Raspberry Pi Connect GPIO input with 5 or 12 volts

The input and output level of a GPIO of Raspberry Pi for “High” is at a voltage of 3.3 volts. But if you have a larger input level then you can not apply this voltage directly to a GPIO. The GPIO would be overloaded and broken.

Raspberry Pi GPIO input wired with button

Basically, you can operate a GPIO pin either as an input or as an output. As an input, it can assume the states “High” and “Low”. For example, from a switch or button. 
In general, Raspberry Pi’s GPIOs are wired with resistors to set inputs to a defined level (+ VCC or GND) or to limit the current.

Control and program Raspberry Pi GPIO with “sysfs” (shell/bash)

The GPIOs are represented on Raspberry Pi as files and can be found in the directory tree under “/ sys / class / gpio”. This is called “sysfs”. If you look at the contents of the files, you get the state out. This is either “1” or “0”.

Raspberry Pi configure GPIO at system startup

Before you can use a GPIO as input or output, it must be configured or set. Unfortunately,  Raspberry Pi does not remember this setting. Therefore, the setting must be executed again each time the system is restarted.

Raspberry Pi Measurement of voltage and current at the GPIO output with high level under load

If you connect a high-level GPIO output to a Raspberry Pi, then you expect a voltage of 3.3 volts. Unfortunately, a GPIO output changes its voltage and current behavior under load.