high resistance input to sniff voltage level of a water temperature sensor

I'd like some advice regarding anyone's experiences with this. I am into vintage cars. And I have been building out my Arduino to start taking over some of the electronics that are, of course, absent. Some of my first projects have been to take over controlling heating and ventilation fans on and around the engine (big engine, tiny space = heat). I'd like to tap the electronic water temperature sensor sending unit on the block that is for my water temp gauge. I figure I can use the high resistance input to sense it and apply a little hysteresis to switch various fans on and off at various temp's.

Ok, my question: has anyone done this in an automotive setting? Do I need to worry about EM interference on the signal? And if so, advice on how to resolve that issue?

Firstly you will need to test the voltage range with a multi meter.

Second assuming it is a petrol engine you will need to shield a lot. You will need to google how to do that in an automotive environment. Shield and ground the case, for a start.

I figure I can use the high resistance input to sense it and apply a little hysteresis to switch various fans on and off at various temp’s.

That should work. But, the Arduino can be damaged by voltages higher than +5V, or negative voltages.

Assuming the sensor puts-out more than 5V, you’ll need a [u]Voltage Divider[/u].

Even if the sensor puts-out 5V or less, it would be a good idea to add a [u]protection circuit[/u] because you are running from a (nominally) 12V system and stuff can happen. If you are using a voltage divider, you already have the resistor and you can just add a diode (or two).

Do I need to worry about EM interference on the signal?

You’ll probably pick-up some noise. You won’t know 'till you try it. Since your signal is slowly-changing DC you can use a low-pass filter… Just a resistor and a capacitor, and again the resistor can be the same resistor used for the voltage divider or protection circuit, so you can just add a capacitor between the Arduino’s analog input and ground.

You can also take several readings and average them to filter-out (average-out) the noise if it’s not too bad.

Automobiles are incredibly harsh, hostile environments for electronics. Protection is mandatory.

Plan on power from the battery going up to 18V, with lots of noise going up much higher. Plan on any sensors having a lot of noise spikes on them, and ground loop currents. Build your circuits accordingly.

I think an opto isolator can be used as a linear amplifier
Google search : Using opto isloator as a linear amplifier