Hello folks,
I am new to the arduino world and already made some experiments based on tutorials.
Arduino rocks btw!
Now I want to read four sensors with external power supply. The signal from the sensor provides 12V if the sensor is triggered. Otherwise it will be 0V. So I used a voltage divider in order to get the 5V for the arduino. I measured them and everyone gave a correct 5V output.
I thought: great, I just connect them to the digital pins but
unfortunately I grilled one of my adruinos by connecting each 5V signal to one corresponding digital pin.
I grounded each sensor with my arduino, but no luck
Is there something I am missing? 
I made a little shematic in order to display my setup. Just imagine four sensors connected to Pin 5, 6, 7 and 8 instead of only one.
help is appreciated.
Hello HToneill,
Thanks for the pictorial diagram.
We need to see what your voltage
divider looks like, and especially the
values of the resistors you used.
How do you know the digital inputs
are damaged?
Herb
Hello herbschwarz,
I am using the L78S05CVas a voltage divider.
This unit provides no more than 5V when a 12V Current is applied.
I have also added two K5M Capacitors between the lines
This is my current voltage divider setup:
I know that this arduino is grilled, because the ardu is not found, when I connect it via USB. Others are found 
That's at least an interesting way to adjust voltage . . .
But a two resistor divider would do what you need, with the first twice the size of the second (three times if running on 3.3v).
Or, just a stiff resistor inline can do it in some circumstances.
What are your inputs coming from? is there a risk of noise or spiking?
There should be no noise or spiking because of the voltage regulator, at least I get stable 4.9 V on my multimeter. (Because of the caps, it decreases slowly over time to 0V, if I cut the 12V power supply)
Unfortunally I got no digital oscilloscope to see this in detail.
Because of a lot of unique sensor voltages I need to use (12V,10V....) I started to use the voltage regulator in order to run around a big pile of resistors.
Do you also think this should be okay?
Or is there some sort of grounding problem or other logical error in my circuit?
This is the power supply for my setup:
That part just isn't designed for what you're doing with it.
TO my limited knowledge, it's not a problem, but I have no real basis for that. But it's an expensive approach.
What is the source? Why not just an inline resistor? Or if the dissipation isn't an issue, a resistor voltage divider?
4.9v won't bother a 5v Arduino, but could be a problem for 3.3v (which are most of mine; there rest were ordered by accident 
and are waiting to be converted.
I use the arduino Mega 2560, so it should be 5V tolerant. 4.9V should also be a good value for them.
I have updated my previous post in order to show the power supply. The sensor can't provide more power than the power supply, or am I wrong here?
The sensor shouldn't be providing power at all, although a bit may flow.
It's output voltage generally shouldn't exceed the input limit of the Arduino or other device. This is what you were doing with 7805, but can also be done more simply.
