Make hardware pin voltage precautions more explicit

[rct]

The maximum voltages for hardware pins are somewhat hard to find in the documentation and are missing from some of the key sections describing hardware interfacing.

1. the Pin Headers section should list max. voltages. In particular, pins that are likely to be used for ad-hoc/experimentation such as GPIO and UART should have explicit mentions.
2. The GPIO section in Making Stuff should mention GPIO voltage ranges and precautions.
3. The Headless CHIP section doesn't mention voltage requirements. "About the Cable" states " If you need a connector, search your favorite shop for ‘USB to UART cable’ - any will do. This section should mention that it needs to be TTL not RS-232 voltages, and whether it needs to be 3v3 connection or whether 5 vdc is also acceptable?

Some other comments regarding these sections.

• The example cable that is listed in the serial section uses female headers. There is no mention of needing male headers, or pieces of paper clips, etc. to connect the two female headers. Also, it might be better to direct people to get a cable that has an FTDI or CP210x chip to avoid some of the drier issues with PL2303 chips/clones.
• The Making Stuff section is pretty minimal, in many cases it explains the SYSFS interfaces, but doesn't mention which pins would be useful for that type of interface.
• The PWM section in Making Stuff is problematic. it currently says can be used to control motors and other devices. It is possible to use GPIO pins to drive motors, but they generally are not fast enough for robust and smooth control. First, generally one shouldn't go connecting motors directly to your GPIO pins without protection and likely need drivers that can supply more current than the GPIO can provide. Second, the fast enough for robust and smooth control doesn't make a lot of sense in the context of motors. It might be better to say that "PWM can be used to cycle a pin on and off rapidly at a range of frequencies or duty cycles. In some cases, PWM can be used to simulate an analog output. For example, an LED driven by PWM can be used to simulate dimming."