Which device is typically used to control one circuit with contacts in another circuit?

• A. Motor
• B. Push Button Switch
• C. Limit Switch
• D. Control Relay

Answer: (D)

The idea is using a device that lets a signal in one circuit control a separate circuit through its contacts, while keeping the circuits electrically isolated. A control relay does this: it has a coil in the control circuit, and when that coil is energized, it magnetically actuates the switch contacts in the separate load circuit. That way, a small control current can turn a larger current on or off in another circuit, providing isolation and safe control. The other options don’t fit as the primary mechanism for controlling one circuit with contacts in another: a motor is a load, a push button is a manual operator but doesn’t inherently provide isolation between circuits, and a limit switch detects position rather than serving as the device that switches another circuit.