Processors
A PhidgetInterfaceKit 8/8/8, shown in Figure 7, connects sensors to the PC. The terminal block on the left ties to eight digital inputs, with ground terminals at either end of the block. The terminal block on the right connects to eight digital outputs, also with grounds on either end, and with pull up resistors on each output. Eight analog output ports are along the bottom, while the top has DC power input and output connectors on the left, a twoport USB hub in the middle, and the USB connector to the PC on the right. You only need to power the device separately if you plan to use the USB hub; otherwise, the interface draws all required power from the PC USB connection.
FIGURE 7: PhidgetInterfaceKit 8/8/8
Figure 8 shows the processor core of our system design. If you don’t want to use a pre-built temperature warning sensor, you can read a thermistor directly into any of the Phidget analog inputs — see text for how to convert a resistance into a voltage reading and how to convert that voltage reading into a temperature measurement.
FIGURE 8: Security system design
Any PC that can run the version of Linux or Windows you choose has enough capability to run an alarm system. Wipe the PC’s hard disk clean, and install just the software you need. Minimizing what’s on the machine will make it more reliable, and reduce the opportunity for software conflicts. Don’t forget to protect the machine with a hardware or software firewall, and good anti-virus software if you’ll connect it to the Internet. Don’t use the machine for anything else — have a second machine if you need to, and keep this one in a closet. E-mail and Web browsing are the most likely ways to bring in spyware and viruses, things you really don’t need to have on a machine you’re counting on as part of a security system. Don’t think of it as a PC, think of it as the processor running the security system.