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. I chose the LM339 comparator because it has been around forever, and still comes in through-hole DIP packages. It is cheap and easy to source. Best of all, it is hard to blow up! The device has an open collector output. What this means is, a resistor is required to pull the output high. By connecting the output pull-up resistor to 3.3V, we now have a level converter that can be safely connected to the BeagleBone.
In the following diagram of the single zone comparator circuit, a 4.7K ohm resistor—R5 is used as a pull-up. The LM339 comparator...