[solved] Shield to detect presence 12 V signal, on multiple channels?

TL:DR
I want to detect the presence, or absence, of a sustained 12 V signal.
To protect the Arduino board from beginner-mistakes, I want to use an I²C- or SPI-based shield rather than a voltage divider. That way I’ll burn the shield, rather than the board, when I mess up.


Experience = Minimal
I’m completely new to Arduino. Only done a little programming (entry level Python). I’m familliar with analogue circuits, breadboarding etc. I need help picking a good shield for my project. Could you dudes point me in the right direction?


Project intro
Currently moving into a new student apartment. A few days ago, a group of kids were sneaking around the area where I’m moving in, checking out windows & taking pictures. I live on the ground floor, so my place is pretty vulnerable. I have blinds & locks, sure, but any window can be opened with a rock.

But I’m not securing a bank vault from the Joker… All I need is something that goes WEE-OOO-WEE-OOO to scare them off if they try something. I have a bunch of sensors from an old alarm system. Sensors work fine, but the alarm central doesn’t work.

Input from each sensor comes in the form of a DC signal.
12 V, <5 µA = No detection
12 V, 5 mA = Detection


Shield?
A votlage divider would be a simple solution, but let’s be honest: As a beginner, I’m more than likely to burn something. With an I²C- or SPI-based shield between the sensor and the Arduino, then the first thing I burn will probably be the shield.

I only need to get a yes/no indication for sustained voltage.

Do you guys have any suggestions or advice on what type of shield to look for?


Additional info
Perpherals: 10 sensors, 1 keypad, 1 LCD (status panel).
Siren controlled via 12 V relay.
Board: ATMEGA2560 R3

I’m packing to move. Will check in periodically, but may be slow to respond. Apologies for that.

You don't need a shield for that, all you need is a simple transistor and a resistor.
When 12V is applied, the transistor turns on and pulls the input pin Low.
Your sketch looks for a low input to turn the siren/relay on.

CrossRoads:
You don't need a shield for that, all you need is a simple transistor and a resistor./.../

Thank you!
That's a perfect solution.
Have a great day!

There is still a good reason to use a shield, and that is to provide a secure means of connecting the wires from your various sensors to the Arduino. Breadboards are okay for debugging simple circuits but not so great for more permanent installations. Look for boards called "screw shields." These have screw terminals and a small breadboarding area for your transistors or R dividers...
S.

I offer an Uno screwshield as an example

CrossRoads:
You don't need a shield for that, all you need is a simple transistor and a resistor.
When 12V is applied, the transistor turns on and pulls the input pin Low.
Your sketch looks for a low input to turn the siren/relay on.

Adding a single LED to the 2.2k resistor gives indication. This way one transistor and one I/O pin can serve all the sensors. Add the combination LED + 2.2K to each sensor.

roy_roger:
That’s a perfect solution.

I’m very curious to know what role you expected I2C or SPI to have played?

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