Sensors
We’ve assumed you’ll want to use hardwired sensors — ones that use pairs of wires between the sensor and the processor — because they’re more reliable (and often less expensive) than wireless ones. Many of them require a power source; if you don’t use an uninterruptible power supply to drive them, you’ll want to be sure you’re warned about power failures.
Figure 2 shows the Winland Vehicle Alert system, consisting of the sensor embedded in PVC pipe, the control box, and the cable to run between the two. The sensor is protected in a pipe filled with sand, so you can directly bury it in a gravel drive. Put the sensor 6–8 inches down, regardless of whether it’s in the drive or along it.
FIGURE 2: Winland Vehicle Alert
Figure 3 shows the interior electronics, including the connection points for the power supply, sensors (you can use two in parallel), and alarm contacts. The contacts are normally open, closing when a moving vehicle is detected. There are two sets of contacts, which would let you drive an X-10 Powerflash module as well as the security processor.
FIGURE 3: Vehicle Alert interior connections
Figure 4 shows your options for how to locate the sensor pipe to monitor a driveway. If you install a pair of alerts (this only works on a sufficiently long driveway), you can use the sequence of contact closures to detect whether the vehicle is coming or going. You’ll need two control boxes to do that, not one box with two sensors, because you need a separate set of alarm contacts for each sensor.
FIGURE 4: Driveway sensor placement
Figure 5 shows the Winland WaterBug, a moisture sensor you can use to detect either flooding or too-low levels in a water tank. The sensor at the bottom of the photo is upside down to show the four water contacts. Any conductivity between any pair of the contacts will trigger the sensor. The sensor detects basement flooding by placing it on the normally dry floor, and detects lack of water by mounting it underwater, making sure the lowest pair of contacts (or all four) are at the minimum desired water line. The contacts on the control module, from left to right, are input power, ground, two for the sensor, contact common, and the normally open and normally closed relay contacts.
FIGURE 5: Winland WaterBug
Figure 6 shows a typical magnetic door switch installed on a sliding glass door. The switch is in the upper corner where the door opens, with the wires trailing into the wall. The magnet is mounted on the door, and slides to the left away from the switch. The installation wasn’t difficult, because careful removal of the molding above the door provided a place to work to get the wires pulled through.
FIGURE 6: Magnetic door switch
Hiding the wires running from sensors to the processor is an art. You’ll probably need to make and repair holes in walls; multiple Internet sites offer help on how to do the repairs.