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Building a Home Security System with BeagleBone by Bill Pretty

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The hardware

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.

The LM339 comparator

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 ...

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