Which device can detect the presence of a magnet?

• A. Magnoflux sensor
• B. Reed switch
• C. Photocell
• D. Pressure switch

Answer: (B)

Magnetic switches change their electrical state when a magnetic field is present. A reed switch does this with a glass-encased pair of ferromagnetic reeds that are normally separated. When a magnet comes near, the magnetic field pulls the reeds together, closing the circuit. When the magnet is moved away, the reeds separate and the circuit opens. This simple, contact-based action makes reed switches a classic choice for detecting the presence of a magnet, as seen in door sensors and position sensing.

Other devices listed don’t fit as neatly for magnet presence. A photocell responds to light, not magnets. A pressure switch responds to mechanical pressure. A magnetic flux sensor (magnoflux) measures magnetic field strength, which is related to magnetism but doesn’t inherently provide a simple on/off contact indicating magnet presence.