Sensors are devices widely used in a multitude of circuits. There are temperature, humidity, smoke, light, and a long etc. They are elements that allow to measure some magnitude and to transform it into a voltage response. The analogical output signal can be easily transformed to digital, so that this type of sensor can be used with digital circuits, LCD screens, an Arduino board, etc.
LM35 is one of the most popular and used sensors of all, as it is a temperature sensor. It comes packaged in a similar package to the transistors we discuss in this blog, such as the 2N2222 and BC547. What it does is measure the ambient temperature and depending on whether it is higher or lower, it will have one or another voltage at its output.