Cheap protection...

[Bernd Felsche]

Mike writes:

>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,

That's novel. A bit pricey though. Works out about twice the cost
per input as opto.

I was aiming at $20 in parts for the lot (well, about $25 including
the case). The uC is about $6, though I might have to spring for $10
on the prototype as the "best type" is not yet available...

>depending on the source impedance you'd probably also want a series
>resistor to the MAX232 input.

What's the source impedance of a car battery? :-)

>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 ?

No. Not in my budget for this month.

All my "spare" cash is tied up in the business at the moment.

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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