Which type of relay uses a light beam to transfer the control signal across isolation?

• A. Reed-controlled AC relays
• B. Opto-isolated relay
• C. Pneumatic relay
• D. Mechanical relay

Answer: (B)

Light-based isolation lets a control signal be transmitted without a direct electrical connection, using an LED and a light sensor to bridge the gap. In an opto-isolated relay, the control side drives an LED, and the light from that LED passes across a nonconductive barrier to a photodetector on the load side. The detector then triggers the relay, so the control circuit and the load circuit stay electrically separate. This separation protects sensitive electronics from voltage spikes, noise, or ground differences while still providing a reliable control signal. Other relay types rely on direct electrical current, magnetic fields, or pneumatic actuation and do not use a light beam to transfer the control signal across isolation.