Part 1: Arduino Home Security

I just moved into a new apartment that was pre-wired for a standard home security system. There’s a motion sensor in the living room and open-sensors on the front door and on each window. The building owner doesn’t pay for monitoring and the system was just sitting there unplugged.

After doing some quick research online, I decided that I could hook up an Arduino to the leads from the different sensors, add an Arduino Ethernet Shield, have the system monitor each sensor, and take action if there was a security breach. This is only Part 1 but I’ll go ahead and spill the beans – it worked (and still works)!

My goals for the project:

1. Have fun
2. Learn how to program the Arduino since I’ve never worked with one before
3. Spend < $100 for an end product that performs reasonably well My first step was to learn how everything was wired up. I hooked up the power to the existing system and grabbed a multimeter. I touched the leads to the door sensor while the door was closed, and the reading was 0 volts. When I opened the door, the reading immediately jumped to ~5 volts.

This told me that my sensors were likely wired as “Normally Open” (NO) rather than “Normally Closed” (NC). This means that the circuit is open and not completed (i.e. 0 volts) unless the sensor is tripped. With an NO sensor, there are two possible reasons for the circuit to be closed: either by the door/window being opened or by someone cutting the wire.

Door sensor:

Main panel wiring:

The main panel has inputs for 6 different sensors, but it appeared that only 5 were connected. A note in the control panel showed that the installer had skipped zone #6 and put the door sensor as #7:

Removing the control panel:

I soon found out that the door sensor was wired straight to the control panel near the doorway which is then connected to the main panel:

After removing the control panel from the wall I found that the other 5 sensor inputs shared a single lead back to the negative power terminal on the control panel. The various sensor connections all terminated in the wall behind the control panel and rode a single 8-wire cable over to the main panel.

Next step: hook it all up!

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