One of the joys of working with basic digital electronics– and logic gate ICs in particular –is that it almost works like building with a set of Lego blocks: One output goes here, which connects to the next input here, and so forth until it does what you wanted.
If you’ve played with chips like these, you’ve probably also come across chips with “open collector” outputs. And if not, they’re worth knowing about. Open-collector outputs form the basis of a number of clever tricks for level-shifting and interfacing between different types of logic, and from logic to other types of electronic circuits.
In what follows, we’ll work with the SN7407N, which is one of the most basic ICs with open-collector outputs. We’ll discuss what it means to have “open collector” outputs, and show some of the different ways that they are used.