Episode: 2028
Title: HPR2028: Some basic info on alarm systems
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2028/hpr2028.mp3
Transcribed: 2025-10-18 13:30:16

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This is HPR episode 2028 entitled Some Basic Info on Alarm Systems.
It is posted by first-time post-chimium and in about 8 minutes long, the summary is a very
basic intro into some alarm equipment.
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Hi, my name is Bill Scarbury.
I heard Kinfellent's call for shows saying that we were getting very, very low on them.
So I figured I would add what a little bit of knowledge I have to the HPR community.
I do alarms for a living and talking to quite a few people smarter than me that I realized
that they really didn't understand how alarms work.
So I figured I would start with something simple explaining how a contact works.
Most of your contacts work on a very simple, normally close principle.
What you have is either an embedded or a surface contact.
It's mainly one half of it is just a glob of plastic with wires inside that go to a vacuum
seal glass file that have two little pieces of metal in it, and on the other side you
have a piece of plastic with a blob of goop inside that holds a bar magnet.
And whenever that magnet is close to the contact itself, which is usually about a half
an inch, it closes the circuit and allows the system to see that the contact is closed.
Now the way that it does it is, it sends out a voltage on one side of the wire, depending
upon the actual panel that voltage can vary, and then you'll have a resistor somewhere
in the line, which should actually be at the device, and the voltage that comes back
to the panel that the panel reads is of course the lower voltage because it's went across
the resistor, and that lets the system know that everything is okay.
If the opening opens, or if somebody tries to short the wires together to circumvent it,
the panel knows that as well, and allows it to know that something is happening and
it needs to respond in the correct way.
A good way to look at it is, as far as the voltage goes, is kind of like a garden hose.
Take a garden hose, you crank it all the way up, and you know you have full water going
through, and put a little kink in it, that's kind of like the resistor at the other side,
so you know how much water is coming out the other end.
Now there are other types of contacts, which are dual polarity, dual throw, which are
used for more secure situations, but it's just a simple closed circuit that is reading
the voltage.
Now that explains the basic idea of a circuit, as far as an alarm system is concerned.
There are other devices that you can use of course, the most commonly known other than
a contact, would be what people commonly refer to as a motion detector, which is a misnomer.
And after 20 years in the business, that misnomer drives me absolutely ape.
What you commonly come across is what is actually known as a passive infrared device, or a
PIR, and the way that they work is they have a heat sensing, do a hakey for the technical
term inside of the unit, and it has a Fresnel lens that allows it to focus the infrared that
is in the room, and what it looks for is a body of heat to move from one focusing area
to the other, so it can begin its initiation circuit.
Now the circuit side itself is just like a contact, it's just the voltage going out on
the wire coming across the terminals, and if it's closed, it sees the resistor, and everything
is happy, if it's open, like if the passive infrared sees motion, then it begins the process
at the control panel to let the person know what's going on, whether it be in the
alarm condition, or just letting somebody know that somebody's out there doing a dance
or something.
Now those are good for indoor environments where there's not a lot of animals.
Most common passive infrareds right now actually do have some digital signal processing that
will handle an animal outside of six linear feet of about 40 pounds or so reliably.
And what happens after that, whether it be the animal or object or what have you gets
within six linear feet, it's kind of like the elephant effect, an elephant out in the
middle of the field, looks like an ant, but if it's standing on your foot, all you see
is an elephant, and the motion detector goes nuts and goes off.
Now another common type of PIR motion detector, whatever you want to call it, is either what
is referred to as a dual technology or a tri-technology motion detector, and it has the
passive infrared built in with the Fresnel lens, but it also has a microwave component
that measures velocity, and it's a lot like your radar gun.
And since a microwave signal out receives that microwave signal back, and if it determines
that it's receiving that signal back quicker than what it should, then it helps process
the alarm.
Most of those type of motion detectors actually have to have both the microwave component
and the passive infrared component to go off conjunction with one another to actually
begin the alarm initiation.
Now those motion detectors are commonly used in a place where there could possibly be
a larger animal or in a non-temperature control environment such as a garage or a warehouse
or something of that nature.
I think this is where I'm going to leave off, it's more than I originally plan on covering,
but if you have any questions about anything, please feel free to send me an email.
My email address is AlarmDude9 at gmail.com, alpha, lima, alpha, romeo, mic, delta, uniform,
delta, echo, the number 9 at gmail.com.
Please put something like HPR or some such in the title because I do have a habit of sticking
everything in the trash if I don't recognize the email address, and if there's something
in particular about alarms, you have a question about you would like me to record on,
please feel free to do that as well.
Thank you very much.
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