AVR-Chat

Put a pull up resistor between Vcc and the microcontroller input pin, then a diode (1n4001)Anode between the junction of the resistor and the microcontroller pin and the cathhode to the Car input (Perhaps a cap between junction of pull up resistor and microcontroller pin as well. This isolates the microntroller from excessive input values and limits and current via the pull up resistor, the cap provides a simple low pass filter. As long as the input doesn't exceed the reverse breakdown voltage all will be well. Auto electrics tend to have lots of nasty stuff "hanging" on the signal and power lines so you need to isolate the microcontroller inputs form this. The opto-isolater idea might be required but i think this might doe the trick.

Regards

--- In AVR
-Chat@yahoogroups.com, David Kelly <dkelly@...> wrote:
>
> 
> On Jul 28, 2009, at 9:51 PM, Jim Wagner wrote:
> 
> > One way is to add a series resistor to each input (say 4.7K to 10K),
> > then add a diode (preferably Schottkey,j but silicon junction will
> > work) from each input to the processor Vcc, cathode to Vcc. That will
> > limit each input at the micro to the current Vcc )(plus one diode  
> > drop).
> 
> 
> There is an Atmel apnote describing how one might monitor AC zero  
> crossings which states there is already a protection diode on all  
> inputs which is good for up to 1 mA. The apnote used a large value  
> 1000V rated resistor in series to connect direct to 120V AC. And then  
> goes on to say something to the effect, "Use at own risk."
> 
> --
> David Kelly N4HHE, dkelly@...
> ========================================================================
> Whom computers would destroy, they must first drive mad.
>