Hardware Reference
In-Depth Information
Pressure mats are not polarity-sensitive, although I have used positive and negative indications in Figure
5-4
to
help you understand how they work. A pressure mat is a type of circuit called normally open, or NO for short. This
type of circuit is expected to be open and an alarm will be triggered when the circuit is closed. Looking at Figure
5-4
,
you can see three distinct parts of the pressure mat.
•
On the top side of the mat you will see the positive contact.
•
On the bottom of the mat you will see the negative contact.
Separating them is the section called void and this is simply just an air gap wide enough to stop the positive and
negative sides from touching when there is no pressure. Because the pressure mat is made of porous material, when
you step on it the air is expelled from the void and the positive and negative sides make contact. The circuit is now
closed and the alarm will trigger. When you remove the pressure from the mat the sides separate again and the circuit
is broken.
Some security mats also come with an additional two or more wires. These wires are often in place to act as a
tamper loop, most often in a normally closed circuit. I won't use them in this chapter as I only want to demonstrate
the use of the mat itself. Now I want to show you the full circuit schematic for the mat; you will soon see how simple it
really is. In Figure
5-5
you can see the two loops. The pressure loop is the loop you will be interfacing with: as you can
see it's a normally open circuit. It will work just like a big switch.
Mat
Tamper Loop
Pressure Loop
Figure 5-5.
A typical pressure mat circuit diagram
Don't worry about the tamper loop: you won't be using it but you can see that it's a normally closed circuit. If you
did want to use the tamper loop it would be the exact opposite circuit from the pressure loop. This type of tamper loop
prevents someone from cutting the mat open and jamming the pressure loop into an open state. If you were to cut the
mat, you would cut though the normally closed tamper loop circuit and the alarm would trigger.
Assembling the Devices
Now that you know more about the pressure mat and the PIR sensor, let's make them work. For this chapter you're
going to need the following items:
•
Pressure mat
•
PIR sensor (I used a Panasonic AMN31111)
•
39K ohm resistor
