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Back to: Advanced Physical Security Integration (APSI)

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Lesson 5.4: Circuit Wiring & Resistor Logic

Module: 5 – Intrusion Detection & Perimeter Security

Prerequisites: Lesson 5.1 (Alarm Panels)

Estimated Time: 45–60 Minutes

1. Learning Objectives

By the end of this lesson, you will be able to:

  • Construct a Normally Closed (NC) loop with a Series resistor for intrusion sensors.
  • Explain why parallel wiring is used for Normally Open (NO) devices (like Fire/Smoke).
  • Calculate the total resistance of a Series vs. Parallel circuit to troubleshoot “Open” or “Short” faults.
  • Diagram the specific wiring for a “Tamper” switch utilizing Double End-of-Line (DEOL) resistors.

2. The Circuit Logic: Series vs. Parallel

Series Circuit (The Chain)

  • Concept: Components are wired one after another. If any link breaks, the whole chain breaks.
  • Usage: Burglar Alarms (NC). We want the alarm to trigger if any sensor opens or if the wire is cut.
  • Math: R_{total} = R_1 + R_2 + …

Parallel Circuit (The Ladder)

  • Concept: Components are wired across the same two rails. Current has multiple paths.
  • Usage: Fire Alarms (NO). Devices are normally “Off” (Open). When smoke is detected, the switch closes, shorting the rails to signal the panel.
  • Math: 1/R_{total} = 1/R_1 + 1/R_2 + …

3. Intrusion Wiring (Normally Closed with EOL)

  • The Standard: Security devices (Door Contacts, Motions) are Normally Closed (NC).
  • The Goal: We want the panel to see a specific resistance (e.g., 2.2 ohms) when the door is closed.
  • The Wiring:
    1. Run a 2-conductor wire from the Panel (Zone 1 & COM) to the Sensor.
    2. At the sensor, connect one wire to the “NC” terminal.
    3. Connect the Resistor to the “C” (Common) terminal.
    4. Connect the return wire to the other leg of the Resistor.
  • Result:
    • Door Closed: Current flows through the resistor. Panel sees 2.2 ohms (Secure).
    • Door Open: Switch opens. Current stops. Panel sees Infinite ohms (Alarm).
    • Wire Cut: Current stops. Panel sees Infinite ohms (Alarm/Trouble).
    • Wire Shorted: Current bypasses the resistor. Panel sees ohms (Tamper).

4. Fire Wiring (Normally Open with EOL)

  • The Standard: Conventional Smoke Detectors / Heat Detectors are Normally Open (NO).
  • The Wiring:
    1. Run wires to Detector 1, then to Detector 2, etc. (Daisy Chain).
    2. The Resistor goes at the very end of the last device, across the terminals.
  • Result:
    • Normal: The panel pushes current down the line. It passes through all devices (open switches) and hits the resistor at the end. Panel sees 2.2 ohms (Secure).
    • Fire: A detector switch closes. It creates a short circuit across the wires before the current gets to the resistor. Panel sees 0 ohms(ALARM).
    • Wire Cut: The path to the resistor is broken. Panel sees Infinite ohms (Trouble/Fault).
    • Crucial: In Fire logic, a cut wire must NOT trigger an alarm; it must trigger a “Trouble” signal so you know to fix it. This is why we use NO loops.

5. Troubleshooting with a Multimeter

  • Scenario: Zone 1 is showing “Open” on the keypad, but the door is physically closed.
  • Step 1: Disconnect the wires from the panel (Zone 1 and COM).
  • Step 2: Set Multimeter to Ohms.
  • Step 3: Measure the wires.
    • Result = Infinite: The wire is cut somewhere in the wall, or the sensor is stuck open (broken reed switch).
    • Result = 2.2 ohms: The wiring is fine. The Panel Input is likely blown/damaged.
    • Result = 0 ohms: Short circuit (Staple through the wire?).

6. Double EOL (DEOL) Logic

  • Why: To distinguish between a “Door Alarm” and a “Tamper Alarm” using only 2 wires.
  • The Wiring:
    • Resistor 1 (Series): 2.2 ohms.
    • Resistor 2 (Parallel across the switch): 2.2 ohms.
  • The Panel Logic:
    • Secure (Door Closed): Loop passes through Resistor 1 only (Resistor 2 is shorted out by the closed switch). Total: 2.2 ohms.
    • Alarm (Door Open): Switch opens. Current is forced to go through Resistor 1 AND Resistor 2. Total: 4.4 ohms.
    • Tamper (Short): Current bypasses everything (burglar shorted the wires). Total: 0 ohms.
    • Trouble (Cut): Infinite ohms.