What is the difference between 74HCT and LS ICs

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 The S series was for Schottky logic and is logically and also electrically compatible to the LS and mostly HC series. LS stands for Low Power Schottky and the difference to the Schottky family was that it uses a lot less power but is not as fast. However for most practical purposes LS is by far fast enough and Schottky was even 40 years ago pretty difficult to get and very pricy and that certainly hasn't gotten better.

For most practical purposes you can probably use HC series too, which are slightly slower but even more low power than LS. The main difference between HC and LS is the logic levels, with HC requiring a minimum of 3.5 V@5Vcc  at the input for the Hi logic level while LS has the standard TTL level of anything above 2.0 V@5Vcc being logic 1.

So you have to be careful what you connect to the input of HC logic circuits. Normal TTL outputs are only guaranteed to drive at least 2.4 V@5Vcc and will not be able to drive HC logic inputs reliable.

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