With perimeter alarm systems, it is the entire loop that is monitored to prevent tampering from disabling it. Switches are used to change the current flowing in the loop. Generally, they are not, themselves, the signals used by the panel.

[EDIT: While traditional systems used relays held in by the current though the loop, it is really the integrity of the loop that needs monitoring. This can include voltage drops or rises. But generally speaking it is the loop and not the switch the alarm system watches.]

 

Oh so I was missing something!! Thanks @Yaakov!

 

Soooo.. in this case then isn't it easiest to just use an analogue differentiator (+ window comparator)... if the current changes you get a pulse out of the differentiator which trips the comparator.. signals your interrupt on your Pi... you then run a quick A to D conversion, and work out what has changed?

 

Soooo.. in this case then isn't it easiest to just use an analogue differentiator (+ window comparator)... if the current changes you get a pulse out of the differentiator which trips the comparator.. signals your interrupt on your Pi... you then run a quick A to D conversion, and work out what has changed?

It probably is, in fact. In principle, we want to know if the loop changes beyond whatever the acceptable variance might be. However, since we have the computer involved, it would be nice to have the raw information and do the comparator's job in software since you can do all sorts of clever things in the digital domain.

Otherwise, comparators are the bomb.

 

It probably is, in fact. In principle, we want to know if the loop changes beyond whatever the acceptable variance might be. However, since we have the computer involved, it would be nice to have the raw information and do the comparator's job in software since you can do all sorts of clever things in the digital domain.

Otherwise, comparators are the bomb.

Hi guys - I had the opportunity to spend 15 hours on my motorcycle yesterday (Phoenix to DFW) so lots of time to think. I am debating including the ADC so that I can get the value into the digital domain for a variety of reasons;

1. Allows me to record voltage at system setup for each circuit that corresponds to an open and closed circuit, call this Vopen & Vclosed
   1. Circuits w/o resistor at the switch Vopen = 0, Vclosed = ~.5V
   2. Circuits with a resistor at the switch + on high side Vopen = ~.00148V - .~.0122V & Vclosed = ~.00296 - ~.02382V
2. When the comparator signals stage change I can now detect if the circuit has been cut or the circuit has been shorted by comparing the recorded Vopen & Vclosed to the measured V when the state changes; < Vopen = circuit cut, > Vclosed = circuit shorted
3. It will allow me to better calibrate the tuning of the digipot for each circuit
4. It may come in handy if I need to consider adjusting the digipot slightly for temperature variations over time

Just wanted to let you know that this is something I'm contemplating. I'm also looking into the analogue differentiator (+ window comparator) that pmd34 mentioned. PMD34 do you have a reference or two to a circuit like what you're describing? I'd like to research this and then come back and ask clarifying questions if that is ok?

Another thing that I debated on my ride was the resolution of the comparator. In my worst case scenario, circuit 7, Vclosed will be .00296V and Vopen will be .00148V. With that value coming into the + of the comparator and the - set at .00160V are there comparators that operate at that low of a level or do i need to boost these?

It probably is, in fact. In principle, we want to know if the loop changes beyond whatever the acceptable variance might be. However, since we have the computer involved, it would be nice to have the raw information and do the comparator's job in software since you can do all sorts of clever things in the digital domain.

Otherwise, comparators are the bomb.

 

It probably is, in fact. In principle, we want to know if the loop changes beyond whatever the acceptable variance might be. However, since we have the computer involved, it would be nice to have the raw information and do the comparator's job in software since you can do all sorts of clever things in the digital domain.

Otherwise, comparators are the bomb.

Hi Yaakov - FYI I did go the route of the comparator and have built a prototype though I am having a challenge with respect to the open drain output of the ADCMP393. I posted the current schematic and images of prototype and issue at https://forum.allaboutcircuits.com/...rain-output-and-pull-ups.159137/#post-1384884

Hope you're doing well!