Hi all, I could use some assistance please.
I have three very poorly documented pressure sensors. The spec sheet showed up in Chinese, but they are definitely 12-24vdc input and provide a 0-5v output signal.
ebay link
I'm powering the sensors with a 24v power supply with a relay in-line to power the sensors off if the Arduino loses power.
It is working as expected, but after some more reading I think I should be worried about spikes from the sensors. I've got it all shut down now and I'm wondering if anyone could tell me if I need to add a resistor (or anything else??) on each of the signal wires, and what size if so? There was almost no drift in the analog readings currently and it is my understanding that adding resistance may increase noise... Is that accurate?
If it isn't obvious, I'm (at best) a novice in this area so I appreciate any guidance you can offer!
Jason
Why are you worried about "spikes"? Could they be on the data line to the Arduino? Could they be negative or positive spikes? Have you looked for spikes with an oscilloscope?
Paul
Put a 10K resistor between the sensor output and the analog input. That will protect against overvoltage.
with a relay in-line to power the sensors off if the Arduino loses power.
Relays are not fast enough to prevent damage. Instead, use a step down converter to power the Arduino from the sensor power supply.
Thank you both. Paul, I wasn't worried about them until after I put everything together and started reading. Ignorance is bliss. 
It was my understanding that the Arduino analog ports are very sensitive to any voltage above 5v and since these are 12-24v sensors that something might go errant there. Maybe not.
Was just asking for validation here since I was piecing together info I was reading about. An 10k resistor on the signal wire was indeed something I had read about....
I would start by reading the specification of the sensor. Many sensors have a higher input voltage then the output will swing. I have a feeling your output is in the range of 0V to 5V, The series resistor will protect the arduino. You can add a low pass filter by putting a cap to ground in the range of 0.01uF to get the fast transients, this will slow down the response. If the voltage goes higher use a voltage divider to limit the voltage. There are many fine calculators on line that will do that for you. Try: Voltage Divider Calculator. I llike to use a 2K or less to ground in the divider circut keeping the input impedance low and swamping the A/D capacitance.
Good Luck, HavE Fun, Gil
Hi,
Welcome to the forum.
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .
OPs circuit...
Tom...
