Alerts
Installing a siren, if you choose to use one, is a matter of placing it where it can be heard and running wires back to the interface. You can drive a siren from one of the digital outputs, butbecause the output can pass no more than 20 milliamps, you’ll need to use a relay or transistor to buffer the higher power needed to drive the siren. Figure 9 shows a sample drive circuit for a DC siren using the same approach we use to drive the fan. Even if it requires more interface hardware to remote it, consider locating the siren away from the PC, and out of reach, to make it harder for an intruder to disable.
FIGURE 9: Siren drive circuit