PWM (pulse width modulation) is a cheap method of creating the required analog voltage levels, and even of generating pseudo analogue signals, using purely digital means.

The purpose of this article is how to generate analog voltage with digital circuit.
Many times, designers are faced with the need of generating analogue voltage levels in purely digital circuits. Although the market provides today a very broad range of dedicated digital-to-analogue converters, putting such a device in the schematic has a negative impact on the overall cost of the system. There are however, cheap methods of creating the required voltage levels, and even of generating pseudo-analogue signals, using purely digital means.