Four-step electronic combination lock

How it works

The circuit recognizes a sequence of four switch presses. In the published arrangement, S1, S2, S3, and S4 must be pressed in order; the other switches reset the sequence. The output then controls the load connected at K1.

Changing the switch wiring changes the combination. This is a learning exercise in state and sequencing rather than a modern secure access-control system.

On the bench

Test first with an indicator in place of a lock. A real door installation requires an independent assessment of emergency release, power failure behavior, and actuator protection.

Resources & provenance

Rewritten from the original circuits/sec/mn3qo9c_sec003.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.