"101" sequence detector circuit: build and simulate it online
A Moore machine that raises Z after it has seen 1, 0, 1 on X (overlapping allowed).
Build the "101" sequence detector in the free online simulator — it checks your circuit automatically.
What you need
- 2 logic switches labelled X, CLK
- 1 probes labelled Z
- 2 flip-flops
- a few gates
Step by step
- Inputs and outputs. From the Logic section of the library, drop digital inputs (0/1 boxes) or logic switches for the inputs, and digital outputs or probes for the outputs; give each its label in the inspector (the checker finds them by label). Switches X, CLK; Probe Z
- Choose your states. Four states: S0 (nothing), S1 (seen 1), S2 (seen 10), S3 (seen 101, Z = 1). Two D flip-flops hold the state.
- Next-state and output logic. Work out D1, D0 from the state table (a K-map helps), and Z from the state. Any correct design passes. From S3, X = 0 goes to S2 (overlap)
- Prove it. The checker feeds 1 0 1 1 0 1 0 0 and reads Z after each clock.
Expected sequence
| # | inputs | clock | Z |
|---|---|---|---|
| 1 | X=1 | ↑ | 0 |
| 2 | X=0 | ↑ | 0 |
| 3 | X=1 | ↑ | 1 |
| 4 | X=1 | ↑ | 0 |
| 5 | X=0 | ↑ | 0 |
| 6 | X=1 | ↑ | 1 |
| 7 | X=0 | ↑ | 0 |
| 8 | X=0 | ↑ | 0 |
Frequently asked questions
How do you build a "101" sequence detector?
A Moore machine that raises Z after it has seen 1, 0, 1 on X (overlapping allowed). In short: inputs and outputs, then choose your states, then next-state and output logic, then prove it.
How does a "101" sequence detector behave clock by clock?
It is checked over 8 steps: X = 1, X = 0, X = 1, X = 1, …. The table on this page shows the expected Z after each clock.
What parts do you need for a "101" sequence detector?
2 flip-flops; a few gates — plus logic switches for the inputs and probes for the outputs.
Can I simulate a "101" sequence detector online for free?
Yes. Open it in the BitForBytes Embedded Lab, a free browser-based circuit and logic simulator: drag in the gates, wire them, and press "Check my circuit" — every clock cycle is simulated and marked for you. No download or sign-up needed.
Part of DSD 39 · Sequence detectors in the free BitForBytes digital system design course.