From a single flip-flop, build a state machine
A sequential circuit "remembers the past" — its output depends on the current inputs plus an internal state. This chain starts from the smallest storage cell and builds up step by step to counters, registers, and state machines. All four simulators let you pulse the clock and watch the state change in real time.
Start with step one →- 1Flip-FlopFlip-Flop (D/JK/T/SR) Smallest unit
The atom of sequential circuits: a storage cell that remembers 1 bit. Send a clock pulse and watch how Q changes under the D/JK/T/SR rules — this is the basis for everything that follows.
Open simulator → - 2CounterCounter (Mod-N) Cascaded flip-flops
Wire several flip-flops together so each clock pulse adds or subtracts 1 from the state, wrapping around automatically at the modulus. The lowest bit toggles fastest, and each higher bit halves the frequency — that's frequency division and counting.
Open simulator → - 3Shift Register Shift the bits
Also a row of flip-flops, but each beat shifts the whole data over by one position. This gives you serial↔parallel conversion (74HC595 serial-in parallel-out, 74HC165 parallel-in serial-out), a common way to expand I/O.
Open simulator → - 4Finite State MachineFSM (Mealy / Moore) Most general form
Define "current state + input → next state + output" any way you like, and you have a finite state machine — counters, sequence detection, and control flows are all special cases of it. Includes two classic machines: a "101" detector and a parity checker.
Open simulator →
Want to brush up on combinational logic first? Check out the other track →
Combinational Logic series: truth table → minimal circuit