From: andrew@cucumber.demon.co.uk   
      
   In article <7c184913-7d4a-434e-992c-daade9a4fb64@googlegroups.com>,   
   	jjk  writes:   
   > Thanks again Andrew. this is making more sense. I never used 3.3v zeners   
   before.   
   > The 3.3v protection I was talking about is to protect output pins of 3.3v   
   devices. The gpio pins on a Broadcom processor on the Raspberry Pi is one   
   example. If an external source that is attached to the pin fails and applies a   
   voltage greater than 3.3v,   
    the Broadcom device can be damaged. There are articles on the net describing   
   the use of a zener for protection and that's what I attempted to do. I guess   
   the safest thing to do is add buffers in between the processor and external   
   devices.   
      
   You can do that. When I've driven pi inputs from 5V outputs, I've used   
   a potential divider (1k8/3k3) to drop the input voltage.   
   If there's a significant risk of the pi's 3.3V supply failing whilst   
   the 5V output is still active, you could add a regular diode from the   
   potential divider's mid-point to the 3.3V supply, to limit the extent   
   the input can exceed the Vcc pin of the pi during such a failure, but   
   the pi isn't particularly expensive, and you could spend more trying   
   to protect one than it costs to replace it if you do damage it.   
      
   --   
   Andrew Gabriel   
   [email address is not usable -- followup in the newsgroup]   
      
   --- SoupGate-Win32 v1.05   
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