Quad 2-Input NOR Gate with Open-Collector Outputs
The Fairchild 74LS33SC is a quad 2-input NOR gate from the 74LS low-power Schottky TTL family, packaged in a 14-pin SOIC for surface-mount assembly. Each of the four independent gates provides a NOR function with an open-collector output stage, allowing wired-OR connections, level shifting to higher-voltage rails, or driving loads such as LEDs and relays directly. This part operates from a standard 5V supply (4.75V to 5.25V) over the commercial temperature range of 0°C to 70°C, making it suitable for indoor equipment, office electronics, and general-purpose logic boards.
Open Collector — What It Means for the Board
The open-collector output on the 74LS33SC is the feature that drives most selection decisions. Unlike a totem-pole output, the open collector can sink current (24 mA per output) but cannot source it — the pull-up resistor must be external. This lets you wire the outputs of multiple gates together (wired-OR) without contention, or pull the output to a voltage higher than Vcc, up to the breakdown limit of the external transistor. For legacy 5V TTL systems with a 5V pull-up, the 24 mA sink capability drives a standard TTL load or a small LED. If you need a higher-voltage interface — say 12V for an industrial sensor input — the open collector handles it with an appropriate pull-up resistor to that rail.
22 ns Propagation Delay — Timing Budget
Maximum propagation delay is 22 ns at 5V supply and 15 pF load.
Sourcing — Active and Quoted to Order
No last-time-buy or obsolescence risk is associated with this part at present. The 74LS series is mature but still widely supported for legacy designs and maintenance spares.