Do you have to build the circuit on an external board every time you decide there is a need for a pull-up or pull-down circuit? If not, then what is the alternative and how can you use it?


This depends on what you are doing.

The IO hardware of the Uno (more specific the Atmega328p, which is the used chip) has internal pullup resistors, that can be used, when the pin is configured as input. This can be done in one line:

pinMode(pin_number, INPUT_PULLUP);

This is true for all AVR based chips.

Other, more complex and capable chips also have an internal pulldown resistor (as I remember this is true for the ESP32 for example). In this case you can do

pinMode(pin_number, INPUT_PULLDOWN);

If you are using special hardware of the Arduino, it depends on the hardware itself, since in most cases the normal IO hardware is internally not connected to the pin, when using this hardware. For example: The internal pullup resistor will not be active, when doing an analog measurement with the ADC (like via analogRead()).

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