Traffic light

A clock, a counter, and a decoder cycle three lamps like a street-corner signal. A ready-made fun circuit you can open in the TorchAnvil simulator.

A tiny finite-state machine

A real traffic controller is, under all the weatherproofing, a little state machine: something counts, something decodes the count into "which lamp is on," and a clock keeps it marching. This sample builds the smallest honest version of that. A 2-bit counter cycles through four states; a decoder turns each state into a single lamp; an OR stitches two of the states together so red gets two slots in the cycle.

How it works

The counter is two T flip-flops in the familiar ripple pattern:

Clock → C0.CLK gives the base beat (period 8, so it's slow enough to read).
HIGH → C0.T and HIGH → C1.T keep both flip-flops toggling.
C0.Q → C1.CLK — the second bit sees a rising edge once per two clock ticks, so together (C1, C0) counts 00, 01, 10, 11, 00….

Then a tiny 2-to-4 decoder turns that count into four state lines Y0…Y3, one per combination of C1 and C0:

Y0 = !C1 · !C0 — state 0
Y1 = !C1 · C0 — state 1
Y2 = C1 · !C0 — state 2
Y3 = C1 · C0 — state 3

Finally the outputs:

Y1 → Green (state 1)
Y2 → Yellow (state 2)
Y0 OR Y3 → Red — red gets states 0 and 3, doubling its time on the road like a real signal. (Each lamp pin only accepts one edge, so we fold the two "red" states through an OR gate first.)

tick	C1	C0	state	lit
0	0	0	Y0	Red
1	0	1	Y1	Green
2	1	0	Y2	Yellow
3	1	1	Y3	Red
4	0	0	Y0	Red (wrap)

Why it matters

Counter plus decoder plus output logic is the template for basically every hardware sequencer you'll ever meet. Washing machines, elevator dispatchers, arcade-cabinet attract-mode animations, even the microcode ROMs inside old CPUs — they all boil down to "advance a counter, look up what to do this tick." Once you've built one, you've built them all.

Try this

Press Run. Watch the lamps cycle Red → Green → Yellow → Red at eight ticks per state.
Try dropping the clock period to 4. The sequence speeds up but the ratio between colors stays the same — that's the decoder doing its job independent of timing.
Reach in and disconnect the OR gate's second input from Y3. Red now lights for one state instead of two; the cycle becomes a plain three-phase loop with a mystery dark beat. Reconnect it to restore the extra red.