A relay machine is a computer or logic device that processes information using electromechanical relays. Each relay uses an electrical signal to move contacts, opening or closing a circuit to switch signals. Relays can be arranged to perform logical operations, store state, or control other parts of a system.
In cyber security, relay machines are not a distinct threat category, but understanding their switching logic helps when assessing legacy control equipment and other hardware systems. Relay-based devices can be physically inspected, yet their circuits may be vulnerable to tampering, miswiring, or unauthorized changes. In attacks on operational technology, an adversary might manipulate control signals or settings; defenders can reduce risk through physical access controls, circuit documentation, inspection, and separation from untrusted networks. Relay machines also appear in retrocomputing projects, where their visible operation makes computing principles easier to study.