Monitoring 12v car circuits (are they on or off) with a Mega2560

[kriboogh]

I'm building a custom car monitoring dashboard with an RPI3 that I want to connect to a Arduino Mega2560 over USB. The idea is for the RPI to read and control the Arduino's IO ports and present them graphically using a QT application. I have a Carnettix 90 Watt PSU that delivers 12v and 5v stable regulated lines. (http://carnetix.com/power-supplies). The car is from the 70s, so it's using plain old 12v electrical circuits for switching on the lights, lowering the windows, the door locks, .... and has a EFI (electronic fuel injection) system with analogue 12v sensors for fuel tank level, water temp, oil pressure ect... (No OBD or CAN-bus or any of that modern stuff).

What I would like to achieve without altering the existing circuits (too much) is to "hook" into those to somehow monitor these circuits. I read in another thread here that was doing something similar (monitoring external circuits) to use some sort of current sensors. So google brought these up:
ACS712 (ACS712 5A Current Sensor Module for Arduino - Blue - Free shipping - DealExtreme)

Couple of questions:

• These sensors seem to deliver an analogue signal (I think?) but I would like to somehow use the digital inputs on the Arduino as I need to monitor a lot of circuits. But what it boils down to is, I basically want the Arduino to know if the button for the headlights is turned on or off.
• I also would like to get feedback if a bulb pops, so I can get a warning I need to replace the front left headlight. I think I also need some current detection for this, but again how do I hook it up to the digital input?
• Can I perhaps achieve this in another way? This being, the 12v car circuit state (in use or not) needs to be translated into a digital I/O input from the Arduino.
• Do I need to fuse it somewhere? (The car circuit already has it's own fuses).

Thanks,
Kris

Ps: As for my level, I'm not an expert with electronic components, I know what a capacitor and transistor are, but don't know how to chose the right ones, let alone, improvise a custom circuit. So if any pre-assembled little add-on boards exist I'll go that way...

[DVDdoug]

Use a [u]voltage divider[/u] (2 resistors) to knock-down 12V to about 5V. (The resistors can add-up to about 10K.)

Then add a couple of [u]protection diodes[/u] (or a 5V Zener diode) to protect from over-voltage spikes or negative "inductive kickback".

... You can probably get-by with just the protection circuit (one resistor and a pair of diodes, or one resistor and a 5V Zener.)

• I also would like to get feedback if a bulb pops, so I can get a warning I need to replace the front left headlight. I think I also need some current detection for this, but again how do I hook it up to the digital input?

Current measurement is trickier, but you can look for a Hall effect current sensor.

• Do I need to fuse it somewhere? (The car circuit already has it's own fuses).

No... A fuse won't protect the Arduino... It protects against burned-up wires or a fire (in case there's a short).

[kriboogh]

Thanks for the suggestion! With one 10k resistor and a 5.1V Zener I'm a 4.99V for my digital input on the Arduino, so that would work right? Voltage levels kan range a bit on the car DC source (the alternator is ranged at 13.9-14.6V), in simulation mode in iCircuit this seems to hold well. Do I need to add any extra protection (so I don't fry the Arduino?) (see drawing, attached)

Any idea if the resistor will get hot or will this effect the existing circuit in any way?

I now know the voltage (5.1) for the Zener, but how do I know what Wattage the zener should have? There are 1/2W, 1W, 250mW...