Transistor–transistor logic

Transistor–Transistor Logic (TTL) is a class of digital circuits built from transistors and resistors. It is called "transistor–transistor logic" because the logic function (e.g., AND) and amplification is performed by transistors. Many integrated circuits have TTL technology. They are used in applications such as computers, industrial controls, test equipment and instrumentation, synthesizers, etc.
TTL gates define a voltage below 0.5V as "0", and a voltage of 4–5V as "1".