Home Automation System - SARCNET

//These are the characteristics of the security system and its sensors

//The security system has 4 physical zones inputs (1-4)

const float Ri = 3240.0;                                //This is the measured internal pullup resistor, in ohms, on each physical zone in the security system:

//In dual-zone mode each physical zone input supports two zones, one low and one high as follows: 1/5, 2/6, 3/7, 4/8

//Each sensor comprises a Normally-Closed contact and a series End-Of-Line resistor.  

const float Rh = 3750.0;                               //The EOL resistance on the high zone sensor, in ohms. The high zone is active when Rl is NOT present.

const float Rl = 6970.0;                                //The EOL resistance on the low zone sensor, in ohms. The low zone is active when Rh is NOT present.

const float Rp = 1.0 / (1.0 / Rh + 1.0 / Rl);    //The EOL resistance with both sensors inactive, in ohms.

const float Vs = 13.55;                                 //The measured security system supply voltage, in volts.

//The dual-zone input voltage is Vz

//Vz equals Vn, Vh, Vl or Vs, depending on which sensors are active as follows: None, High, Low or Both sensors

//Here are the formulas for the dual-zone input voltage in each case, in order of decreasing voltage:

const float Vl = Vs * Rl / (Ri + Rl);                 //Low

const float Vh = Vs * Rh / (Ri + Rh);              //High

const float Vn = Vs * Rp / (Ri + Rp);              //None

//To identify active zones we require 3 threshold voltages between Vs & Vl, Vl & Vh and Vh & Vn: Vsl, Vlh and Vhn. They are:

const float Vsl = (Vs + Vl) / 2.0;

const float Vlh = (Vl + Vh) / 2.0;

const float Vhn = (Vh + Vn) / 2.0;

//The active zones can then be tested as follows:

//Both: if Vz >= Vsl

//Low:  else if Vz >= Vlh

//High: else if Vz >= Vhn

//None: Otherwise

//The constant K is used to calculate the actual zone input voltage from the analog read value

//Given the Arduino supply voltage and the input resistive-divider values

const float Vcc = 3.3;                                   //The Arduino Pro Mini supply voltage

const float Amax = 1023.0;                           //The analogRead values range between 0 and Max for voltages between 0 and Vcc

const float R1 = 47000.0;                             //The chosen voltage divider resistor connected to the zone input

const float R2a = R1 / (Vs / Vcc - 1);             //Calculate the required voltage divider resistor connected to ground (= 15132)

const float R2 = 15000.0;                             //The chosen voltage divider resistor connected to ground

const float Vmax = Vcc * (R1 + R2) / R2;      //The maximum permitted zone input voltage

const float K = Vmax / Amax;                       //A constant to convert analogRead values to zone input voltage