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I wouldnt use an optoisolator. There are multiple other ways to level clamp a voltage that dont involve opto's. In my experience an optoisolator is used to establish / maintain isolated grounds. We're obviously sharing the same ground. They also arnt the cheapest things in the world.
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I have a lot of optos around, somewhere, if I ever find them. Here is a new one for $1.89,
http://www.alliedelec.com/Search/Pro...2&DESC=NTE3041
so it IS a lot more than zeners. Since I was assuming dcb was making this design for production, that added cost WOULD make a big difference, so I figured that was why it had not been chosen.
I agree with you there are other ways to do it, I don't have your knowledge about all the options. My training is in mechanical engineering and heat transfer, so I tend to go for overkill in electronics that I design. You have to admit that an opto is a simple and SURE way. Nothing is going to get thru that opto to upset the guino.
Also, I was also thinking in terms of powering the duino with a dc to dc converter I have somewhere, I would not connect any grounds to the car body under that condition.
And again, it was NOT my intention to attack or criticize dcb personally.
Laboring under the assumption that dcb was developing this as a product for sale, I was role-playing the part of a paying customer- what would that customer expect in a device that cost real money? So dcb is (rightly)
annoyed because he assumes I am whining about something I am getting for free.
Meanwhile, I am figuring dcb has put this out publicly here as a product development, and that he WANTS to hear about all the things that could POSSIBLY BE PROBLEMS under any conditions (because that is a brilliant thing to do- put out your design for review by thousands of design people, many of whom HAVE shipped real consumer or industrial product) before he has 2000 of these made in China, has sold them all, and has put in an order for 5000 more units.
So when dcb says "$3.00 chip" I am concerned he is not taking the (potential) issue seriously and his PRODUCT is in jeapordy.
I say (role-playing the part of a future, hypothetical, paying CUSTOMER) "I have built this in and it is hard for me to take it out of my car and find the cables and learn how to re-program it, I don't have the experience." Because I feel that for a PRODUCT, the customer cannot be expected to change out chips.
Anyway, sorry for the unintentional increase of peoples blood pressures.