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→3v3 vs 5v0
# Use micro test clips (J-clips, grabber clips) to attach to individual header pins. The micro test clips can be attached to wires inserted into the breadboard, or you can use double-ended test clip cables and attach to devices plugged into the breadboard.
== 3v3 Logic level pain: 3.3 volt vs 5v0 5 volt signalling ==
"TTL" digital signals were traditionally 5v signals -- 5v indicated a "1", 0v indicated a "0". More recent devices use 3v3 or 1v8 signals.
The Raspberry Pi uses 3v3 signalling. Arduinos[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 as high.
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.