There are two lines, called logic families,of analog IC's and while they can generally be interoperable "off the shelf," they work best with ICs of the same family and sometimes IC's of mixed logic families can require a voltage translation to get working correctly. Often, though, our designs will have need to intermix different logic families. This isn't always a problem when there is no inherent dependency on signaling between the logic IC's, and in some cases even signaling between them won't pose a problem. However, in some cases, like when signaling between a CMOS IC and a TTL IC, there can be a problem because each logic family defines the valid range of voltage that make up a valid HIGH and valid LOW and the logic families don't agree on the range.
This instructable will briefly explain the two logic families of IC's you will most likely be encountering and using in your circuit designs and how to ensure that the HIGH or LOW output of the other is translated and interpreted correctly between a TTL and CMOS device. This process is called voltage level shifting and is the subject of this instructable.
Turn the page and read up on the two logic families that you have probably already implemented in your designs.