Cheap protection...

[Buchholz, Steven]

... I forget the actual numbers, but not too long ago I was surprised to
read that blowing the horn puts spikes that can be somewhere in the 200v
region on the power lines ...

I don't think the problem is the cost of replacing the CPU ... it is more
the hassle of having to fix something that's broken.  

I've seen optocouplers that come in small SMT packages ... I'll bet the cost
isn't that much.  AAMOF, I picked up a really cool demo kit from
http://www.lumex.com that included optocouplers and a whole selection of
lamps, LEDs & displays.  

It isn't clear to me that you need to have a voltage divider to protect the
input ... a series resistor, perhaps with diodes to the rails should be
sufficient.  Some parts already have the diodes built in.  I'd also put
ferrite bead filters on the signal inputs.  You may want to look around to
see if someone doesn't offer an input device that has built-in protection.  

Steve Buchholz
San Jose, CA (USA)

> -----Original Message-----
> 
> 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?
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