12V analog input readings

I am building my home alarms systems.
what is the solution to read 12V analog inputs with an arduino ?
I don't want to use a resistor dividers circuit, is there a hat card or a solution to plug 12V analog input and be able to process it with arduino?

I need about 4 analog inputs for 4 alarms zones
Each alarms zones with contains reed switch and passive infrared sensor.

addon: if that card could have relay to power siren it would be great !


Your post was MOVED to its current location as it is more suitable.

Why not ?

These type of sensors are digital, not analog, you could use a high value resistor in series to tap into the 12V signal, but I would use an opto-isolator. You could build these on to a protoshield, as well as the relay, this would give you a more permanant solution, you could even design a PCB and hive it made to give you a more reliable and robust project.

If that's too simple for you then use optocouplers. Perhaps you can find a board for that solution?

If there is such a solution, it will probably use voltage dividers!

ok for optocouplers, but is optcouplers able to manage analog readings ?

There are some analog opto-couplers but they are more involved using opamps for a feedback loop.

However from the initial posting describing reed switches and PIR sensors I don't see why analog is needed.

i need analog input because reed switch and pir connectivity can give 3 states :

  • 0Ohms
  • Infinite

Alarms systems works like this. It is a loop with all sensors with a End Of Line Resistors

Can you give a schematic of such a sensor? There will be a standard receiver circuit for these which would be good to know but can be reverse engineered.


"i need analog input because reed switch and pir connectivity can give 3 states :


A "supervised" loop, alarms if the loop is shorted or cut, AC or DC?

That seems unlikely: where do the PIR sensors get power? Or is this just the reed relay part of the circuit?

Where is the 12V?

It is 12V DC

There are another 2 wires to power PIR (not on the schematic)
Z1+ is also 12VDC

Can we have the actual schematic then - where are you supposed to sense the voltage if the sensors are only connected to the supply rails? Or is the 12V current-sensed?

And include which are the connections going to the Arduino, that's not obvious at all.

here is what i want to do

After some research on internet
i think this is the right solution for me

That is a voltage divider and it is the correct way to do this.

Also is that schematic showing a fire alarm panel? I have a friend who builds fire alarm systems in his room.

This not for a Fire alarm but for a security Alarm
I think for Fire alarm Sensor are in parallel and Normally open

Find here the update for the security Alarm
Let me know what do you think of it

It's okay except that condition is determined from voltage, not R. Voltages are calculated from all resistors, not only EOL resistor.