Open collector and drain circuits
Open collector/drain outputs can be used to:


shift a voltage level


Here's an example of a voltage level switching inverter. The input voltages (0 = 0 V, 1 = 1.8 V) can be different from the output voltages (0 = 0 V, 1 = 5 V).







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combine (OR) a couple of outputs


Here's an example: this circuit combines (ORs) a, b and c and outputs NOT( a OR b OR c). This is for example useful if I have 3 separate ICs which all have an open drain output which becomes activated when the IC is on. Suppose I want to know when all ICs are off, then I could use this circuit. As soon as any of the ICs becomes active, out will be pulled low. Only when out = 1 (supply voltage) there is no IC active.







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