Of the bases for bulbs, which one does not belong?

• A. Single pin
• B. Bayonet
• C. Bi-pin
• D. Screw base

Answer: (A)

Bases for bulbs are defined by how the bulb connects to the socket and where the electrical contacts are made. The screw base is the familiar Edison-style connection, where a threaded metal shell and a center contact provide the two electrical paths. The bayonet base uses side pins to align and lock into the socket, with the electrical contacts typically on the base and shell to complete the circuit. The bi-pin base has two pins that insert into corresponding socket contacts, delivering the two connections directly at the pins.

The single-pin base isn’t a standard bulb base for general lighting sockets. In typical lighting, you need two electrical connections to power a bulb, and the common base designs provide that in their own mechanical way (threaded shell plus center contact, side-pin bayonet, or two bottom pins). Because only the single-pin design deviates from these conventional, widely used configurations, it does not belong with the others.