Cheap protection...

[Bernd Felsche]

I'm a little baffled here:

Where can I find some examples of cheap signal "isolation" circuits
for automotive application?

To "cut my teeth" on uC practical applications, I'm designing a
uC-based system (fancy interior light delay with acoustic warning)
with 5 inputs from switches and "12V" from various automotive
circuits. The uC is running at 5V.

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.

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.

Should I include a (4.7V) Zener on the input to the trigger?

These component counts are crazy! I'm looking at about 40 for a
simple controller; most (around 30) for input protection.

-- 
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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