Alarm Contact

**Ron Marcus** · - 11th October 2005, 14:53

Your description lost a little in the translation. Are you looking for two statyes out of your loop, or three? For two states, make your pin an input, and it it is on a port with weak pull up, enable it. If not, place a 47K resistor between the pin and B+. Drop the resistor to ground to 1K and when the switch is opened, the port will read high, when closed, it will read low.

Hope this helps,
Ron

**Pesticida** · - 11th October 2005, 17:59

Hi,thanks for the help,
Sorry my english is very bad,Yes what I mean is three states Switch is:
Closed = Normal State,
Open = Alarm State,
Between the wires is a Shortcircuit = Sabotage

I want that the PIC recognize when a 10 K is in the loop or not,when is not a 10 K then is alarm ,when i have on the port ~0 Ohm then is sabotage and when I have open loop then is Alarm.

BigWumpus what commands I need to measure the voltage,what you mean with 0,3 and 0,7?

Thanks a lot !!!
I hope you can help me.
Bye Pesti.

**Darrel Taylor** · - 13th October 2005, 03:59

I think this is what BigWumpus meant. But the tolerances are a bit tighter than the 0.3 and 0.7 times VDD. Catches more hackers. And still leaves room for 5% (or worse) resistors.

And, it's better to take a Reading when the system is Initialized, and use an even tighter tolerance from that reference point to determine if a TAMPER has occured.
<br>

**BobK** · - 13th October 2005, 12:41

Good morning to all responding to this post.

The purpose of an "end of line resistor" is to supervise the loop. What you are working on here is fine but the supervision resistor needs to go between the ground point and the switch. This way if the wiring to the switch gets scraped while installing or the staple goes thru the cable and shorts out the contact it will be detected properly. Most modern day systems use either 1K or 2.2K resistors as the EOLR. A few use 1.5K.

BobK (31 years as alarm installer)

**Luciano** · - 13th October 2005, 15:09

Hi,

Darrel's schematic:

The components R1 and S1 are grouped into a dotted-line box,
corresponding to a physical unit. (R1 is inside an enclosure with S1).

* * *

It dosen't matter on which contact of the switch S1 the resistor R1 is soldered.

The system is secure only if the switch S1 and the resistor R1 are
in the same enclosure and coated with resin. (One physical unit).

If you put more than one switch in series on the same line, the system won't be secure.

Code:

Not secure system

+5 --R2--┐
         |
AN0 -----┴-----------S1--------S2-------S3---------S4R1--┐ 
                                                         |                                                 
GND -----------------------------------------------------┘ 


Secure system

+5 --R2--┐
         |
AN0 -----┴-----------------------------------------S1R1--┐ 
                                                         |                                                 
GND -----------------------------------------------------┘

(R2 is inside the microcontroller enclosure).

Best regards,

Luciano

**Pesticida** · - 13th October 2005, 17:49

Thanks to all for the answers,but what command I now need to measure this Voltage I have a PIC16F870 with 20Mhz .
What happend when i have a wire ~ 100-300 Meter long.
I dont know how sensitive is a PIC controller.

Thanks a lot People!!!

Bye Pesti.

**Pesticida** · - 13th October 2005, 19:30

I think that I can use the Alarm circuit like on my draw !?

I have three States,Cicuit :
Magnetic Contact is closed: No Alarm (5K6 Ohm)
Magnetic Contact is Open: Alarm (6K4 Ohm)
Cable Tamper or Alarm Box Tamper is open : Sabotage
ShortCircuit on the loop : Sabotage

The Contacts are inside a door and on the Security side Zone.
I think so is ok or ?
Bye Pesti.