Build a traffic light circuit from flip-flops and gates

A traffic light is a state machine: it's always in one of a fixed list of states and moves to the next on a timer. You can build one with no program at all from four D flip-flops and two gates. A divide-by-2 turns a 2 Hz clock into a 1 Hz tick; a Johnson counter of three flip-flops steps through six states, one a second; and gates decode the states into lamps.

The sequence

• Red for two seconds
• Red and amber for one: get ready
• Green for two seconds
• Amber alone for one: stop if you can
• Back to red, for ever: a six-second cycle

1. The divider

CLK1 ticks twice a second. FF1 has its Q̄ fed back to its D, so it toggles on each tick and its output changes once a second. Its Q̄ clocks everything else.

2. The ring

FF2, FF3 and FF4 form a twisted ring on that 1 Hz clock: FF2's Q to FF3's D, FF3's Q to FF4's D, and FF4's Q̄ back to FF2's D. From 000 it fills and empties, six states, one per second.

3. The lights

Read the lamps off the ring's last two flip-flops:

Red covers the three states where FF4 is 0, amber lights wherever FF3 and FF4 differ, and green where both are 1. Two gates and one spare output give the whole sequence.

Parts list

Red needs no gate at all: it's FF4's Q̄ output.

Check it

Watch for two ticks of CLK1 per step: red, red, red and amber, green, green, amber, then back to red and the same again. If the light never changes, check the twist: FF4's Q̄, not its Q, must feed FF2. If everything runs twice as fast, the ring is clocked from CLK1 instead of the divider.

Real junction controllers are microcontrollers now, with sensors and pedestrian buttons, but inside they're still state machines stepping through a list like this one.