Sensing an external voltage which switches between 12V and 0V


I'm a complete newbie to Arduino and electronics, and I have a project that I would like to make. I've not done any electronics since 1994 when I covered some basics for physics A-Level. So I bought and read a 'dummies' guide and did some revision.

What I would like to do is have an Arduino monitor some signals from an external device. The device is designed to output these signals at 12V (100mA) and if the condition is triggered then the voltage drops to 0.

I think I have worked out how this should be done, and was wondering if anyone could check my workings and confirm if I am going down the right road?

Please point out any errors in my workings!

So this is my current plan:

For each signal output the voltage is 12v (actually measured with a multimeter to be 13.6V across the output pins and the aux 0v pin of the device)

Therefore for each output I need to have a voltage divider to get the voltage down to the 5V that the Arduino digital inputs are expecting.

The current output from the signal outputs is 200mA therefore the total resistance of the voltage bridge should be 120 Ohms (using V=IR where V=12 and I = 0.2A)

To get 5V out (or near enough) the value of R2 should be twice the value of R1 (=> the voltage out to the arduino would be 1/3 of total voltage = 13.6/3 = 4.5V)

Therefore if R Total = 120 Ohms then R1 = 80 Ohms and R2 = 40 Ohms

I would also need to connect the Arduino Ground pin to the 0V pin on the device supplying the signal voltages. Right?

Does it matter that the voltage to the Arduino Digital input pins would be at 4.5v rather than 5V?

So - how did I do?

I'd go for much higher resistances, in the Ks, rather than the tens, otherwise you're just burning current in your potential divider.
4k7 and 8k2 would give you about 4.4 volts.

Ahhh - I see. The 200mA is the maximum current that the device is designed to allow out through the outputs, not what should be going through them.

So if the current is highly restricted we aren't wasting power and just allowing a trickle through.

We don't want current, as we are dealing with signals, even with a tiny current the signal is still available as a voltage.

So - theoretical question - how do you decide how high the total resistance should be?

I'd pick total resistance so that ~0.5 to 1mA are flowing.
So about 12K total.
Enough current flow to not be susceptible to picking up noise, yet not so much as to load down the source.

Great. Thanks for all the help.