Alarm panel related question?

[Snops]

(In a nutshell, How can I make proper burglar alarm panel zone input circuit?)

Hi everyone. Last couple of months I have been designing burglar alarm system for personal use.
I came acrosss a small problem and I hope you guys could help me.

My problem is that I dont know how to design zone circuitry properly. I know that they are basically just voltage divider circuit.
Let assume that we have zone 1 (Z1) and com (COM) screw terminals as usual. Z1 is +12V and COM is 0V/GND.
(I draw simple schematic to make it easier to figure out what I'm trying to ask)

For example lets use LM339 comparator. (Is there any specific IC designed to do this kinda thing?)
LM339 is used as window comparator, high threshold +7V, low threshold +5V. Between those two voltages everything is ok. Voltage divider gives out +6V.
If burglar shorts sensor wires, comparator input is at 0v and thats fine since "Input common mode voltage range includes ground"
but when sensor trips (contact opens) there will be +12V in comparators input. Is that a problem since LM339 V+ is also at +12V?
Does that even matter since we are not detecting near the V+ rail?
Do comerical panels even use separate IC or do they just use 5V zone voltage and drive it directly into analog input of MCU?

If my approach is completely wrong or if anoyone knows exactly how comerical alarm panels do this thing, let me know.

This was probably most confusing question, evar! Thanks.

[helius]

Take a look at the EEVBlog video here:
https://www.eevblog.com/forum/blog/eevblog-682-ness-d16x-alarm-panel-repair/msg547135/

at 9:00 it shows the input circuit.

[SeanB]

It will work, and is typical for an alarm input. you might want to add a series 10k resistor to protect the comparator input from induced voltage, along with a pair of catch diodes to ground and Vcc, along with a slight reorganisation of the comparator resistor network to use 3 resistors in series, which will both track better and save a resistor. Typically alarm inputs use the comparators, though the more modern units use more levels in the comparator and use a CMOS analogue switch to scan the alarm inputs in sequence, so as to use only a single quad comparator to have 3 different levels to detect.

[Snops]

Take a look at the EEVBlog video here:
https://www.eevblog.com/forum/blog/eevblog-682-ness-d16x-alarm-panel-repair/msg547135/

at 9:00 it shows the input circuit.

Thanks. Ive watched that episode many times trying to figure out all the stuff i need to know. Ive also done quite a bit research to figure out other manufacturers panel details aswell. Unfortunately there isnt that much data available and I understand why 

It will work, and is typical for an alarm input. you might want to add a series 10k resistor to protect the comparator input from induced voltage, along with a pair of catch diodes to ground and Vcc, along with a slight reorganisation of the comparator resistor network to use 3 resistors in series, which will both track better and save a resistor. Typically alarm inputs use the comparators, though the more modern units use more levels in the comparator and use a CMOS analogue switch to scan the alarm inputs in sequence, so as to use only a single quad comparator to have 3 different levels to detect.

Thanks alot. I will add input protection, but that schematic was quickly drawn just to make it easier to explain. I was thinking about multi end-of-line resistor possibility, but then I thought that I don't realy need that much loop protection. Maybe in rev f 

Im planning to use switch mode power supply as main power for panel at around 13.7V. That will charge and float sla backup battery and provide power for sensors and zone loops. Then 5V linear regulator for mcu (probably AVR). Does that sound adequate?

[SeanB]

Power is fine, just use a spare ADC input to monitor battery voltage periodically to sense low battery, and if you want use a switch ( P channel FET or like alarm panels do a 5V regulator floated up by 8V to charge the battery via a diode, with a transistor to periodically pull the output to 5V to see if the battery is connected and capable of supplying power) to detect battery disconnection or failure. If the MCU has brownout restart turn it on, or add an external reset supervisor to reliable restart the MCU on brownout of the supply. WDT is a good addon, but you probably will want to disable it for initial code work and test it as a final measure, probably by stopping the MCU clock with a resistor to ground and see if it resets.

[Snops]

Very good design tips! I´m most likely going to use just plain old atmega328 as main mcu. Easy to program, quite cheap, alot of support from arduino community if needed. It has built in brownout detection and watchdog timer so thats covered. 

Now when I thought about it I might just use that multiplexer solution. So I need only one comparator and one multiplexer. Otherwise I would need like 4 quad comparators (8 zones) and more input pins from mcu.