Part 2B – the alarm system hardware

First of all, what is a zone? A zone is an area that is being protected and/or a collection of sensors that are protecting the zone. For example, ground floor windows could be a zone. Ground floor PIRs could be another zone. There are also special zones, such as fire alarm zones. In a commercial installation, fire zones and carbon monoxide (CO) detectors must have their own zone.

But first of all, the alarm system, in this case the BeagleBone, must be able to monitor the world around it. It does this by using ICs called comparators. You can think of a comparator as an op-amp with extremely high gain. Like an op-amp, when the voltage on the positive input (+) is higher than the voltage on the negative input (-), the output goes high. When the opposite is true, the output goes low.

We will be using an LM339 comparator. This device has an open collector output. What this means is that a resistor is required to pull the output high. By connecting the output...