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→Logic level pain: 3.3 volt vs 5 volt signalling
The Raspberry Pi uses 3v3 signalling. [http://www.arduino.cc/ Arduino]-compatible devices generally use 5v. Other devices vary.
Sometimes, no signal conversion is necessary between 3v3 and 5v signalling systems. For example, a 3v3 output driving a 5v input will often work, because the 3v3 "high" ("1") level is more than half of the 5v signalling level and is therefore counted properly registers as a highsignal.
However, connecting a 5v output to a 3v3 input presents problems. A simple solution can be to use a voltage divider (pair of resistors) to tap 3v3 from the 5v signal. When a bidirectional signal is used, an active bidirectional voltage converter is required.
