[sdiy] Protecting microcontroller inputs from - voltage?

[Ken Stone]

>>   Use a diode clamp.  That is, a diode from the I/O pin to ground, with 
>> the cathode on the I/O pin.  Check the microcontroller electrical spec, 
>> but most will tolerate -1V or so.  If even less drop is needed, use a 
>> Schottky diode.  In practice, a 1N4148 will work fine.  Caveat: put a 
>> small resistor in series with the I/O pin and whatever it is sensing.
>
>On which side of the resistor would you put the diode ?

The diode goes directly to the IC pin. The resistor goes between the pin and
whatever you are connecting to. The purpose of the resistor is to limit the
current flow through the diode in the "protected" condition. If you put the
resistor on the other side, you'll probably blow the diode up when the input
goes negative, or at very least, you will be sending whatever is doing the
driver to it's current limit.
_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au  
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