Traffic Light Controller in Logisim-evolution
The traffic light controller is the most popular digital-design course project: a finite state machine that cycles two directions through green → yellow → red with realistic timing.
Step 1: Define the states
A minimal two-direction controller needs four states:
- North-South green, East-West red
- North-South yellow, East-West red
- North-South red, East-West green
- North-South red, East-West yellow
Encode them with two D flip-flops (00–11) and draw the transition diagram — each state simply advances to the next, which keeps the excitation logic small. This is the same method as our sequence detector tutorial.
Step 2: Slow the clock down
A raw clock ticks far too fast to watch. Divide it with a Counter: let the counter run and use one of its high bits (or its carry output) as the slow state clock. Larger counter → longer green phases. See the counter tutorial and digital clock tutorial for the technique.
Step 3: Decode the outputs
Each state code drives six lamps (two directions × red/yellow/green) through simple combinational logic, or use two Hex Digit Displays as a compact stand-in while debugging. Remember Simulate → Ticks Enabled (Ctrl-K), or nothing moves.
Extensions instructors like
- Pedestrian button: an input that forces an early transition to the yellow state.
- Night mode: a control input that blinks only the yellow lamps.
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Related: sequence detector · digital clock