU
programming with counters

Kids needed 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 withou using MCU and rather some combination logic.

Schematics of the traffic light Traffic light schematics (PNG, PDF)

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, 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.

All components on a perfboard All components on a perfboard

Bottom of a perfboard Bottom of a perfboard

LEDs assembly LEDs assembly