RPi GPIO Interface Circuits
This page shows a number of common circuits used for interfacing the Raspberry Pi's GPIO pins to various other electronic circuits.
Please note: this page is currently under construction
This circuit may be used to connect a digital signal from a 5V device to a GPIO pin on the Pi, which requires a 3.3V input level. The Ov point in the schematic should be connected to one of the Ground lines on the Pi's GPIO connector, and also to the 0v (or Ground) signal on the input device.
The important thing here is the ratio of R1 to R2; R1 should be just over half R2's value to ensure 5V is divided down to 3.3V. The values shown here should be suitable for most uses.
(See the [http://en.wikipedia.org/wiki/Voltage_divider Wikipedia article on voltage dividers).
Using an NPN transistor
Using a FET
Note: If you wish to produce new or modified schematics matching the ones above, you can use Inkscape using symbols from File:Circuit_Symbols.svg.