How can an ARDUINO act as a manual potentiometer?
What do you want to do?
Control an arduino with a potentiometer? Connect analog input pin to wiper, one side to Vcc, other side to Gnd, and then read it with analogRead().
Use an Arduino to dim an LED, or control the speed of a motor (like one might do with a pot manually)? Use PWM (see analogWrite() ). This works just fine for most loads, but not all.
Use an Arduino to generate an analog voltage? Use a DAC (an external IC, controlled over SPI or I2C) or digital potentiometer between Vcc and Gnd.
Use an arduino controlled device as a "virtual" potentiometer, changing the resistance between a wiper pin and the two end pins? You can do this with a "digital potentiometer" controlled by Arduino (the digital potentiometer is another small IC, which you control over I2C, SPI, or as described in the datasheet).
Note that neither a DAC nor digipot can handle much current (typically not even enough for an LED); If you need an analog voltage with reasonable current handling capability, you'd need an opamp in voltage follower configuration, or some other solution.
DAC current output varies widely by DAC. There’s plenty here that will drive 20mA outputs: