Cheap protection...

[Mike (Perth, Western Australia)]

At 11:28 AM 26/3/2000 +0800, you wrote:
>Cost is a significant factor - the uC is worth only a few bucks, so
>I don't want to spend hundreds protecting it! Opto-couplers are a
>bit OTT (IMHO) and large.

Yeah true - cost is a bummer and yes Opto's shouldn't be necessary due
to common ground (provided its a 'good' ground).

>So far, a simple voltage divider with forward signal diode, and
>filter capacitor feeding into a (hex) Schmitt trigger (74C14) is how
>far I've got. But the voltage can vary from about 8V to perhaps in
>excess of 16V, but I want those levels to be "true" always. The
>Schmitt will sense 3.5 to 5V as true when running at 5V.

Well, another option is to use a protected MAX232 RS-232 input/output
chip, this already has lots of ESD protection and overvoltage too,
depending on the source impedance you'd probably also want a series
resistor to the MAX232 input.

BTW: The standard MAX232 will protect fairly well, though there are a
heap of 15Kv ESD rated devices, which also have wider input range and
can handle negative transients quite well.

Hey Bernd, did you get that cheap pump from Dick SMith - rockingham ?

Rgds

:) Mike


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