Keith, you can't NOT choose to install the diodes that are built into the processor! Those are the ones I meant.

Oh! OK, I misunderstood.

 

If you are really worried about protection for the Arduino input, use an optoisolator.
That way, there is no electrical connection between the 12V and the Arduino.
Almost all the industrial control equipment we have designed use this.

the alarm system and the arduino circuit will have common ground so they will not be isolated.

 

I often use optos as easy non isolated level shifters anyway. It also makes it easy to change the input polarity.

 

I often use optos as easy non isolated level shifters anyway. It also makes it easy to change the input polarity.

doesn't it have the same issue as the transistor original solution using a bjt (more instability of the uC input)?

 

Here is a rough circuit.
A as an input switch with B shorted for current sourcing (PNP sensor), and B as the input with A shorted with current sinking (NPN sensor).
Arduino input with active pullups used.

 

An alarm system,
as in a house / office one
in my experience tends to involve long wires,

In my experience of these
they pick up all sorts of horrible common mode noise

If you connection is anything apart form all in the same box,
As others have said
Id strongly suggest that you use an opto isolated input
and do not have common "ground"

 

You could use a Opto 6N138 for full isolation like this...

Note that this particular circuit open switch -> high output, which is probably what you want for a security system (Just in case someone comes along with some cutters...)



Also note that I've seen some *very* smart security circuits spoken about on AAC, that overcome lots of different ways people typically hack a security systems

 

An alarm system,
as in a house / office one
in my experience tends to involve long wires,

In my experience of these
they pick up all sorts of horrible common mode noise

If you connection is anything apart form all in the same box,
As others have said
Id strongly suggest that you use an opto isolated input
and do not have common "ground"

Ok it looks like I have to tell more about what I am doing.
I am doing an add on for a home alarm system.
I bring power and signal with the same cable from the central unit to the Arduino Circuit. Basically I have an AI camera connected to the central unit that act as a sensor and I want to add an Arduino circuit controlled speaker that say something before the alarm goes off. Can I adopt with the solution in the attached image?

 

Ok it looks like I have to tell more about what I am doing.
I am doing an add on for a home alarm system.
I bring power and signal with the same cable from the central unit to the Arduino Circuit. Basically I have an AI camera connected to the central unit that act as a sensor and I want to add an Arduino circuit controlled speaker that say something before the alarm goes off. Can I adopt with the solution in the attached image?

use an opto
Why would you not want to ?

 

Something as simple as this?
Don't I have to worry about noise?

 

Something as simple as this?
Don't I have to worry about noise?

perfect

 

I've completed the circuit and it looks like it works in real life.
For this version of the circuit I've opted for a voltage divide insted of an opto (I will try that solution too).
Do you spot any major flaw in the design?
When the speaker is working the current draw doesn't exceed 0.2A and the speaker is supposed to work for several minutes just when the alarm goes off.
Thanks
p.s. I've used a trimmer in the voltage divider because I deal with alarm systems that give both 12V and 15V signal and power to my board.

 

I've completed the circuit and it looks like it works in real life.
For this version of the circuit I've opted for a voltage divide insted of an opto (I will try that solution too).
Do you spot any major flaw in the design?
When the speaker is working the current draw doesn't exceed 0.2A and the speaker is supposed to work for several minutes just when the alarm goes off.
Thanks
p.s. I've used a trimmer in the voltage divider because I deal with alarm systems that give both 12V and 15V signal and power to my board.

Terrible
What happens to the voltage into the zener / gpio when pot is at Max voltage .

What is the current through the zener when voltage is turned up to 15 volts , and pot is at either end ?

 

What is the purpose of C1 and C2?

Bob

 

Terrible
What happens to the voltage into the zener / gpio when pot is at Max voltage .

What is the current through the zener when voltage is turned up to 15 volts , and pot is at either end ?

ok ok...
I have done an update.
Better now?

 

What is the purpose of C1 and C2?

Bob

Noise filter since this board may be some meters away from the main unit.

 

I calculate a cutoff frequency of 0.03Hz. Just the 100n should be sufficient.

Does this actually work? I calculate that it would take seconds to change state with the 100u.

Bob

 

I calculate a cutoff frequency of 0.03Hz. Just the 100n should be sufficient.

Dies this actually work? I calculate that it would ll take secind ti change state with the 100u.

Bob

Yes it works.
When the alarm goes off the main unit is programmed to give a high signal to the Arduino board that last several seconds.

 

But you don’t see a delay before the Arduino sees the signal from the alarm?

Bob

 

But you don’t see a delay before the Arduino sees the signal from the alarm?

Bob

yes