Open-collector usually refers to a type of output used by logic gates, but it can refer to any type of output that uses a transistor to sink load current, and are found in everything from TTL logic gates to optocouplers
Most integrated circuits use a totem-pole style output which can both source (supply) and sink (act as a load for) current. An open-collector output is the lower half of a classical totem pole. It acts only to sink current. It can pull a signal down to the ground of the circuit containing it, but it cannot generate any voltage or current. Open-collector outputs are usually rated to sink much more current than a totem pole output, and as such are used as low-side drivers to operate higher current loads (lamps, LEDs, buzzers, relays, etc).
See also: open-drain output
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